Terbium(3+) as a probe of nucleic acids structure. Does it alter the DNA conformation in solution?

Luminescent Lanthanides in Biorelated Applications: From Molecules to Nanoparticles and Diagnostic Probes to Therapeutics

Lanthanides are particularly effective in their clinical applications in magnetic resonance imaging and diagnostic assays. They have open-shell 4f electrons that give rise to characteristic narrow, line-like emission which is unique from other fluorescent probes in biological systems. Lanthanide luminescence signal offers selection of detection pathways based on the choice of the ion from the visible to the near-infrared with long luminescence lifetimes that lend themselves to time-resolved measurements for optical multiplexing detection schemes and novel bioimaging applications. The delivery of lanthanide agents in cells allows localized bioresponsive activity for novel therapies. Detection in the near-infrared region of the spectrum coupled with technological advances in microscopies opens new avenues for deep-tissue imaging and surgical interventions. This review focuses on the different ways in which lanthanide luminescence can be exploited in nucleic acid and enzyme detection, anion recognition, cellular imaging, tissue imaging, and photoinduced therapeutic applications. We have focused on the hierarchy of designs that include luminescent lanthanides as probes in biology considering coordination complexes, multimetallic lanthanide systems to metal-organic frameworks and nanoparticles highlighting the different strategies in downshifting, and upconversion revealing some of the opportunities and challenges that offer potential for further development in the field.

Experimental detection of conformational transitions between forms of DNA: problems and prospects

Under different conditions, the DNA double helix can take different geometric forms. Of the large number of its conformations, in addition to the "canonical" B form, the A, C, and Z forms are widely known, and the D, Hoogsteen, and X forms are less known. DNA locally takes the A, C, and Z forms in the cell, in complexes with proteins. We compare different methods for detecting non-canonical DNA conformations: X-ray, IR, and Raman spectroscopy, linear and circular dichroism in both the infrared and ultraviolet regions, as well as NMR (measurement of chemical shifts and their anisotropy, scalar and residual dipolar couplings and inter-proton distances from NOESY (nuclear Overhauser effect spectroscopy) data). We discuss the difficulties in applying these methods, the problems of theoretical interpretation of the experimental results, and the prospects for reliable identification of non-canonical DNA conformations.

Eu3+ as a luminescence probe in DNA studies: Structural and conformational implications

Lanthanide ions are widely used as luminescent probes for structural studies of various biomolecules, including DNA. Latest developments of circularly polarized luminescence (CPL) methodology further boosted interest to luminescence techniques. However, an effect of the lanthanide probes themselves on the DNA structure and conformation was investigated only partially and not for all lanthanides. In the present work, we performed a detailed spectroscopic study of Eu³⁺ complexes with native double-stranded DNA and compared them to the relevant complexes with single-stranded DNA. We employed infrared (IR), vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectroscopic methods to investigate Eu³⁺ effect on DNA structure and conformational transitions. It was shown that Eu³⁺ ions can induce significant alteration of the native DNA structure at the concentrations often used in luminescence studies. While no DNA denaturation was observed at these metal ion concentrations, significant unstacking of the base pairs and disordering of the sugar-phosphate backbone, partial appearance of the A-form backbone geometry, and DNA transition into condensed ψ-type form took place. Eu³⁺ binding to single-stranded DNA was more pronounced than the binding to double-stranded DNA. We detected the main Eu³⁺ binding sites and determined the metal ion concentration range in which DNA geometry remains largely unaltered. The results obtained in the current study could be used for tuning the luminescence and CPL structural studies of DNA utilizing Eu³⁺ ions as probes.

Europium (III) as a Circularly Polarized Luminescence Probe of DNA Structure

We report as a proof-of-concept the first application of circularly polarized luminescence (CPL) measured with a Raman optical activity (ROA) spectrometer to differentiate several DNA structures without need of sensitizing complexes. The ROA/CPL approach provides sufficiently high CPL intensity to use hydrated Eu³⁺ ions, thus avoiding DNA structural changes associated with binding of sensitizers and overcoming the sensitizer quenching issue. We showed that deoxyguanosine monophosphate (dGMP), single- and double-stranded DNA provide different CPL spectra, which could be used for their discrimination. Our results demonstrate that ROA/CPL method is a promising approach to measure CPL spectra of complex biomolecules when the use of sensitizers is not possible. The method can be extended to other biomolecules, such as proteins, lipids, sugars, etc.

Tunable near white light photoluminescence of lanthanide ion (Dy3+, Eu3+and Tb3+) doped DNA lattices

We constructed lanthanide ion doped double-crossover DNA lattices grown on a silica substrate and studied their photoluminescence characteristics.

Terbium fluorescence as a sensitive, inexpensive probe for UV-induced damage in nucleic acids

Much effort has been focused on developing methods for detecting damaged nucleic acids. However, almost all of the proposed methods consist of multi-step procedures, are limited, require expensive instruments, or suffer from a high level of interferences. In this paper, we present a novel simple, inexpensive, mix-and-read assay that is generally applicable to nucleic acid damage and uses the enhanced luminescence due to energy transfer from nucleic acids to terbium(III) (Tb(3+)). Single-stranded oligonucleotides greatly enhance the Tb(3+) emission, but duplex DNA does not. With the use of a DNA hairpin probe complementary to the oligonucleotide of interest, the Tb(3+)/hairpin probe is applied to detect ultraviolet (UV)-induced DNA damage. The hairpin probe hybridizes only with the undamaged DNA. However, the damaged DNA remains single-stranded and enhances the intrinsic fluorescence of Tb(3+), producing a detectable signal directly proportional to the amount of DNA damage. This allows the Tb(3+)/hairpin probe to be used for sensitive quantification of UV-induced DNA damage. The Tb(3+)/hairpin probe showed superior selectivity to DNA damage compared to conventional molecular beacons probes (MBs) and its sensitivity is more than 2.5 times higher than MBs with a limit of detection of 4.36±1.2 nM. In addition, this probe is easier to synthesize and more than eight times cheaper than MBs, which makes its use recommended for high-throughput, quantitative analysis of DNA damage.

Critical behavior of megabase-size DNA toward the transition into a compact state

We studied the changes in the higher-order structure of a megabase-size DNA (S120-1 DNA) under different spermidine (SPD) concentrations through single-molecule observations using fluorescence microscopy (FM) and atomic force microscopy (AFM). We examined the difference between the folding transitions in S120-1 DNA and sub-megabase-size DNA, T4 DNA (166 kbp). From FM observations, it is found that S120-1 DNA exhibits intra-chain segregation as the intermediate state of transition, in contrast to the all-or-none nature of the transition on T4 DNA. Large S120-1 DNA exhibits a folding transition at lower concentrations of SPD than T4 DNA. AFM observations showed that DNA segments become aligned in parallel on a two-dimensional surface as the SPD concentration increases and that highly intense parallel alignment is achieved just before the compaction. S120-1 DNA requires one-tenth the SPD concentration as that required by T4 DNA to achieve the same degree of parallel ordering. We theoretically discuss the cause of the parallel ordering near the transition into a fully compact state on a two-dimensional surface, and argue that such parallel ordering disappears in bulk solution.

DNA-induced aggregation and fusion of phosphatidylcholine liposomes in the presence of multivalent cations observed by the cryo-TEM technique

By means of cryoelectron transmission microscopy (cryo-TEM), we were able to demonstrate the formation of ternary complexes (TC): DNA-phosphatidylcholine liposome-divalent metal cations. Addition of Ba2+ to TC led to visualization of DNA compacting on the liposome surface. Staining the TC by Tb3+ cations revealed the changed secondary structure of DNA located between fused liposomes. Cryo-TEM and liposome turbidity data were analyzed during TC formation. Liposome aggregation and the liposome fusion induced by DNA in TC were observed. Because TC displayed the property of DNA cationic liposome complexes as well as their own unique properties, we were able to consider cationic lipoplexes as a particular case of TC. The involvement of TC and direct DNA-lipid interactions in the formation nuclear pore complexes were assumed.

Probing metal binding in the 8-17 DNAzyme by TbIII luminescence spectroscopy

Metal-dependent cleavage activities of the 8-17 DNAzyme were found to be inhibited by Tb(III) ions, and the apparent inhibition constant in the presence of 100 microM of Zn(II) was measured to be 3.3+/-0.3 microM. The apparent inhibition constants increased linearly with increasing Zn(II) concentration, and the inhibition effect could be fully rescued with addition of active metal ions, indicating that Tb(III) is a competitive inhibitor and that the effect is completely reversible. The sensitized Tb(III) luminescence at 543 nm was dramatically enhanced when Tb(III) was added to the DNAzyme-substrate complex. With an inactive DNAzyme in which the GT wobble pair was replaced with a GC Watson-Crick base pair, the luminescence enhancement was slightly decreased. In addition, when the DNAzyme strand was replaced with a complete complementary strand to the substrate, no significant luminescence enhancement was observed. These observations suggest that Tb(III) may bind to an unpaired region of the DNAzyme, with the GT wobble pair playing a role. Luminescence lifetime measurements in D(2)O and H(2)O suggested that Tb(III) bound to DNAzyme is coordinated by 6.7+/-0.2 water molecules and two or three functional groups from the DNAzyme. Divalent metal ions competed for the Tb(III) binding site(s) in the order Co(II)>Zn(II)>Mn(II)>Pb(II)>Ca(II) approximately Mg(II). This order closely follows the order of DNAzyme activity, with the exception of Pb(II). These results indicate that Pb(II), the most active metal ion, competes for Tb(III) binding differently from other metal ions such as Zn(II), suggesting that Pb(II) may bind to a different site from that for the other metal ions including Zn(II) and Tb(III).

DNA structural transitions induced by divalent metal ions in aqueous solutions

Using methods of IR spectroscopy, light scattering, gel-electrophoresis DNA structural transitions are studied under the action of Cu2+, Zn2+, Mn2+, Ca2+ and Mg2+ ions in aqueous solution. Cu2+, Zn2+, Mn2+ and Ca2+ ions bind both to DNA phosphate groups and bases while Mg2+ ions-only to phosphate groups of DNA. Upon interaction with divalent metal ions studied (except for Mg2+ ions) DNA undergoes structural transition into a compact form. DNA compaction is characterized by a drastic decrease in the volume occupied by DNA molecules with reversible formation of DNA dense particles of well-defined finite size and ordered morphology. The DNA secondary structure in condensed particles corresponds to the B-form family. The mechanism of DNA compaction under Mt2+ ion action is not dominated by electrostatics. The effectiveness of the divalent metal ions studied to induce DNA compaction correlates with the affinity of these ions for DNA nucleic bases: Cu2+>>Zn2+>Mn2+>Ca2+>>Mg2+. Mt2+ ion interaction with DNA bases (or Mt2+ chelation with a base and an oxygen of a phosphate group) may be responsible for DNA compaction. Mt2+ ion interaction with DNA bases can destabilize DNA causing bends and reducing its persistent length that will facilitate DNA compaction.

DNA interaction with Mn2+ ions at elevated temperatures: VCD evidence of DNA aggregation

Interaction of natural calf thymus DNA with Mn(2+) ions was studied at room temperature and at elevated temperatures in the range from 23 degrees C to 94 degrees C by means of IR absorption and vibrational circular dichroism (VCD) spectroscopy. The Mn(2+) concentration was varied between 0 and 1.3M (0 and 10 [Mn]/[P]). The secondary structure of DNA remained in the frame of the B-form family in the whole ion concentration range at room temperature. No significant DNA denaturation was revealed at room temperature even at the highest concentration of metal ions studied. However at elevated temperatures, DNA denaturation and a significant decrease of the melting temperature of DNA connected with a decrease of the stability of DNA induced by Mn(2+) ions occurred. VCD demonstrated sensitivity to DNA condensation and aggregation as well as an ability to distinguish between these two processes. No condensation or aggregation of DNA was observed at room temperature at any of the metal ion concentrations studied. DNA condensation was revealed in a very narrow range of experimental conditions at around 2.4 [Mn]/[P] and about 55 degrees C. DNA aggregation was observed in the presence of Mn(2+) ions at elevated temperatures during or after denaturation. VCD spectroscopy turned out to be useful for studying DNA condensation and aggregation due to its ability to distinguish between these two processes, and for providing information about DNA secondary structure in a condensed or aggregated state.

Vibrational CD (VCD) and atomic force microscopy (AFM) study of DNA interaction with Cr3+ ions: VCD and AFM evidence of DNA condensation

The interaction of natural calf thymus DNA with Cr(3+) ions was studied at room temperature by means of vibrational CD (VCD) and infrared absorption (ir) spectroscopy, and atomic force microscopy (AFM). Cr(3+) ion binding mainly to N(7) (G) and to phosphate groups was demonstrated. Psi-type VCD spectra resembling electronic CD (ECD) spectra, which appear during psi-type DNA condensation, were observed. These spectra are characterized mainly by an anomalous, severalfold increase of VCD intensity. Such anomalous VCD spectra were assigned to DNA condensation with formation of large and dense particles of a size comparable to the wavelength of the probing ir beam and possessing large-scale helicity. Atomic force microscopy confirmed DNA condensation by Cr(3+) ions and the formation of tight DNA particles responsible for the psi-type VCD spectra. Upon increasing the Cr(3+) ion concentration the shape of the condensates changed from loose flower-like structures to highly packed dense spheres. No DNA denaturation was seen even at the highest concentration of Cr(3+) ions studied. The secondary structure of DNA remained in a B-form before and after the condensation. VCD and ir as well as AFM proved to be an effective combination for investigating DNA condensation. In addition to the ability of VCD to determine DNA condensation, VCD and ir can in the same experiment provide unambiguous information about the secondary structure of DNA contained in the condensed particles.

Probing non-selective cation binding in the hairpin ribozyme with Tb(III)

Catalysis by the hairpin ribozyme is stimulated by a wide range of both simple and complex metallic and organic cations. This independence from divalent metal ion binding unequivocally excludes inner-sphere coordination to RNA as an obligatory role for metal ions in catalysis. Hence, the hairpin ribozyme is a unique model to study the role of outer-sphere coordinated cations in folding of a catalytically functional RNA structure. Here, we demonstrate that micromolar concentrations of a deprotonated aqueous complex of the lanthanide metal ion terbium(III), Tb(OH)(aq)(2+), reversibly inhibit the ribozyme by competing for a crucial, yet non-selective cation binding site. Tb(OH)(aq)(2+) also reports a likely location of this binding site through backbone hydrolysis, and permits the analysis of metal binding through sensitized luminescence. We propose that the critical cation-binding site is located at a position within the catalytic core that displays an appropriately-sized pocket and a high negative charge density. We show that cationic occupancy of this site is required for tertiary folding and catalysis, yet the site can be productively occupied by a wide variety of cations. It is striking that micromolar Tb(OH)(aq)(2+) concentrations are compatible with tertiary folding, yet interfere with catalysis. The motif implicated here in cation-binding has also been found to organize the structure of multi-helix loops in evolutionary ancient ribosomal RNAs. Our findings, therefore, illuminate general principles of non-selective outer-sphere cation binding in RNA structure and function that may have prevailed in primitive ribozymes of an early "RNA world".

Effect of glycine on DNA structural transitions induced by multivalent cationic compounds

We have investigated the effect of glycine (an organic osmolyte) on several DNA transitions induced by Tb3+, spermidine3+ and spermine4+ addition, using light scattering, circular dichroism, UV spectroscopy and electric linear dichroism techniques. DNA condensation and B-Z transition by the three compounds is perturbed by glycine: more Tb3+, spermidine3+ and spermine4+ must be added to obtain the same extent of condensation or Z-form as compared to the behaviour in the absence of this organic osmolyte. However, according to the light scattering experiments, glycine has also a structural effect on the DNA condensation that could be explained by an influence of the medium dielectric constant on the morphology of particles formed or on the rate of the condensation process. Contrary to these transitions, the particular B-B'-psi transition resulting from the addition of Tb3+ to a DNA solution is not observed in the presence of glycine. Since the chelation of Tb3+ by the phosphate group and the N-7 of guanine is presumably responsible for this transition, the glycine effect could probably be explained by a perturbation of this chelation by the change in solvent polarity and the chelating ability of the organic osmolyte.

Dielectric constant and ionic strength effects on DNA precipitation

We have investigated the effect of different zwitterionic compounds on DNA precipitation induced by spermine4+. Glycine, beta-alanine, 4-aminobutyric acid, and 6-aminocaproic acid have shown an increasing capacity to attenuate DNA precipitation. This protection effect has been correlated with the dielectric constant increase of their corresponding solutions. Calculations based on these experimental data and counter-ion condensation theory have confirmed the importance of this parameter for DNA-ion interactions and precipitation mechanisms. We have also observed a resolubilization of DNA in the presence of 6-aminocaproic acid at high spermine4+ concentration and in the presence of glycine at high spermidine3+ concentration. This could be explained by an increase of screening effect with polyamine concentration.

Osmotic effectors and DNA structure: effect of glycine on precipitation of DNA by multivalent cations

We have investigated the effect of glycine (an organic osmolyte) on DNA precipitation induced by spermine4+, spermidine3+ and Tb3+ addition, using circular dichroism (CD), UV spectroscopy (UV), and electric linear dichroism (ELD) techniques. DNA precipitation by the three compounds is perturbed by glycine: more spermine4+, spermidine3+ and Tb3+ must be added to obtain the same extent of precipitation as compared to the behaviour in absence of this organic osmolyte. It seems that glycine has a general effect on the DNA environment. Calculations based on experimental results and Manning's counterion condensation theory show that glycine could modify the electrostatic environment of DNA as a consequence of a change in dielectric constant.

Effect of organic effectors on chromatin solubility, DNA-histone H1 interactions, DNA and histone H1 structures

We have extended our previous investigations on the effect of organic osmolytes (glycine, proline, taurine, mannitol, sorbitol and trimethylammonium oxide (TMAO)) on chromatin solubility, to the study of their influence on DNA stability and DNA-histone interactions. Our aim was to understand the molecular origin of the protection effects observed. To this end, we determined the amount of histone H1 required to precipitate DNA or H1-depleted chromatin, at various salt concentrations, in the presence of the above mentioned organic compounds. We found a shift of the H1/DNA ratio required to reach 50% precipitation, towards higher values. Taurine was the most efficient compound followed by mannitol and glycine, then sorbitol and proline. On the contrary, TMAO favoured the precipitation process. We attempted to interpret these results on the basis of Manning's counterion condensation theory. Changes in histone H1 structure folding and in DNA melting temperature Tm were also analyzed. Glycine, taurine, sorbitol and TMAO increased the degree of secondary structure folding of the protein while mannitol and sorbitol had no effect. Taurine, glycine and proline decreased the Tm of DNA, TMAO largely destabilized DNA, but mannitol and sorbitol had no effect. Measurements of NaCl activity in the presence of organic osmolytes did not reveal sufficiently large changes to account for their protection effect against chromatin precipitation. The osmotic coefficient j of the organic effectors solutions increased in the order: taurine < glycine < sorbitol < mannitol < proline << TMAO. For the two latter compounds, the j values increased above 1 at high concentration. We consider that the organic compounds investigated may be classified into three categories: (i) class I (zwitterionic compounds: glycine, proline, taurine) would produce sodium ions release from the DNA surface; (ii) class II (the very polar molecule TMAO) would increase sodium counterions condensation on DNA together with histone H1 folding; (iii) class III compounds (mannitol and sorbitol) would possibly produce a modification of NaCl activity but no definite explanation could be found for the complex behavior of these compounds.

Interaction of calf-thymus DNA with trivalent La, Eu, and Tb ions. Metal ion binding, DNA condensation and structural features

The interaction of calf-thymus DNA with La3+, Eu3+ and Tb3+ has been investigated in aqueous solution at pH 6.5, using metal/DNA(P) molar ratios(r) 1/80, 1/40, 1/20, 1/10, 1/4 and 1/2. Correlations between FTIR spectral changes and DNA structural properties have been established. At low metal/DNA(P) (r) 1/80, the metal ions bind mainly to the PO2- groups of the backbone, resulting in increased base-stacking interaction and duplex stability. At (r) 1/40 and 1/20, metal ion binding to the PO2- and the guanine N-7 site (chelation) predominates with minor perturbations of the A-T base pairs. Evidence for this comes from the displacement of the band at 1712 cm-1 (T,G) towards a lower frequency and the PO2- antisymmetric band at 1222 cm-1 towards a higher frequency. At higher metal/DNA(P) ratio, r > 1/20, DNA begins to condensate and drastic structural changes occur, which are accompanied by the shift and intensity changes of several G-C and A-T absorption bands. No major departure from B-DNA conformation was observed before and after DNA condensation even though some local structural modifications were observed. A comparison with the Cu-DNA complexes (denaturated DNA) shows some degree of helical destabilization of the biopolymer in the presence of lanthanide ions.

Linear dichroism spectroscopy of nucleic acids

This review will consider solution studies of structure and interactions of DNA and DNA complexes using linear dichroism spectroscopy, with emphasis on the technique of orientation by flow. The theoretical and experimental background to be given may serve, in addition, as a general introduction into the state of the art of linear dichroism spectroscopy, particularly as it is applied to biophysical problems.

Interaction of La (III) and Tb (III) ions with purine nucleotides: evidence for metal chelation (N-7-M-PO3) and the effect of macrochelate formation on the nucleotide sugar conformation

The interaction of the La (III) and Tb (III) ions with adenosine-5'-monophosphate (5'-AMP), guanosine-5'-monophosphate (5'-GMP), and 2'-deoxyguanosine-5'-monophosphate (5'-dGMP) anions with metal/nucleotide ratios of 1 and 2 has been studied in aqueous solution in acidic and neutral pHs. The solid complexes were isolated and characterized by Fourier transform ir and 1H-nmr spectroscopy. The lanthanide (III)-nucleotide complexes are polymeric in nature both in the solid and aqueous solutions. In the metal-nucleotide complexes isolated from acidic solution, the nucleotide binding is via the phosphate group (inner sphere) and an indirect metal-N-7 interaction (outer-sphere) with the adenine N-1 site protonated. In the complexes obtained from neutral solution, metal chelation through the N-7 and the PO3(2-) group is prevailing. In aqueous solution, an equilibrium between the inner and outer sphere metal-nucleotide interaction has been observed. The ribose moiety shows C2'-endo/anti pucker in the free AMP anion and in the lanthanide (III)-AMP complexes, whereas the GMP anion with C2'-endo/anti sugar conformation exhibits a mixture of the C2'-endo/anti and C3'-endo/anti sugar puckers in the lanthanide (III)-GMP salts. The deoxyribose has O4'-endo/anti sugar pucker in the free dGMP anion and a C3'-endo/anti, in the lanthanide (III)-dGMP complexes.

Contribution of asymmetric ligand binding to the apparent permanent dipole moment of DNA

Despite its antiparallel symmetry, DNA often appears to possess a permanent electric dipole moment in transient electro-optical experiments. We propose that this may be due to the asymmetric binding of charged ligands to the DNA. We have used the fluctuating dipole theory of Kirkwood and Shumaker to calculate the contribution of asymmetric ligand binding to the electro-optic orientation function, and Monte Carlo computer simulation to calculate the reversing pulse behavior, as a function of ligand binding density. The results indicate that the effect should be observable even against the background of the sizable induced dipole moment produced by polarization of the counterion atmosphere.

Reinterpretation of fluorescence of terbium ion-DNA complexes

Terbium (Tb3+) fluorescence was used to investigate local non-denaturation perturbations of double-helical DNA structure induced in this nucleic acid by various physical and chemical agents. It has been shown that the interaction of Tb3+ with DNA into which single-strand or double-strand breaks have been introduced by DNase I or by low doses of ionizing radiation does not influence the fluorescence of the lanthanide cation. On the other hand, interaction of terbium with DNA modified by the antitumour drug cis-diamminedichloroplatinum(II) at low levels of binding and by low doses of ultraviolet radiation (wavelength 254 nm) has been shown to result in substantial enhancement of the fluorescence of this cation. It has been proposed that the terbium fluorescent probe can also be exploited successfully for the purpose of analysing the guanine bases present in distorted double-stranded regions of DNA, in which only the vertical stacking of the base-pairs is altered.

Comparative study of the condensation of chicken erythrocyte and calf thymus chromatins by di- and multivalent cations

The condensation of chicken erythrocyte (CE) and calf thymus (CT) chromatins upon addition of di- and multivalent cations has been studied using turbidity, precipitation and electric dichroism measurements. For all the cations investigated (Mg2+, Tb3+, Co(NH3)6(3+), spermidine Spd2+ and spermine Sp4+) condensation of CE chromatin occurred before the onset of aggregation, while aggregation of CT chromatin started before condensation with all cations except Mg2+ and Tb3+. Precipitation of CE chromatin required lower di- and multivalent cations concentrations than CT chromatin. The electric dichroism data for both chromatins, at low ionic strength in the absence of di- or multivalent cations, indicated that the nucleoprotein molecules were not totally decondensed but that a "precondensed" state was already present. A positive electric dichroism was observed for the most condensed chromatin fibers, in agreement with the "cross-linker" models. Tb3+ led to less compact condensed particles as judged from the electric dichroism observations, but electron microscopy revealed that "30 nm fibers" were formed. Very little aggregation was produced by Tb3+. On the contrary, spermine produced very large networks of condensed molecules, but large spheroidal particles were also observed. The condensation of CE chromatin happened without changes of solution conductivity upon cation salt addition, regardless of the condensing cation, indicating a cooperative uptake of the ions during this process.

Terbium(III)-induced conformational transition in poly(dG-dC) fluorescence and circular dichroic studies

Poly(dG-dC) in 60% aqueous alcohol exhibits the characteristic inversion of the circular dichroism spectrum associated with the formation of left-handed helix. Upon complexation with Tb3+, poly(dG-dC) in this medium induces marked enhancement of the Tb(III) fluorescence emission at 488 and 545 nm, when excited at 290 nm. The degree of fluorescence enhancement is dependent on the concentration of Tb(III) at a fixed poly(dG-dC) concentration. Neither poly(dG-dC) in water nor poly(dA-dT) in water or 60% alcohol, causes any significant fluorescence spectral changes of Tb3+. Tb(III)-poly(dG-dC) in 60% alcohol shows circular dichroic spectra associated with a broad positive molar ellipticity ranging from 6000 to 10 000 degree X cm2 X dmol-1 between 270 and 280 nm, and a small negative band around 240 nm.

An electro-optical study of the mechanisms of DNA condensation induced by spermine

Condensation of DNA by spermine has been studied by electric dichroism, electric birefringence and rotational relaxation times at 1 mM ionic strength. Using Manning's theory, we found that condensation occurs for a fraction of neutralized phosphate charges (r) equal to 0.90, in good agreement with previous studies using spermidine, synthetic polyamines and trivalent cations (e.g. Co(NH3)36 +, Tb3 +). Our results are compatible with the presence in solution of torus-shaped condensed structures in a narrow range of spermine concentration; further addition of the polyamine produced precipitation due to the self-aggregation of several toroids. For spermine concentrations lower than that required for collapse, important changes of the orientation mechanism in the electric field and of DNA stiffness were observed. Whereas free DNA was mainly oriented by a fast-induced polarizability mechanism, DNA-spermine complexes displayed an important permanent dipole component, in the spermine concentration range where extension of the DNA molecules was present. The birefringence relaxation times suggested that, in the first step, the stiffness of the DNA molecules increased, and then, at higher spermine concentration, bending of the DNA molecules occurred so that condensation into toroidal particles became possible.