Delocalization-Assisted Transport through Nucleic Acids in Molecular Junctions

The flow of charge through molecules is central to the function of supramolecular machines, and charge transport in nucleic acids is implicated in molecular signaling and DNA repair. We examine the transport of electrons through nucleic acids to understand the interplay of resonant and nonresonant charge carrier transport mechanisms. This study reports STM break junction measurements of peptide nucleic acids (PNAs) with a G-block structure and contrasts the findings with previous results for DNA duplexes. The conductance of G-block PNA duplexes is much higher than that of the corresponding DNA duplexes of the same sequence; however, they do not display the strong even-odd dependence conductance oscillations found in G-block DNA. Theoretical analysis finds that the conductance oscillation magnitude in PNA is suppressed because of the increased level of electronic coupling interaction between G-blocks in PNA and the stronger PNA-electrode interaction compared to that in DNA duplexes. The strong interactions in the G-block PNA duplexes produce molecular conductances as high as 3% G₀, where G₀ is the quantum of conductance, for 5 nm duplexes.

Spin-selective electron transmission through self-assembled monolayers of double-stranded peptide nucleic acid

Monolayers of chiral molecules can preferentially transmit electrons with a specific spin orientation, introducing chiral molecules as efficient spin filters. This phenomenon is established as chirality-induced spin selectivity (CISS) and was demonstrated directly for the first time in self-assembled monolayers (SAMs) of double-stranded DNA (dsDNA)¹ . Here, we discuss SAMs of double-stranded peptide nucleic acid (dsPNA) as a system which allows for systematic investigations of the influence of various molecular properties on CISS. In photoemission studies, SAMs of chiral, γ-modified PNA show significant spin filtering of up to P = (24.4 ± 4.3)% spin polarization. The polarization values found in PNA lacking chiral monomers are considerably lower at about P = 12%. The results confirm that the preferred spin orientation is directly linked to the molecular handedness and indicate that the spin filtering capacity of the dsPNA helices might be enhanced by introduction of chiral centers in the constituting peptide monomers.

Development of Cu2+-Based Distance Methods and Force Field Parameters for the Determination of PNA Conformations and Dynamics by EPR and MD Simulations

Peptide nucleic acids (PNAs) are a promising group of synthetic analogues of DNA and RNA that offer several distinct advantages over the naturally occurring nucleic acids for applications in biosensing, drug delivery, and nanoelectronics. Because of its structural differences from DNA/RNA, methods to analyze and assess the structure, conformations, and dynamics are needed. In this work, we develop synergistic techniques for the study of the PNA conformation. We use CuQ₂, a Cu²⁺ complex with 8-hydroxyquinoline (HQ), as an alternative base pair and as a spin label in electron paramagnetic resonance (EPR) distance methods. We use molecular dynamics (MD) simulations with newly developed force field parameters for the spin labels to interpret the distance constraints determined by EPR. We complement these methods by UV-vis and circular dichroism measurements and assess the efficacy of the Cu²⁺ label on a PNA duplex whose backbone is based on aminoethylglycine and a duplex with a hydroxymethyl backbone modification. We show that the Cu²⁺ label functions efficiently within the standard PNA and the hydroxymethyl-modified PNA and that the MD parameters may be used to accurately reproduce our EPR findings. Through the combination of EPR and MD, we gain new insights into the PNA structure and conformations as well as into the mechanism of orientational selectivity in Cu²⁺ EPR at X-band. These results present for the first time a rigid Cu²⁺ spin label used for EPR distance measurements in PNA and the accompanying MD force fields for the spin label. Our studies also reveal that the spin labels have a low impact on the structure of the PNA duplexes. The combined MD and EPR approach represents an important new tool for the characterization of the PNA duplex structure and provides valuable information to aid in the rational application of PNA at large.

SAT0188 HIGH-DOSE STEROIDS ARE IMPORTANT CONTRIBUTORS TO THE INFECTION BURDEN OF PATIENTS WITH LUPUS NEPHRITIS

Background:

Infection remains a major cause of morbidity and mortality in patients with systemic lupus erythematosus (SLE) and lupus nephritis (LN) treated with systemic immunosuppression (IS).

Objectives:

We sought to describe the infection profile in patients with LN treated with aggressive immunosuppression (induction and maintenance therapy) and to identify the associated risk factors.

Methods:

Patients with LN followed in the Nephrology Department of Fundeni Clinical Institute, were retrospectively reviewed for any infection that occurred from initiation of induction therapy. Infections were graded (1-5) according to the Common Terminology Criteria for Adverse Events. Infection site and type of microorganism were also recorded. Univariate and multivariate Cox proportional hazard regression analysis were performed in order to identify independent risk factors for infection.

Results:

The study cohort comprised 101 patients (86.1% females) with a mean age of 34 ± 14 years. Forty-eight patients (47.5%) had at least one infection with a total 92 episodes of infection occurring during a median follow-up of 17 months (IQR:8.5-52.5 months). The majority of patients (31 of 48) had infections during the first 12 months since IS treatment initiation. The most common site was lung infection (in 24.8% of patients), followed by urinary tract (20.8% of patients) and cutaneous/mucosal infections (11% of patients). Thirty-eight percent of patients had bacterial infections. Nineteen percent of patients had severe infections (grade 3 or higher) with 3.3% of infection-related deaths (3 patients). The most common induction regimen was cyclophosphamide in addition to corticosteroids (48.5%), with 44.6% of patients receiving pulse methylprednisolone and 45.5% of patients receiving more than 30 mg/d of prednisone as the maximum oral dose. In univariate Cox regression analysis, female gender (HR 3.34; 95% CI, 1.03-10.8, p=0.04), pulse methylprednisolone (HR 2.9; 95% CI, 1.6-5.24, p=0.001), high-dose (≥30 mg/day) oral corticosteroids (HR 4.22; 95% CI, 2.21-8.02,p=0.001) and SLEDAI score (HR 1.047; 95% CI, 1.012-1.084, p=0.008) were risk factors for infection. In multivariate Cox regression analysis, female gender (HR 6.35; 95%CI, 1.86-21.64,p=0.003), high-dose oral corticosteroids (HR 4.7; 95% CI, 2.25-9.87, p=0.003) and SLEDAI score (HR 1.046; 95% CI, 1.003-1.09, p=0.034) remained independent predictors of infection risk. Of the risk factors associated with severe infections (grade 3 or higher), in univariate analysis we identified younger age (HR 0.96, 95%CI, 0.92-0.99, p=0.035), neurological involvement (HR 2.59; 95%, 0.86-7.83, p=0.09), pulse methylprednisolone (HR 5.42; 95% CI, 1.79-16.35, p=0.003) and high-dose oral corticosteroids (HR 8.32; 95% CI, 2.4-28.77, p=0.001) as risk factors for infection. After multivariate adjustment, neurological involvement (HR 4.33; 95%, 1.29-14.51, p=0.01) and high-dose oral corticosteroids (HR 7.6; 95% CI, 1.6-35.39, p=0.01) were identified as independent predictors of infection risk.

Conclusion:

A high-dose oral corticosteroid regimen increased the risk for any infection and for severe infections by 4.7-fold and 7.6-fold, respectively. In addition, female gender and a higher SLEDAI score were identified as risk factors for any infection, while neurological involvement was associated with an increased risk for severe infections.

References:

[1]Jung JY, Yoon D, Choi Y, Kim HA, Suh CH. Associated clinical factors for serious infections in patients with systemic lupus erythematosus. Sci Rep. 2019;9(1):9704.

Disclosure of Interests: :

None declared

P1687 When the easiest answer is the right one

A 42 year-old woman presented with moderate exertion dyspnea within the past months. She was hypertensive, with end stage-chronic kidney disease (CKD) on hemodialysis for 8 years, initially by arteriovenous fistula and then by central venous catheter. She associated thrombosis of both common femoral veins.

Clinical examination revealed mild leg edema, hepatomegaly, III/VI systolic murmur, left brachiocephalic arteriovenous fistula with aneurismal dilation and subclavian central venous catheter. Resting ECG showed atrial flutter with variable block and biventricular hypertrophy. Lab studies highlighted mild anemia, glomerular filtration rate (MDRD) 13.5 ml/min/1.73m2, hyperhomocysteinemia.

Transthoracic echocardiography revealed left ventricle (LV) with normal diameters and mild concentric hypertrophy, mild systolic LV dysfunction and diastolic dysfunction (Fig A), dilation of both atria, mild mitral regurgitation, dilated RV with moderate global dysfunction, moderate tricuspid regurgitation and secondary pulmonary hypertension (PH) with estimated systolic pulmonary artery pressure of 79 mmHg (Fig C), moderate pulmonary regurgitation, moderate dilatation of pulmonary artery. Vascular Doppler ultrasonography showed thrombosis of the arteriovenous fistula and of right jugular vein. To confirm PH, right heart catheterization was necessary. The intervention could not be realized on femoral approach due to thrombosis of both common femoral veins. We considered the jugular approach, but weighing the risks/benefits balance, with one functional jugular vein, we decided not going for it, as the jugular approach was the only vascular approach for dialysis. We performed left catheterization right after a dialysis session and an LV end-diastolic pressure of 13 mmHg was measured. The possible causes of PH considered in this case were the CKD by itself, a pulmonary thromboembolic component in a patient with hyperhomocisteinemia and thrombosis of both femoral veins and a left heart component given the borderline value of LV end-diastolic pressure.

The patient received treatment with beta-blockers, blockers of the renin-angiotenins-aldosterone system, calcium channel blockers and oral anticoagulation. She was evaluated after two months describing improvement of symptoms. The resting ECG showed sinus rhythm and the echocardiography revealed improved parameters of LV diastolic function (Fig B), with reduction of systolic pulmonary artery pressure (Fig D) and improvement of right heart parameters.

PH is a frequent complication in patients with CKD on hemodialysis. Possible mechanism of its development include RV overload with increased pulmonary flow or creating a fistula with an increased flow through arteriovenous shunt. Thromboembolic complications and hyperhomocysteinemia are other frequent findings in these patients. Despite all the above, the left heart involvement with LV diastolic dysfunction remains a common and amendable cause of PH in this setting.

Abstract P1687 Figure.

The Disobeying 'Soldier': Use of an Achiral Group to Modulate Chiral Induction in PNA Duplexes

Peptide nucleic acid (PNA) is a synthetic analogue of DNA in which the natural nucleobases A, G, C, and T are linked to an achiral, charge neutral, pseudopeptide backbone. PNA strands can form double helices similar to DNA whose helical sense can be modulated by applying the 'sergeants-and-soldiers' principle. Attachment of a chiral amino acid (sergeant) at the C-terminus of PNA leads to the amplification of chirality of the sergeant onto the achiral PNA monomers (soldiers), resulting in an enantiomeric excess of either left- or right-handed PNA duplexes. In the present study we looked at the effect of an achiral N-terminal terpyridine (soldier) on the helicity of the double helix that contains <small>L</small>-lysine. We have found that terpyridine interferes with the chiral induction effect of the <small>L</small>-lysines, an effect that can be reverted upon coordination of Cu2+ ions to terpyridine.

Metal Coordination to Ligand-Modified Peptide Nucleic Acid Triplexes

A challenging goal in nanotechnology is the precise and programmable arrangement of specific elements in nanosystems in the three-dimensional space. The use of ligand-modified nucleic acids represents an accurate and selective tool to achieve this goal when it comes to metal ion organization. The synthesis of peptide nucleic acid (PNA) monomers that contain ligands instead of nucleobases makes possible the creation of metal-mediated alternative base pairs and triplets at specific locations in PNA duplexes and triplexes, respectively. We report the formation of four- and six-coordinate metal complexes between PNA triplexes modified with 2,2'-bipyridine (Bpy) or 8-hydroxyquinoline (Q) ligands and 3d metal ions. These metal complexes function as alternative base triplets or pairs in that they increase the thermal stability of the triplexes if the stability constants of the metal complexes are relatively high. The increase in the triplex melting temperature correlates with the stability constants of the metal complexes with ligand-containing PNA determined by UV-vis titrations. The metal complexes coordinate two or three ligands although three bidentate ligands are in close proximity of each other within a triplex. Metal coordination to ligand-modified PNA triplexes was further studied by electron paramagnetic resonance (EPR) spectroscopy and circular dichrosim (CD) spectroscopy. EPR spectroscopy indicated the formation of a square planar [CuQ₂] complex between Cu²⁺ and Q-containing PNA triplex. Taken together, the spectroscopic results indicate that in the presence of 1 equiv of Fe²⁺ or Ni²⁺ the majority, but not all, of the Bpy-containing PNA triplexes contain [MBpy₃] complexes, with a minority of them being metal free. We attribute this behavior to a supramolecular chelate effect exerted by the triplex, which favors the formation of tris-ligand complexes, that is balanced by the steric interactions between the metal complex and the adjacent nucleobase triplets, which decrease the stability of the complex and triplex. In contrast, the very high stability of square planar [MQ₂] complexes of Cu²⁺ and Ni²⁺ leads to formation of bis-ligand complexes instead of tris-ligand complexes with Q₃-containing PNA triplexes. The metal-containing PNA triplexes have a terminal l-lysine and adopt a left-handed chiral structure in solution. The handedness of the PNA triplex determines that of the metal complexes formed with the Bpy-containing PNA triplexes.

Histological predictors of renal outcome in lupus nephritis: the importance of tubulointerstitial lesions and scoring of glomerular lesions

Introduction Lupus nephritis (LN) affects nearly 60% of patients with systemic lupus erythematosus and up to 30% of them will progress to end-stage renal disease (ESRD), despite receiving aggressive immunosuppressive therapy. The prognostic value of ISN/RPS classification is controversial. Therefore, we aimed to identify clinical and pathological predictors of outcome in LN patients independent of this classification. Material and methods Thirty-seven patients with LN who underwent percutaneous kidney biopsy between 1997 and 2016 were included in this study. Twenty clinical and twenty histological variables were tested for their association with a composite end-point of doubling of serum creatinine, ESRD and death. Univariate and multivariate Cox proportional hazard regression analysis were performed to identify independent predictors of outcome. Results During a median follow-up period of 48 months (IQR: 17.5-120 months), 21.6% of patients reached the composite end-point. The overall survival rate of our cohort was 89% at one year, 86% at five years, 74% at 10 years and 64% at 20 years. Patients with Class IV LN showed the worst prognosis with 44% survival at 10 years, while those who additionally showed crescents and global sclerosis on kidney biopsy had an even lower survival of 21% and 0% at 10 years, respectively. After multivariate adjustment, we identified estimated glomerular filtration rate at baseline (HR, 0.91 per ml/min /1.73 m²; 95% CI, 0.84 to 0.99), 24-hour proteinuria at baseline (HR, 2.04 per g/d; 95% CI, 1.19 to 3.5), crescents (HR, 1.068 per %; 95% CI, 1.003 to 1.091), global sclerosis (HR, 1.036 per %; 95% CI, 0.984 to 1.091), presence of adhesions (HR, 9.2; 95% CI, 1.38 to 61.2) and tubulitis (HR, 13.1; 95% CI; 1.3 to 131) as independent predictors of outcome in our cohort of LN. Conclusions Our study identified glomerular (crescents, global sclerosis, adhesions) and tubulointerstitial (tubulitis) lesions, in addition to clinical variables (renal function, 24-hour proteinuria), as important predictors of renal outcome, independent of the ISN/RPS classification. We suggest that the ISN/RPS classification could be improved by a quantitative assessment of glomeruli with active and chronic lesions and by a greater emphasis given to tubulointerstitial lesions.

Chirality Dependent Charge Transfer Rate in Oligopeptides

It is shown that "spontaneous magnetization" occurs when chiral oligopeptides are attached to ferrocene and are self-assembled on a gold substrate. As a result, the electron transfer, measured by electrochemistry, shows asymmetry in the reduction and oxidation rate constants; this asymmetry is reversed between the two enantiomers. The results can be explained by the chiral induced spin selectivity of the electron transfer. The measured magnetization shows high anisotropy and the "easy axis" of magnetization is along the molecular axis.

Chiral Molecule-Enhanced Extinction Ratios of Quantum Dots Coupled to Random Plasmonic Structures

Devices based on self-assembled hybrid colloidal quantum dots (CQDs) coupled with specific organic linker molecules are a promising way to simply realize room-temperature, spectrally tunable light detectors. Nevertheless, this type of devices usually has low quantum efficiency. Plasmonics has been shown as an efficient tool in guiding and confining light at nanoscale dimensions. As plasmonic modes exhibit highly confined fields, they locally increase light-matter interactions and consequently enhance the performance of CQD-based photodetectors. Recent publications presented experimental results of large extinction enhancement from a monolayer of CQDs coupled to random gold nanoislands using a monolayer of organic alkyl linkers. We report here that a twofold larger extinction enhancement in the visible spectrum is observed when a monolayer of helical chiral molecules connects the CQDs to the gold structure instead of a monolayer of achiral linkers. We also show that this effect provides insight into the chirality of the molecules within the monolayer. In future work, we plan to evaluate the potential of these results to be used in the construction of a more efficient and sensitive photon detector based on surface QDs, as well as to supply a simple way to map the chirality of a single chiral monolayer.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex

The radical bridged compound [(Ni(TPMA))₂-μ-bmtz^•-](BF₄)₃·3CH₃CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 Ni^II centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm^-1 (-2J_Ni-radS_NiS_rad). DFT calculations support the existence of strong ferromagnetic exchange.

Effects of the Backbone and Chemical Linker on the Molecular Conductance of Nucleic Acid Duplexes

Scanning tunneling microscope break junction measurements are used to examine how the molecular conductance of nucleic acids depends on the composition of their backbone and the linker group to the electrodes. Molecular conductances of 10 base pair long homoduplexes of DNA, aeg-PNA, γ-PNA, and a heteroduplex of DNA/aeg-PNA with identical nucleobase sequence were measured. The molecular conductance was found to vary by 12 to 13 times with the change in backbone. Computational studies show that the molecular conductance differences between nucleic acids of different backbones correlate with differences in backbone structural flexibility. The molecular conductance was also measured for duplexes connected to the electrode through two different linkers, one directly to the backbone and one directly to the nucleobase stack. While the linker causes an order-of-magnitude increase in the overall conductance for a particular duplex, the differences in the electrical conductance with backbone composition are preserved. The highest molecular conductance value, 0.06G₀, was measured for aeg-PNA duplexes with a base stack linker. These findings reveal an important new strategy for creating longer and more complex electroactive, nucleic acid assemblies.

Comparative Incorporation of PNA into DNA Nanostructures

DNA has shown great promise as a building material for self-assembling nanoscale structures. To further develop the potential of this technology, more methods are needed for functionalizing DNA-based nanostructures to increase their chemical diversity. Peptide nucleic acid (PNA) holds great promise for realizing this goal, as it conveniently allows for inclusion of both amino acids and peptides in nucleic acid-based structures. In this work, we explored incorporation of a positively charged PNA within DNA nanostructures. We investigated the efficiency of annealing a lysine-containing PNA probe with complementary, single-stranded DNA sequences within nanostructures, as well as the efficiency of duplex invasion and its dependence on salt concentration. Our results show that PNA allows for toehold-free strand displacement and that incorporation yield depends critically on binding site geometry. These results provide guidance for the design of PNA binding sites on nucleic acid nanostructures with an eye towards optimizing fabrication yield.

Luminescence quenching by photoinduced charge transfer between metal complexes in peptide nucleic acids

A new scaffold for studying photoinduced charge transfer has been constructed by connecting a [Ru(Bpy)3](2+) donor to a bis(8-hydroxyquinolinate)2 copper [CuQ2] acceptor through a peptide nucleic acid (PNA) bridge. The luminescence of the [Ru(Bpy)3](2+*) donor is quenched by electron transfer to the [CuQ2] acceptor. Photoluminescence studies of these donor-bridge-acceptor systems reveal a dependence of the charge transfer on the length and sequence of the PNA bridge and on the position of the donor and acceptor in the PNA. In cases where the [Ru(Bpy)3](2+) can access the π base stack at the terminus of the duplex, the luminescence decay is described well by a single exponential; but if the donor is sterically hindered from accessing the π base stack of the PNA duplex, a distribution of luminescence lifetimes for the donor [Ru(Bpy)3](2+*) is observed. Molecular dynamics simulations are used to explore the donor-PNA-acceptor structure and the resulting conformational distribution provides a possible explanation for the distribution of electron transfer rates.

A three-step kinetic model for electrochemical charge transfer in the hopping regime

Single-step nonadiabatic electron tunneling models are widely used to analyze electrochemical rates through self-assembled monolayer films (SAMs). For some systems, such as nucleic acids, long-range charge transfer can occur in a "hopping" regime that involves multiple charge transfer events and intermediate states. This report describes a three-step kinetic scheme to model charge transfer in this regime. Some of the features of the three-step model are probed experimentally by changing the chemical composition of the SAM. This work uses the three-step model and a temperature dependence of the charge transfer rate to extract the charge injection barrier for a SAM composed of a 10-mer peptide nucleic acid that operates in the hopping regime.

The single-molecule conductance and electrochemical electron-transfer rate are related by a power law

This study examines quantitative correlations between molecular conductances and standard electrochemical rate constants for alkanes and peptide nucleic acid (PNA) oligomers as a function of the length, structure, and charge transport mechanism. The experimental data show a power-law relationship between conductances and charge transfer rates within a given class of molecules with the same bridge chemistry, and a lack of correlation when a more diverse group of molecules is compared, in contrast with some theoretical predictions. Surprisingly, the PNA duplexes exhibit the lowest charge-transfer rates and the highest molecular conductances. The nonlinear rate-conductance relationships for structures with the same bridging chemistries are attributed to differences in the charge-mediation characteristics of the molecular bridge, energy barrier shifts and electronic dephasing, in the two different experimental settings.

Binding of Eu(III) to 1,2-hydroxypyridinone-modified peptide nucleic acids

Substitution of a nucleobase pair with a pair of 1,2-hydroxypyridinone (1,2-HOPO) ligands in the center of a 10-base-pair peptide nucleic acid (PNA) duplex provides a strong binding site for Eu(III) as evidenced by UV thermal melting curves, UV titrations, and luminescence spectroscopy. Eu(III) excitation spectra and luminescence lifetime data are consistent with Eu(III) bound to both 1,2 HOPO ligands in a PNA-HOPO duplex as the major species present in solution.

Charge transfer through modified peptide nucleic acids

We studied the charge transfer properties of bipyridine-modified peptide nucleic acid (PNA) in the absence and presence of Zn(II). Characterization of the PNA in solution showed that Zn(II) interacts with the bipyridine ligands, but the stability of the duplexes was not affected significantly by the binding of Zn(II). The charge transfer properties of these molecules were examined by electrochemistry for self-assembled monolayers of ferrocene-terminated PNAs and by conductive probe atomic force microscopy for cysteine-terminated PNAs. Both electrochemical and single molecular studies showed that the bipyridine modification and Zn(II) binding do not affect significantly the charge transfer of the PNA duplexes.

Coordination-driven inversion of handedness in ligand-modified PNA

Peptide nucleic acid (PNA) is a synthetic analogue of DNA, which has the same nucleobases as DNA but typically has a backbone based on aminoethyl glycine (Aeg). PNA forms duplexes by Watson Crick hybridization. The Aeg-based PNA duplexes adopt a chiral helical structure but do not have a preferred handedness because they do not contain a chiral center. An L-lysine situated at the C-end of one or both strands of a PNA duplex causes the duplex to preferably adopt a left-handed structure. We have introduced into the PNA duplexes both a C-terminal L-lysine and one or two PNA monomers that have a γ-(S)-methyl-aminoethyl glycine backbone, which is known to induce a preference for a right-handed structure. Indeed, we found that in these duplexes the γ-methyl monomer exerts the dominant chiral induction effect causing the duplexes to adopt a right-handed structure. The chiral PNA monomer had a 2,2':6',2''-terpyridine (Tpy) ligand instead of a nucleobase and PNA duplexes that contained one or two Tpys formed [Cu(Tpy)(2)](2+) complexes in the presence of Cu(2+). The CD spectroscopy studies showed that these metal-coordinated duplexes were right-handed due to the chiral induction effect exerted by the S-Tpy PNA monomer(s) except for the cases when the [Cu(Tpy)(2)](2+) complex was formed with Tpy ligands from two different PNA duplexes. In the latter case, the metal complex bridged the two PNA duplexes and the duplexes were left-handed. The results of this study show that the preferred handedness of a ligand-modified PNA can be switched as a consequence of metal coordination to the ligand. This finding could be used as a tool in the design of functional nucleic-acid based nanostructures.

Metal binding to ligand-containing peptide nucleic acids

The substitution of nucleobases in nucleic acid duplexes with ligands that have high affinity for transition metal ions creates metal-binding sites at specific locations within the duplexes. Several studies on the incorporation of metal ions into DNA and peptide nucleic acid (PNA) duplexes have suggested that the stability constant of the metal complex formed within the duplexes is a primary determinant of the thermal stability of the duplexes. To understand this relationship, we have synthesized two PNA monomers that carry the same ligand, namely 8-hydroxyquinoline, but have this ligand attached differently to the PNA backbone. The PNA monomers have been incorporated into PNA duplexes. UV and CD spectroscopy and calorimetric studies of the 8-hydroxyquinoline-PNA duplexes showed that the effect of the stability of the metal complex on the PNA duplexes was significantly modulated by the steric relationship between the complex and the duplex. This information is useful for the construction of hybrid inorganic-nucleic acid nanostructures.

Evidence for a near-resonant charge transfer mechanism for double-stranded peptide nucleic acid

We present evidence for a near-resonant mechanism of charge transfer in short peptide nucleic acid (PNA) duplexes obtained through electrochemical, STM break junction (STM-BJ), and computational studies. A seven base pair (7-bp) PNA duplex with the sequence (TA)(3)-(XY)-(TA)(3) was studied, in which XY is a complementary nucleobase pair. The experiments showed that the heterogeneous charge transfer rate constant (k(0)) and the single-molecule conductance (σ) correlate with the oxidation potential of the purine base in the XY base pair. The electrochemical measurements showed that the enhancement of k(0) is independent, within experimental error, of which of the two PNA strands contains the purine base of the XY base pair. 7-bp PNA duplexes with one or two GC base pairs had similar measured k(0) and conductance values. While a simple superexchange model, previously used to rationalize charge transfer in single stranded PNA (Paul et al. J. Am. Chem. Soc. 2009, 131, 6498-6507), describes some of the experimental observations, the model does not explain the absence of an enhancement in the experimental k(0) and σ upon increasing the G content in the duplexes from one to two. Moreover, the superexchange model is not consistent with other studies (Paul et al. J. Phys. Chem. B 2010, 114, 14140), that showed a hopping charge transport mechanism is likely important for PNA duplexes longer than seven base pairs. A quantitative computational analysis shows that a near-resonant charge transfer regime, wherein a mix of superexchange and hopping mechanisms are expected to coexist, can rationalize all of the experimental results.

The crystal structure of non-modified and bipyridine-modified PNA duplexes

Peptide nucleic acid (PNA) is a synthetic analogue of DNA that commonly has an N-aminoethyl glycine backbone. The crystal structures of two PNA duplexes, one containing eight standard nucleobase pairs (GGCATGCC)(2), and the other containing the same nucleobase pairs and a central pair of bipyridine ligands, have been solved with a resolution of 1.22 and 1.10 Å, respectively. The non-modified PNA duplex adopts a P-type helical structure similar to that of previously characterized PNAs. The atomic-level resolution of the structures allowed us to observe for the first time specific modes of interaction between the terminal lysines of the PNA and the backbone and the nucleobases situated in the vicinity of the lysines, which are considered an important factor in the induction of a preferred handedness in PNA duplexes. Our results support the notion that whereas PNA typically adopts a P-type helical structure, its flexibility is relatively high. For example, the base-pair rise in the bipyridine-containing PNA is the largest measured to date in a PNA homoduplex. The two bipyridines bulge out of the duplex and are aligned parallel to the major groove of the PNA. In addition, two bipyridines from adjacent PNA duplexes form a π-stacked pair that relates the duplexes within the crystal. The bulging out of the bipyridines causes bending of the PNA duplex, which is in contrast to the structure previously reported for biphenyl-modified DNA duplexes in solution, where the biphenyls are π stacked with adjacent nucleobase pairs and adopt an intrahelical geometry. This difference shows that relatively small perturbations can significantly impact the relative position of nucleobase analogues in nucleic acid duplexes.

Cliniconeuropathologic correlates of human immunodeficiency virus in the era of antiretroviral therapy

The objective of this study was to examine the spectrum of human immunodeficiency virus (HIV) brain pathology and its clinical correlates in the antiretroviral era. We carried out a cross-sectional survey, analyzing prospective clinical and neuropathological data collected by the National NeuroAIDS Tissue Consortium (NNTC), comprising 589 brain samples from individuals with advanced HIV disease collected from 1999 onwards. We assessed gender, ethnicity/race, mode of transmission, age, year of death, nadir CD4, plasma viral load, last antiretroviral regimen, presence of parenchymal HIV brain pathology, HIV-associated neurocognitive disorder, and major depressive disorder. We compared cohort demographic variables with Centers for Disease Control and Prevention US HIV/AIDS statistics and examined associations of parenchymal HIV brain pathology with demographic, clinical, and HIV disease factors. With regard to Centers for Disease Control and Prevention US data, the NNTC was similar in age distribution, but had fewer females and African Americans and more Hispanics and men who have sex with men. Only 22% of the brains examined were neuropathologically normal. Opportunistic infections occurred in 1% to 5% of the cohort. Parenchymal HIV brain pathology was observed in 17.5% of the cohort and was associated with nadir CD4 and plasma viral load. Brains without parenchymal HIV brain pathology often had other noninfectious findings or minimal nondiagnostic abnormalities that were associated with HIV-associated neurocognitive disorder. Clinically, 60% of the cohort reported a lifetime episode of major depressive disorder and 88% had a HIV-associated neurocognitive disorder. No pathological finding correlated with major depressive disorder. Both antiretroviral treatment regimen and elevated plasma HIV viral load were associated with presence of parenchymal HIV brain pathology; however, multivariate analyses suggest a stronger association with plasma viral load. The frequency of HIV brain pathology was lower than previous pre-antiretroviral reports, and was predicted by lower nadir CD4 and higher plasma viral load. Noninfectious pathologies and minimal changes correlated with HIV-associated neurocognitive disorder, suggesting a shift in pathogenesis from florid HIV replication to other, diverse mechanisms.

The structure of a gamma-modified peptide nucleic acid duplex

This paper presents the results of an NMR spectroscopy and distance-restrained molecular dynamics (MD) study of a gamma-methylated, palindromic, 8-base pair peptide nucleic acid (gamma-PNA) duplex. The goal of this study was to examine the impact of the gamma-backbone modification on the structure of the PNA duplex. The 2D NMR information involving the backbone methyl group, especially the NOEs between the methyl protons and those of the amide and methylene groups of the backbone, led to distance restraints useful in the elucidation of the structure of the backbone of gamma-PNA. Integration of the NOE peaks resulted in 138 inter-proton distance restraints, which were used in ten independent simulated annealing followed by 2 ns restrained MD runs. These simulations led to the conclusion that the gamma-PNA duplex adopts a general P-form helical structure similar to that observed for non-modified PNA but with a smaller base pair rise, which is an A-like helical feature, and a slight helical bending towards the major groove (PDB ID ). These properties of the gamma-PNA duplex may be induced by the gamma-methyl group. A similar effect of the methyl group was revealed by a previous NMR study of single stranded gamma-PNA [A. Dragulescu-Andrasi, S. Rapireddy, B. M. Frezza, C. Gayathri, R. R. Gil and D. H. Ly, J. Am. Chem. Soc., 2006, 128, 10258-10267]. It appears that the steric constraint exerted by the gamma-methyl on the backbone orientation is relatively independent of the base pairing and stacking and thus is likely to manifest when other substituents are introduced at the gamma-position of the PNA.

Distance dependence of the charge transfer rate for peptide nucleic acid monolayers

Charge transfer studies have been performed for self-assembled monolayers of single-stranded and double-stranded peptide nucleic acids (PNAs) having a C-terminus cysteine and an N-terminus ferrocene group as a redox reporter. The decay of the charge transfer rate with distance was strong for short single-stranded PNA molecules and weak for long single-stranded and double-stranded PNAs. Possible mechanisms for this "softening" of the distance dependence are discussed. The nature of the mechanism change can be explained by a transition of the charge transport mechanism from superexchange-mediated tunneling for short PNAs to a "hopping" mechanism for long PNAs.

Role of nucleobase energetics and nucleobase interactions in single-stranded peptide nucleic acid charge transfer

Self-assembled monolayers of single-stranded (ss) peptide nucleic acids (PNAs) containing seven nucleotides (TTTXTTT), a C-terminus cysteine, and an N-terminus ferrocene redox group were formed on gold electrodes. The PNA monomer group (X) was selected to be either cytosine (C), thymine (T), adenine (A), guanine (G), or a methyl group (Bk). The charge transfer rate through the oligonucleotides was found to correlate with the oxidation potential of X. Kinetic measurements and computational studies of the ss-PNA fragments show that a nucleobase mediated charge transport mechanism in the deep tunneling superexchange regime can explain the observed dependence of the kinetics of charge transfer on the PNA sequence. Theoretical analysis suggests that the charge transport is dominantly hole-mediated and takes place through the filled bridge orbitals. The strongest contribution to conductance comes from the highest filled orbitals (HOMO, HOMO-1, and HOMO-2) of individual bases, with a rapid drop off in contributions from lower lying filled orbitals. Our studies further suggest that the linear correlation observed between the experimental charge transfer rates and the oxidation potential of base X arises from weak average interbase couplings and similar stacking geometries for the four TTTXTTT systems.

Solution structure of a peptide nucleic acid duplex from NMR data: features and limitations

This paper describes the results of a 1D and 2D NMR spectroscopy study of a palindromic 8-base pair PNA duplex GGCATGCC in H2O and H2O-D2O solutions. The (1)H NMR peaks have been assigned for most of the protons of the six central base pairs, as well as for several amide protons of the backbone. The resulting 36 interbase and base-backbone distance restraints were used together with Watson-Crick restraints to generate the PNA duplex structure in the course of 10 independent simulated annealing runs followed by restrained molecular dynamics (MD) simulations in explicit water. The resulting PNA structures correspond to a P-type helix with helical parameters close to those observed in the crystal structures of PNA. Based on the current limited number of restraints obtained from NMR spectra, alternative structures obtained by MD from starting PNA models based on DNA cannot be ruled out and are also discussed.

Metal binding to bipyridine-modified PNA

Substitution of natural nucleobases in PNA oligomers with ligands is a strategy for directing metal ion incorporation to specific locations within a PNA duplex. In this study, we have synthesized PNA oligomers that contain up to three adjacent bipyridine ligands and examined the interaction with Ni2+ and Cu2+ of these oligomers and of duplexes formed from them. Variable-temperature UV spectroscopy showed that duplexes containing one terminal pair of bipyridine ligands are more stable upon metal binding than their nonmodified counterparts. While binding of one metal ion to duplexes that contain two adjacent bipyridine pairs makes the duplexes more stable, additional metal ions lower the duplex stability, with electrostatic repulsions being, most likely, an important contributor to the destabilization. UV titrations showed that the presence of several bipyridine ligands in close proximity of each other in PNA oligomers exerts a chelate effect. A supramolecular chelate effect occurs when several bipyridines are brought next to each other by hybridization of PNA duplexes. EPR spectroscopy studies indicate that even when two Cu2+ ions coordinate to a PNA duplex in which two bipyridine pairs are next to each other, the two metal-ligand complexes that form in the duplex are far enough from each other that the dipolar coupling is very weak. EXAFS and XANES show that the Ni2+-bipyridine bond lengths are typical for [Ni(bipy)2]2+ and [Ni(bipy)3]2+ complexes.

Properties of Prussian Blue Materials Manifested in Molecular Complexes: Observation of Cyanide Linkage Isomerism and Spin-Crossover Behavior in Pentanuclear Cyanide Clusters

Pentanuclear, cyanide-bridged clusters [M(tmphen)2]3[M'(CN)6]2 (M/M' = Zn/Cr (1), Zn/Fe (2), Fe/Fe (3), Fe/Co (4), and Fe/Cr (5); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) were prepared by combining [M'III(CN)6]3- anions with mononuclear complexes of MII ions with two capping tmphen ligands. The clusters consist of a trigonal bipyramidal (TBP) core with three MII ions in the equatorial positions and two M'III ions in the axial positions. Compounds 1-4 are isostructural and crystallize in the monoclinic space group P21/c. Complex 5 crystallizes in the enantiomorphic space group P3221. The magnetic properties of compounds 1 and 2 reflect the contributions of the individual [CrIII(CN)6]3- and [FeIII(CN)6]3- ions. The FeII ions in compounds 3 and 4 exhibit a gradual, temperature-induced spin transition between high spin (HS) and low spin (LS), as determined by the combination of Mössbauer spectroscopy, magnetic measurements, and single-crystal X-ray studies. The investigation of compound 5 by these methods and by IR spectroscopy indicates that cyanide linkage isomerism occurs during cluster formation. The magnetic behavior of 5 is determined by weak ferromagnetic coupling between the axial CrIII centers mediated by the equatorial diamagnetic FeII ions. Mössbauer spectra collected in the presence of a high applied field have allowed, for the first time, the direct experimental observation of uncompensated spin density at diamagnetic metal ions that bridge paramagnetic metal ions.

Influence of metal coordination on the mismatch tolerance of ligand-modified PNA duplexes

Recent studies on metal incorporation in ligand-modified nucleic acids have focused on the effect of metal coordination on the stability of metal-containing duplexes or triplexes and on the metal binding selectivity but did not address the effect of the sequence of the nucleic acid in which the ligands are incorporated. We have introduced 8-hydroxyquinoline Q in 10-mer PNA strands with various sequences and have investigated the properties of the duplexes formed from these strands upon binding of Cu(2+). Variable-temperature UV-vis spectroscopy shows that, in the presence of Cu(2+), duplexes are formed even from ligand-modified Q-PNA strands that have a large number of mismatches. Spectrophotometric titrations demonstrate that at any temperature, one Cu(2+) ion binds a pair of Q-PNA strands that each contain one 8-hydroxyquinoline, but below the melting temperature, the PNA duplex exerts a supramolecular chelate effect, which prevents the transformation in the presence of excess Cu(2+) of the 1:2 Cu(2+):Q-PNA complexes into 1:1 complexes. EPR spectroscopy gives further support for the existence in the duplexes of [CuQ(2)] moieties that are similar to the corresponding square planar synthetic complex formed between Cu(2+) and 8-hydroxyquinoline. As PNA duplexes show a preferred handedness due to the chiral induction effect of a C-terminal l-lysine, which is transmitted through stacking interactions within the duplex, only if the metal-containing duplex has complementary strands, does it show a chiral excess measured by CD spectroscopy. The strong effect of the metal-ligand moiety is suggestive of an increased correlation length in PNA duplexes that contain such moieties. These results indicate that strong metal-ligand alternative base pairs significantly diminish the importance of Watson-Crick base pairing for the formation of a stable PNA duplex and lead to high mismatch tolerance, a principle that can be used in the construction of hybrid inorganic-nucleic acid nanostructures.

A Charge-Transfer-Induced Spin Transition in a Discrete Complex: The Role of Extrinsic Factors in Stabilizing Three Electronic Isomeric Forms of a Cyanide-Bridged Co/Fe Cluster

A series of bimetallic, trigonal bipyramidal clusters of type {[Co(N-N)(2)](3)[Fe(CN)(6)](2)} are reported. The reaction of {Co(tmphen)(2)}(2+) with [Fe(CN)(6)](3)(-) in MeCN affords {[Co(tmphen)(2)](3)[Fe(CN)(6)](2)} (1). The cluster can exist in three different solid-state phases: a red crystalline phase, a blue solid phase obtained by exposure of the red crystals to moisture, and a red solid phase obtained by desolvation of the blue solid phase in vacuo. The properties of cluster 1 are extremely sensitive to both temperature and solvent content in each of these phases. Variable-temperature X-ray crystallography; (57)Fe Mossbauer, vibrational, and optical spectroscopies; and magnetochemical studies were used to study the three phases of 1 and related compounds, Na{[Co(tmphen)(2)](3)[Fe(CN)(6)](2)}(ClO(4))(2) (2), {[Co(bpy)(2)](3)[Fe(CN)(6)](2)}[Fe(CN)(6)](1/3) (3), and {[Ni(tmphen)(2)](3)[Fe(CN)(6)](2)} (4). The combined structural and spectroscopic investigation of 1-4 leads to the unambiguous conclusion that 1 can exist in different electronic isomeric forms, {Co(III)(2)Co(II)Fe(II)(2)} (1A), {Co(III)Co(II)(2)Fe(III)Fe(II)} (1B), and {Co(II)(3)Fe(III)(2)} (1C), and that it can undergo a charge-transfer-induced spin transition (CTIST). This is the first time that such a phenomenon has been observed for a Co/Fe molecule.

A Charge-Transfer-Induced Spin Transition in the Discrete Cyanide-Bridged Complex {[Co(tmphen)2]3[Fe(CN)6]2}

A charge-transfer-induced spin transition (CTIST) is observed in the discrete cyanide-bridged complex, {[Co(tmphen)2]3[Fe(CN)6]2}. Single-crystal X-ray diffraction, 57Fe Mössbauer spectroscopy, and magnetic susceptibility were used collectively to describe the oxidation states of the Co and Fe ions in this cluster as a function of temperature. This pentanuclear complex represents the first example of a CTIST at the discrete molecular level.

Metal incorporation in modified PNA duplexes

Substitution of bipyridine for a nucleobase leads to modified peptide nucleic acid (PNA) single strands that are bridged in the presence of Ni2+ into a duplex containing a combination of hydrogen and coordinative bonds. CD experiments demonstrate that the duplex adopts a structure similar to that of an unmodified 10-bp PNA duplex, and UV melting experiments show a very sensitive dependence of the duplex stability on the substitution of a nucleobase pair with a pair of ligands or a metal-ligand alternative base pair.

Biochemical, Mössbauer, and EPR studies of the diiron cluster of phenol hydroxylase from Pseudomonas sp. strain CF 600

Phenol hydroxylase of Pseudomonas sp. strain CF600 comprises three components: DmpP is an FAD- and [2Fe-2S]-containing reductase; DmpM is a cofactorless activator protein; and DmpLNO is the oxygenase. Single turnover experiments established that DmpLNO contains the active site, but requires DmpM for efficient turnover: the steady-state turnover rate reaches a maximum at 1.5 DmpM:1 DmpLNO. Chemical cross-linking experiments showed that DmpM interacts with the large subunit of the DmpLNO oxygenase complex. Mössbauer studies revealed that the active site of the oxygenase can accommodate two types of diiron clusters, each of these cluster types having two equivalent sites. Cluster form I, representing typically around 85% of total Fe, has DeltaE(Q) = 1.73 mm/s and delta = 0.54 mm/s, while cluster II exhibits DeltaE(Q) = 0.79 mm/s and delta = 0.48 mm/s. Studies in strong applied magnetic fields suggest that the two iron sites of cluster I are bridged by an oxo group while sites in cluster II appear to be hydroxo-bridged. Reduction of the samples with dithionite yields the diferrous forms of the clusters. Air oxidation of the reduced samples leads to an increase of the cluster II fraction, accompanied by a corresponding decrease in catalytic activity. The reduced oxygenase samples exhibit at X-band an integer spin EPR signal centered, in parallel mode, at g = 16.6. Quantitative analysis showed that 19% of the clusters contribute to the EPR signal, suggesting that cluster II is the EPR-active species. Incubation with dithiothreitol (DTT) inactivated the oxygenase by a mechanism apparently involving H(2)O(2) generation. In addition, Mössbauer studies of DTT-inactivated enzyme showed that all ferric iron belonged to one diamagnetic diferric cluster with parameters that indicate that DTT coordinates to the cluster.

High-nuclearity sulfide-rich molybdenum[bond]iron[bond]sulfur clusters: reevaluation and extension

High-nuclearity Mo[bond]Fe[bond]S clusters are of interest as potential synthetic precursors to the MoFe(7)S(9) cofactor cluster of nitrogenase. In this context, the synthesis and properties of previously reported but sparsely described trinuclear [(edt)(2)M(2)FeS(6)](3-) (M = Mo (2), W (3)) and hexanuclear [(edt)(2)Mo(2)Fe(4)S(9)](4-) (4, edt = ethane-1,2-dithiolate; Zhang, Z.; et al. Kexue Tongbao 1987, 32, 1405) have been reexamined and extended. More accurate structures of 2-4 that confirm earlier findings have been determined. Detailed preparations (not previously available) are given for 2 and 3, whose structures exhibit the C(2) arrangement [[(edt)M(S)(mu(2)-S)(2)](2)Fe(III)](3-) with square pyramidal Mo(V) and tetrahedral Fe(III). Oxidation states follow from (57)Fe Mössbauer parameters and an S = (3)/(2) ground state from the EPR spectrum. The assembly system 2/3FeCl(3)/3Li(2)S/nNaSEt in methanol/acetonitrile (n = 4) affords (R(4)N)(4)[4] (R = Et, Bu; 70-80%). The structure of 4 contains the [Mo(2)Fe(4)(mu(2)-S)(6)(mu(3)-S)(2)(mu(4)-S)](0) core, with the same bridging pattern as the [Fe(6)S(9)](2-) core of [Fe(6)S(9)(SR)(2)](4-) (1), in overall C(2v) symmetry. Cluster 4 supports a reversible three-member electron transfer series 4-/3-/2- with E(1/2) = -0.76 and -0.30 V in Me(2)SO. Oxidation of (Et(4)N)(4)[4] in DMF with 1 equiv of tropylium ion gives [(edt)(2)Mo(2)Fe(4)S(9)](3-) (5) isolated as (Et(4)N)(3)[5].2DMF (75%). Alternatively, the assembly system (n = 3) gives the oxidized cluster directly as (Bu(4)N)(3)[5] (53%). Treatment of 5 with 1 equiv of [Cp(2)Fe](1+) in DMF did not result in one-electron oxidation but instead produced heptanuclear [(edt)(2)Mo(2)Fe(5)S(11)](3-) (6), isolated as the Bu(4)N(+)salt (38%). Cluster 6 features the previously unknown core Mo(2)Fe(5)(mu(2)-S)(7)(mu(3)-S)(4) in molecular C(2) symmetry. In 4-6, the (edt)MoS(3) sites are distorted trigonal bipramidal and the FeS(4) sites are distorted tetrahedral with all sulfide ligands bridging. Mössbauer spectroscopic data for 2 and 4-6 are reported; (mean) iron oxidation states increase in the order 4 < 5 approximately 1 < 6 approximately 2. Redox and spectroscopic data attributed earlier to clusters 2 and 4 are largely in disagreement with those determined in this work. The only iron and molybdenum[bond]iron clusters with the same sulfide content as the iron[bond]molybdenum cofactor of nitrogenase are [Fe(6)S(9)(SR)(2)](4-) and [(edt)(2)Mo(2)Fe(4)S(9)](3-)(,4-).

Cellular therapies for neurodegenerative diseases

We are confident that neurodegenerative diseases will be managed by an array of pharmacologic and cellular therapies. These may involve oral or intravenous preparations, surgical cell deliveries, or infusion through cerebral delivery systems. Achievements in neuroscience over the past three decades have redefined some of the rules for basic and clinical research in neurotransplantation. Restorative neurosurgical procedures will develop from different directions, and it is likely that a combination of approaches will be necessary to maximize patient outcomes.

Influence of extrinsic factors on electron transfer in a mixed-valence Fe(2+)/Fe(3+) complex: experimental results and theoretical considerations

The crystal structure of the mixed-valence complex (NEt(4))[Fe(2)(salmp)(2)].xMeCN(crystal) (x = 2,3) [1].xMeCN(crystal) was determined at temperatures between 153 and 293 K. The complex shows distinct Fe(2+) and Fe(3+) sites over this temperature interval. Variable temperature Mössbauer spectra confirm the valence-localized character of the complex. In contrast, spectroscopic investigation of powder samples generated from [1].xMeCN(crystal) indicate the presence of a valence-averaged component at temperatures above 150 K. To elucidate this apparent contradiction we have conducted a variable-temperature Mössbauer investigation of different forms of 1, including [1].xMeCN(crystal), [1].2DMF(crystal), [1].yMeCN(powder), and solution samples of 1 in acetonitrile. The low-temperature Mössbauer spectra of all forms are virtually identical and confirm the valence-localized nature of the S = (9)/(2) ground state. The high-temperature spectra reveal a subtle control of electron hopping by the environment of the complexes. Thus, [1].xMeCN(crystal) has valence-localized spectra at all explored temperatures, [1].2DMF(crystal) exhibits a complete collapse into a valence-averaged spectrum over a narrow temperature range, the powder exhibits partial valence averaging over a broad temperature interval, and the solution sample shows at 210 K the presence of a valence-averaged component in a minor proportion. The spectral transformations are characterized by a coexistence of valence-localized and valence-averaged spectral components. This phenomenon cannot be explained by intramolecular electron hopping between the valence-localized states Fe(A)(2+)Fe(B)(3+) and Fe(A)(3+)Fe(B)(2+) in a homogeneous ensemble of complexes, but requires relaxation processes involving at least three distinguishable states of the molecular anion. Hopping rates for [1].2DMF(crystal) and [1].yMeCN(powder) have been determined from spectral simulations, based on stochastic line shape theory. Analysis of the temperature dependences of the transfer rates reveals the existence of thermally activated processes between (quasi) degenerate excited states in both forms. The preexponential factors in the rate law for the hopping processes in the [1].yMeCN(powder) and [1].2DMF(crystal) differ dramatically and suggest an important influence of the asymmetry of the complex environment (crystal) on intramolecular electron hopping. The differences between the spectra for the crystalline sample [1].xMeCN and those for powders generated under vacuum from these crystals indicate that solvate depletion has a profound effect on the dynamic behavior. Finally, two interpretations for the three states involved in the relaxation processes in 1 are given and critically discussed (salmp = bis(salicyledeneamino)-2-methylphenolate(3-)).