Self-assembled complexes of oligopeptides and metalloporphyrins: measurements of the reorganization and electronic interaction energies for photoinduced electron-transfer reactions

Excited-state proton-coupled electron transfer within ion pairs

The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.

Metal complexes as "protein surface mimetics"

A key challenge in chemical biology is to identify small molecule regulators for every single protein. However, protein surfaces are notoriously difficult to recognise with synthetic molecules, often having large flat surfaces that are poorly matched to traditional small molecules. In the surface mimetic approach, a supramolecular scaffold is used to project recognition groups in such a manner as to make multivalent non-covalent contacts over a large area of protein surface. Metal based supramolecular scaffolds offer unique advantages over conventional organic molecules for protein binding, including greater stereochemical and geometrical diversity conferred through the metal centre and the potential for direct assessment of binding properties and even visualisation in cells without recourse to further functionalisation. This feature article will highlight the current state of the art in protein surface recognition using metal complexes as surface mimetics.

Characterization of mesoscale coiled-coil peptide-porphyrin complexes

Photoelectronically conductive self-assembling peptide-porphyrin assemblies have great potential in their use as biomaterials, owing largely to their environmentally responsive properties. We have successfully designed a coiled-coil peptide that can self-assemble to form mesoscale filaments and serve as a scaffold for porphyrin interaction. In our earlier work, peptide-porphyrin-based biomaterials were formed at neutral pH, but the structures were irregular at the nano- to microscale size range, as judged by atomic force microscopy. We identified a pH in which mesoscale fibrils were formed, taking advantage of the types of porphyrin interactions that are present in well-characterized J-aggregates. We used UV-visible spectroscopy, circular dichroism spectropolarimetry, fluorescence spectroscopy, and atomic force microscopy to characterize these self-assembling biomaterials. We propose a new assembly paradigm that arises from a set of unique porphyrin-porphyrin and porphyrin-peptide interactions whose structure may be readily modulated by changes in pH or peptide concentration.

Evidence for concerted electron proton transfer in charge recombination between FADH- and 306Trp• in Escherichia coli photolyase

Proton-coupled electron-transfer (PCET) is a mechanism of great importance in protein electron transfer and enzyme catalysis, and the involvement of aromatic amino acids in this process is of much interest. The DNA repair enzyme photolyase provides a natural system that allows for the study of PCET using a neutral radical tryptophan (Trp(•)). In Escherichia coli photolyase, photoreduction of the flavin adenine dinucleotide (FAD) cofactor in its neutral radical semiquinone form (FADH(•)) results in the formation of FADH(-) and (306)Trp(•). Charge recombination between these two intermediates requires the uptake of a proton by (306)Trp(•). The rate constant of charge recombination has been measured as a function of temperature in the pH range from 5.5 to 10.0, and the data are analyzed with both classical Marcus and semi-classical Hopfield electron transfer theory. The reorganization energy associated with the charge recombination process shows a pH dependence ranging from 2.3 eV at pH ≤ 7 and 1.2 eV at pH(D) 10.0. These findings indicate that at least two mechanisms are involved in the charge recombination reaction. Global analysis of the data supports the hypothesis that PCET during charge recombination can follow two different mechanisms with an apparent switch around pH 6.5. At lower pH, concerted electron proton transfer (CEPT) is the favorable mechanism with a reorganization energy of 2.1-2.3 eV. At higher pH, a sequential mechanism becomes dominant with rate-limiting electron-transfer followed by proton uptake which has a reorganization energy of 1.0-1.3 eV. The observed 'inverse' deuterium isotope effect at pH < 8 can be explained by a solvent isotope effect that affects the free energy change of the reaction and masks the normal, mass-related kinetic isotope effect that is expected for a CEPT mechanism. To the best of our knowledge, this is the first time that a switch in PCET mechanism has been observed in a protein.

Self assembly of coiled-coil peptide-porphyrin complexes

We are interested in the controlled assembly of photoelectronic materials using peptides as scaffolds and porphyrins as the conducting material. We describe the integration of a peptide-based polymer strategy with the ability of designed basic peptides to bind anionic porphyrins in order to create regulated photoelectronically active biomaterials. We have described our peptide system in earlier work, which demonstrates the ability of a peptide to form filamentous materials made up of self-assembling coiled-coil structures. We have modified this peptide system to include lysine residues appropriately positioned to specifically bind meso-tetrakis(4-sulfonatophenyl)porphine (TPPS(4)), a porphyrin that contains four negatively charged sulfonate groups at neutral pH. We measure the binding of TPPS(4) to our peptide using UV--visible and fluorescence spectroscopies to follow the porphyrin signature. We determine the concomitant acquisition of helical secondary structure in the peptide upon TPPS(4) binding using circular dichroism spectropolarimetry. This binding fosters polymerization of the peptide, as shown by absorbance extinction effects in the peptide CD spectra. The morphologies of the peptide/porphyrin complexes, as imaged by atomic force microscopy, are consistent with the coiled-coil polymers that we had characterized earlier, except that the heights are slightly higher, consistent with porphyrin binding. Evidence for exciton coupling in the copolymers is shown by red-shifting in the UV--visible data, however, the coupling is weak based on a lack of fluorescence quenching in fluorescence experiments.

Photo Processes on Self-Associated Cationic Porphyrins and Plastocyanin Complexes 1. Ligation of Plastocyanin Tyrosine 83 onto Metalloporphyrins and Electron-Transfer Fluorescence Quenching

The spectroscopic properties of the self-associated complexes formed between the anionic surface docking site of spinach plastocyanin and the cationic metalloporphyrins, in which the tyrosine 83 (Y83) moiety is placed just below the docking site, tetrakis(N-methyl-4-pyridyl)porphyrin (Pd(II)TMPyP(4+) and Zn(II)TMPyP(4+)), have been studied and reported herein. The fluorescence quenching phenomenon of the self-assembled complex of Zn(II)TMPyP(4+)/plastocyanin has also been discovered. The observed red-shifting of the Soret and Q-bands of the UV-visible spectra, ca. 9 nm for Pd(II)TMPyP(4+)/plastocyanin and ca. 6 nm for the Zn(II)TMPyP(4+)/plastocyanin complexes, was explained in terms of exciton theory coupled with the Gouterman model. Thus, the hydroxyphenyl terminus of the Y83 residue of the self-associated plastocyanin/cationic porphyrin complexes was implicated in the charge-transfer ligation with the central metal atoms of these metalloporphyrins. Moreover, ground-state spectrometric-binding studies between Pd(II)TMPyP(4+) and the Y83 mutant plastocyanin (Y83F-PC) system proved that Y83 moiety of plastocyanin played a critical role in the formation of such ion-pair complexes. Difference absorption spectra and the Job's plots showed that the electrostatic attractions between the cationic porphyrins and the anionic patch of plastocyanin, bearing the nearby Y83 residue, led to the predominant formation of a self-associated 1:1 complex in the ground-state with significantly high binding constants (K = (8.0 +/- 1.1) x 10(5) M(-1) and (2.7 +/- 0.8) x 10(6) M(-1) for Pd(II)TMPyP(4+) and zinc variant, respectively) in low ionic strength buffer, 1 mM KCl and 1 mM phosphate buffer (pH 7.4). Molecular modeling calculations supported the formation of a 1:1 self-associated complex between the porphyrin and plastocyanin with an average distance of ca. 9 A between the centers of mass of the porphyrin and Y83 positioned just behind the anionic surface docking site on the protein surface. The photoexcited singlet state of Zn(II)TMPyP(4+) was quenched by the Y83 residue of the self-associated plastocyanin in a static mechanism as evidenced by steady-state and time-resolved fluorescence experiments. Even when all the porphyrin was complexed (more than 97%), significant residual fluorescence from the complex was observed such that the amplitude of quenching of the singlet state of uncomplexed species was enormously obscured.

Spectroscopic investigations of metalloporphyrin-oligopeptide systems: evidence for peptide aggregation

Anionic pentapeptides consisting of a string of four glutamic acid residues terminated by either tyrosine (Glu4Tyr) or tryptophan (Glu4Trp) were synthesized, and their aggregation properties in buffered (pH = 7.0) aqueous solutions were investigated using two different approaches. In the first approach, the effects of the concentration of peptide used as its own probe (intrinsic probe) on its fluorescence emission, circular dichroism, surface tension, and solution pH yielded similar critical peptide concentrations of around 175 microM. This particular concentration was taken as evidence for peptide aggregation. In the second approach, peptide aggregation was investigated using cationic metalloporphyrins, tetrakis(N-methyl-4-pyridyl)porphyrin (Pd(II)TMPyP(4+) and Zn(II)TMPyP(4+)), as extrinsic probes. The effect of peptide concentration on porphyrin ground-state absorption confirmed peptide aggregation, but at a lower critical peptide concentration near 125 microM. This difference was attributed to the possible distortion introduced by the association of one (or more) large metalloporphyrin molecule with the peptide aggregates. Evidence for peptide aggregation was also demonstrated from the effect of peptide concentration on Pd(II)TMPyP(4+) triplet-state decay. The fast component (k(f), associated with electron transfer from the target Tyr and Trp residues to the porphyrin triplet state) was found to be independent of peptide concentration, implying no noticeable effect of peptide aggregation on the electron-transfer event. This was attributed to the fact that species formed by excitation of porphyrin associated with ion-pair complexes or bound to peptide aggregates and the diffusion together of the separate T(1) and peptide entities in the bulk phase are kinetically similar. On the other hand, the slower component (k(s)) of the decay, which is associated with the diffuse formation of an encounter complex between the free peptide and T(1) porphyrin (bulk phase), was peptide-dependent and displayed a critical peptide concentration near 125 microM, above which it became practically independent of peptide concentration. This invariance of k(s) was taken as an indication that the free peptide concentration in the bulk phase remains constant above 125 microM, the concentration at which peptide molecules prefer to associate as aggregates.

Analytical Application of Oligopyrrole Macrocycles

Progress of modern analytical chemistry is closely related with advancement in other fields such as organic chemistry and biochemistry. Successful solution of current scientific problems is inconceivable without close cooperation of different chemical disciplines. As an example of such hot and very intricate theme research in the field of molecular recognition of biologically active compounds can serve, where numerous methods of analytical chemistry, organic chemistry and biochemistry can suitably be utilized, elaborated and brought into consonance. This multidisciplinary overlap logically leads to the advent of new scientific fields with their own tools, methodologies and subjects of exploration - bioanalytical chemistry and nanotechnology. This review covers different aspects of analytical application of oligopyrrole macrocycles (mainly porphyrins and sapphyrins). These compounds are widely used in analytical chemistry due to their outstanding optical properties. In our contribution oligopyrrole macrocycles are considered as signaling and structural parts of chemical receptors and selectors in various applications. Introduction of different moieties into meso-position of macrocyclic rings allows to obtain e.g., sterically well-organized receptors for recognition of biologically important analytes, new chromatographic materials, and powerful tools in electrochemical research. Finally, future trends in the field are outlined briefly.