A comparative study of calf thymus DNA binding to Cr(III) and Cr(VI) ions. Evidence for the guanine N-7-chromium-phosphate chelate formation

Chromium(VI) salts are well known to be mutagens and carcinogens and to easily cross the cell membranes. Because they are powerful oxidizing agents, Cr(VI) reacts with intracellular materials to reduce to trivalent form, which binds DNA. This study was designed to investigate the interaction of calf thymus DNA with Cr(VI) and Cr(III) in aqueous solution at pH 6.5-7.5, using Cr(VI)/DNA(P) molar ratios (r) of 1:20 to 2:1 and Cr(III)/DNA(P) molar ratios (r) of 1:80 to 1:2. UV-visible and Fourier transform infrared (FTIR) difference spectroscopic methods were used to determine the metal ion-binding sites, binding constants, and the effect of cation complexation on DNA secondary structure. Spectroscopic results showed no interaction of Cr(VI) with DNA at low anion concentrations (r = 1:20 to 1:1), whereas some perturbations of DNA bases and backbone phosphate were observed at very high Cr(VI) contents (r > 1) with overall binding constant of K = 508 M(-1). Cr(III) chelates DNA via guanine N-7 and the nearest PO(2) group with overall binding constant of K = 3.15 x 10(3) M(-1). Evidence for cation chelate formation comes from major shiftings and intensity variations of the guanine band at 1717 and the phosphate asymmetric stretching vibration at 1222 cm(-1). At low Cr(III) concentration (r = 1:40), the number of Cr(III) ions bound to DNA were 6-7 cations/500 base pairs, and this increased to 30-35 cations/500 base pairs at high metal ion content (r = 1:4). DNA condensation occurred at high cation concentration (r = 1:10). No major alteration of DNA conformation was observed, and the biopolymer remained in the B family structure upon chromium complexation.     

A Comparative study of Fe(II) and Fe(III) interactions with DNA duplex: major and minor grooves bindings

The involvement of the Fe cations in autoxidation in cells and tissues is well documented. DNA is a major target in such reaction, and can chelate Fe cation in many ways. The present study was designed to examine the interaction of calf-thymus DNA with Fe(II) and Fe(III), in aqueous solution at pH 6.5 with cation/DNA (P) (P = phosphate) molar ratios (r) of 1:160 to 1:2. Capillary electrophoresis and Fourier transform infrared (FTIR) difference spectroscopic methods were used to determine the cation binding site, the binding constant, helix stability and DNA conformation in Fe-DNA complexes. Structural analysis showed that at low cation concentration (r = 1/80 and 1/40), Fe(II) binds DNA through guanine N-7 and the backbone PO(2) group with specific binding constants of K(G) = 5.40 x 10(4) M(1) and K(P) = 2.40 x 10(4) M(1). At higher cation content, Fe(II) bindings to adenine N-7 and thymine O-2 are included. The Fe(III) cation shows stronger interaction with DNA bases and the backbone phosphate group. At low cation concentration (r = 1:80), Fe(III) binds mainly to the backbone phosphate group, while at higher metal ion content, cation binding to both guanine N-7 atom and the backbone phosphate group is prevailing with specific binding constants of K(G) = 1.36 x 10(5) M(-1) and K(P) = 5.50 x 10(4) M(-1). At r = 1:10, Fe(II) binding causes a minor helix destabilization, whereas Fe(III) induces DNA condensation. No major DNA conformational changes occurred upon iron complexation and DNA remains in the B-family structure.     

Interaction of Calf-Thymus DNA With Trivalent La,Eu, and Tb Ions. Metal Ion Binding,DNA Condensation and Structural Features

Abstract

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 PO^? ₂ 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 PO^? ₂ 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 PO^? ₂ 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 eventhough some local structural modifications were observed. A comparison with the Cu-DNA complexes (denaturated DNA) shows some degree of helical destabilizition of the biopolymer in the presence of lanthanide ions.     

The effects of Cu2+ and Pb2+ on the solution structure of calf thymus DNA: DNA condensation and denaturation studied by fourier transform IR difference spectroscopy

The interaction of calf thymus DNA with Cu²⁺and Pb²⁺ was studied in aqueous solution at pH 6.5 with metal/DNA (P) (P = phosphate) molar ratios (r) 1/80, 1/40, 1/20, 1/10, 1/4, 1/2, and 1, using Fourier Transform ir (FTIR) spectroscopy. Correlations between the ir spectral changes, metal ion binding mode, DNA condensation, and denaturation, as well as conformational features, were established. Spectroscopic evidence has shown that at low metal/DNA (P) molar rations 1/80 and 1/40, copper and lead ions bind mainly to the PO of the backbone, resulting in increased base-stacking interaction and duplex stability. The major copper ion base binding via G-C base pairs begins at r > 1/40, while the lead ion base binding occurs at r > 1/20 with the A-T base pairs. The denaturation of DNA begins at r = 1/10 and continues up to r = 1/2 in the presence of copper ions, whereas a partial destabilization of the helical structure was observed for the lead ion at high metal ion concentration (r = 1/2). Metal-DNA binding also results in DNA condensation. No major departure from the B-family structure was observed, upon DNA interaction with these metal ions. © 1993 John Wiley & Sons, Inc.     

Silver(I) complexes with DNA and RNA studied by Fourier transform infrared spectroscopy and capillary electrophoresis

Ag(I) is a strong nucleic acids binder and forms several complexes with DNA such as types I, II, and III. However, the details of the binding mode of silver(I) in the Ag-polynucleotides remains unknown. Therefore, it was of interest to examine the binding of Ag(I) with calf-thymus DNA and bakers yeast RNA in aqueous solutions at pH 7.1-6.6 with constant concentration of DNA or RNA and various concentrations of Ag(I). Fourier transform infrared spectroscopy and capillary electrophoresis were used to analyze the Ag(I) binding mode, the binding constant, and the polynucleotides' structural changes in the Ag-DNA and Ag-RNA complexes. The spectroscopic results showed that in the type I complex formed with DNA, Ag(I) binds to guanine N7 at low cation concentration (r = 1/80) and adenine N7 site at higher concentrations (r = 1/20 to 1/10), but not to the backbone phosphate group. At r = 1/2, type II complexes formed with DNA in which Ag(I) binds to the G-C and A-T base pairs. On the other hand, Ag(I) binds to the guanine N7 atom but not to the adenine and the backbone phosphate group in the Ag-RNA complexes. Although a minor alteration of the sugar-phosphate geometry was observed, DNA remained in the B-family structure, whereas RNA retained its A conformation. Scatchard analysis following capillary electrophoresis showed two binding sites for the Ag-DNA complexes with K(1) = 8.3 x 10(4) M(-1) for the guanine and K(2) = 1.5 x 10(4) M(-1) for the adenine bases. On the other hand, Ag-RNA adducts showed one binding site with K = 1.5 x 10(5) M(-1) for the guanine bases.     

A laser Raman spectroscopic study of the interaction of calf-thymus DNA with Cu(II) and Pb(II) ions: metal ion binding and DNA conformational changes.

The interaction of calf-thymus DNA with Cu(II) and Pb(II) ions has been investigated in H2O and D2O solutions at physiological pH, using laser Raman spectroscopy. The results confirm the destabilizing effect of Cu2+ ions, which are shown to bind strongly to the guanine and cytidine bases, perturbing the A-T base pairs and disrupting the double-helical structure of DNA, whose conformation is markedly altered by these interactions. Earlier claims that Pb2+ ions destabilize DNA are not supported by the present study. These ions are found to interact only weakly with the nucleic bases, binding to the N7 position of the guanine bases and also interacting with the A-T pairs. Both types of ions are found to interact with the charged phosphate groups of DNA, although these sites are preferred over the nucleic bases by Pb2+ ions.     

The effect of HCl on the solution structure of calf thymus DNA: A comparative study of DNA denaturation by proton and metal cations using fourier transform IR difference spectroscopy

The interaction of HCl with calf thymus DNA was investigated in aqueous solution at pH 7-2 with H⁺/DNA(P)(P:phosphate) molar ratios (r) of 1/80, 1/40, 1/20, 1/10, 1/4, 1/2, and 1, using Fourier Transform (FTIR) difference spectroscopy. Correlations between spectral changes, proton binding mode, DNA denaturation, and conformational variations are established. A comparison was also made between their spectra of denaturated DNA, in the presence of proton and Cu ions with similar cation concentrations. The FTIR difference spectroscopic results have shown that at low proton concentrations of r = 1/80 and 1/40 (pH 7–5), no major spectral changes occur for DNA, and the presence of H⁺ results in an increased base-stacking interaction and helical stability. At higher proton concentrations of r > 1/40, the proton binding to the cytosine and adenine bases begins with major destabilization of the helical duplex. As base protonation progresses, a B to C conformational conversion occurs with major DNA spectral changes. Protonation of guanine bases occurs at a high cation concentration r > 1/2 (pH < 3) with a major increase in the intensity of several DNA in-plane vibrations. Copper ion complexation with DNA exhibits marked similarities with proton at high cation concentrations (r > 1/10), whereas at low metal ion concentrations, copper–PO₂ and copper–guanine N-7 bindings are predominant. No major DNA conformational transition was observed on copper ion complexation. © 1995 John Wiley & Sons, Inc.     

RNA-Ascorbate Interaction

Abstract

Ascorbic acid and divalent iron salts have been widely used to investigate the effects of reactive oxygen species in different biological targets such as nucleic acids, proteins and lipids. This study was designed to examine the interaction of yeast RNA with vitamin C in aqueous solution at physiological pH with drug/RNA(P)(P=phosphate) molar ratios of r= 1/80, 1/40, 1/20, 1/10, 1/4 and 1/2. Absorption spectra and Fourier transform infrared (FTIR) difference spectroscopy were used to determine the ascorbate binding mode, binding constant, sequence selectivity and RNA secondary structure in aqueous solution.

Spectroscopic evidence showed that at low drug concentration (r=1/80 and 1/40), no major ascorbate-RNA interaction occurs, while at higher drug concentrations (r>1/40), a major drug-RNA complexation was observed through both G-C and A-U base pairs and the backbone phosphate groups with k=31.80 M^?1. Evidence for this comes from large perturbations of the G-C vibrations at 1698 and 1488 cm^?1 and the A-U bands at 1654 and 1608 cm^?1 as well as the phosphate antisymmetric stretch at 1244 cm^?1. At r>1/10, minor structural changes occur for the ribose-phosphate backbone geometry with RNA remaining in the A- family structure. The drug distributions around double helix were about 55% with G-C, 33% A-U and 12% with PO₂ groups. A comparison between ascorbate-RNA and ascorbate-DNA complexes showed minor differences. The ascorbate binding (H-bonding) is via anion CO and OH groups.     

Structure and Dynamics of Distamycin A with d(CGCAAATTGGC):d(GCCAATTTGCG) at Low Drug: DNA Ratios

Abstract

Two-dimensional NMR has been used to study the interaction of distamycin A with d(CGCAAA- TTGGC):d(GCCAATTTGCG) at low and intermediate drug: DNA ratios (<2.0). Drug-DNA contacts were identified by nuclear Overhauser effect spectroscopy, which also served to monitor exchange of the drug between different binding sites. At low drug: DNA ratios (0.5), distamycin A binds in two orientations within the five central A-T base pairs and has a preference (2.2:1) for binding with the formyl end directed toward the 5′ side of the A-rich strand. The pattern of drug-DNA contacts corresponding to the preferred binding orientation are consistent with the drug sliding between adjacent AAAT and AATT binding sites at a rate that is fast on the NMR time scale. Similarly, the pattern of NOEs associated with the less favored orientation are consistent with the drug sliding between adjacent AATT and ATTT sites, again in fast exchange. Off-rates for the drug from the major and minor binding orientations were measured to be 2.4 =1.5 and 3.3 = 1.5 s^?1 respectively, at 35°C. At intermediate drug: DNA ratios (1.3) exchange of the drug between the two one-drug and the two sites of a two-drug complex is observed. Off-rates for both drugs from the 2:1 complex were measured to be 1.0 =0.5 s^?1 (35°C).     

The crystal and molecular structure of diethylenetriammonium hexachloro-antimonate(III)

A compound of the type [DenH₃]SbCl₆ (DenH₃ = diethylenetriammonium cation) was prepared and characterized by means of structural and vibrational measurements. The structure consists of monomeric SbCl₆^3? anions and triprotonated diethylenetriam-monium cations. The SbCl₆^3? anion has a strongly distorted octahedral geometry, presenting three short (2.415–2.495 Å) and three long (2.836–3.114 Å) SbCl bonds. The presence of multiple hydrogen bonds, mainly involving the counterion and the three long-bonded chlorine atoms, is considered to be responsible for the octahedral distortion. Vibrational properties of the complex are discussed in the light of its known crystal structure.     

Thermodynamic parameters of complexation of lanthanoid(III) with ascorbic acid

The thermodynamic parameters of formation of the 1:1 complex between trivalent lanthanide cations and ascorbic acid have been determined for 0.1 M and 2.0 M ionic strengths at 25 °C by potentiometric and calorimetric titration. Comparison of these values with data for other organic ligands indicates that in the complex LnAsc²⁺ the ascorbate functions as an inner sphere monodentate ligand more similar to benzoate than to kojate.     

Guanine complexes with dysprosium(III), thorium(IV) and uranium(IV) chlorides

By refluxing mixtures of guanine (guH) and DyCl₃, ThCl₄ or UCl₄ in ethanol-triethyl orthoformate, solid complexes of the Dy(guH)₂(gu)Cl₂ and M(gu)₂Cl₂ (M = Th, U) types were isolated. The insolubility of the new complexes in organic media, combined with the coordination number six suggested by the spectral evidence, favors polymeric configurations. Most likely structures involve a linear, chainlike, single-bridged polymeric backbone

The Dy³⁺ complex is probably a linear polymer, also containing terminal unidentate guanine ligands, whilst for M = Th⁴⁺, U⁴⁺ highly polymeric structures arising from cross-linking between linear polymeric

units seems most likely. IR evidence rules out participation of the O(6) oxygen of guanine in coordination, despite the hard acid character of the metal ions under study. Guanine apparently coordinates exclusively through ring nitrogens in the new metal complexes; N(9) and N(7). N(9) are, respectively, the most likely binding sites of terminal unidentate and bridging bidentate guanine. The chloro ligands present in the complexes seem to be exclusively terminal.     

Hexaamminecobalt(III) binding environments on double-helical DNA.

Previous cation nmr evidence suggests that univalent cations such as Na+ bind to DNA in a diffuse, nonspecific manner, whereas di- and trivalent cations show distinct binding heterogeneity. Here are reported 59Co- and 23Na-nmr measurements of the %GC dependence of the DNA binding behavior of the trivalent hexaamminecobalt(III) cation. When Co(NH3)6Cl3 titrations are performed on one mammalian and three bacterial DNAs, evidence is found for at least three distinct classes of bound Co(NH3)6(3+). A comparison of titration curves for all four DNAs demonstrates that an increase in GC content correlates with an increase in the fraction of specific Co(NH3)6(3+). binding sites. For M. lysodeikticus DNA (72% GC), a slowly exchanging class of bound 59Co(NH3)6(3+) is apparent. This class of sites is saturated at very low binding densities (between 0.02 and 0.03 cobalt cations per DNA phosphate). At higher binding densities (greater than 0.03), the signal due to slowly exchanging 59Co(NH3)6(3+) disappears into the noise, and a single 59Co(NH3)6(3+) signal is observed. Within the sensitivity limitations of these measurements, no evidence for slowly exchanging bound 59Co(NH3)6(3+) could be found for any of the other DNAs, for which a single, rapidly exchanging 59Co(NH3)6(3+) signal is observed at all binding densities. For this rapidly exchanging signal, for all four DNAs, the measured 59Co(NH3)6(3+) nmr parameters depend significantly on (a) binding density and (b) GC content of the DNA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hydrogen-bonded Trimers of DNA Bases and their Interaction with Metal Cations: Ab initio Quantum-chemical and Empirical Potential Study

Abstract

Neutral (G.GC, A. AT, G.AT, T. AT, and C (imino).GC) and protonated (CH⁺.GC and AH⁺.GC) hydrogen-bonded trimers of nucleic acid bases were characterized by ab initio methods with the inclusion of electron correlation. In addition, the influence of metal cations on the third-strand binding in Purine-Purine-Pyrimidine (Pu.PuPy) reverse-Hoogsteen triplets has been studied. The ab initio calculations were compared with those from recently introduced force fields (AMBER4.1, CHARMM23, and CFF95). The three-body term in neutral trimers is mostly negligible, and the use of empirical potentials is justified. The only exception is the neutral G.GC Hoogsteen trimer with a three-body term of -4 kcal/mol. Protonated trimers are stabilized by molecular ion—;molecular dipole attraction and the interaction within the complex is nonadditive, with the three-body term on the order of -3 kcal/mol. There is a significant induction interaction between the third-strand protonated base and guanine. The calculations indicate an enhancement of the third-strand binding in the G.GC reverse-Hoogsteen trimer due to metal cation coordination to the N7/06 position of the third-strand guanine. Interactions between metal cations and complexes of DNA bases are in general highly nonadditive; the three-body term is above -10 kcal/mol in a complex of a divalent cation (Ca²⁺) with the GG reverse-Hoogsteen pair. The pairwise additive empirical potentials qualitatively underestimate the binding energy between cation and base.     

The structures of bis(1H,5H-S-methylisothiocarbonohydrazidium) di-μ-chloro-octachlorodibismuthate(III) tetrahydrate and tris(1H-S-methylisothiocarbonohydrazidium) esachlorobismuthate(III)

The structures of bis(1H⁺,5H⁺-S-methylisothiocarbonohydrazidium) di-μ-chlorooctachlorodibismuthate(III) tetrahydrate: (C₂H₁₀N₄S)₂(Bi₂Cl₁₀)· 4H₂O (compound [I]) and of tris(1H⁺-S-methylisothiocarbonohydrazidium) esachlorobismuthate(III): (C₂H₉N₄S)₃(BiCl_5.67I_0.33) (compound [II]) were determined from single crystal X-ray diffractometer data. Both compounds crystallize as triclinic (P ); crystals [I] with Z = 1 formula unit in a cell of constants: A = 10.621(3), B = 9.989(5), C = 7.439(3) Å, α = 88.31(2), β = 84.51(2), γ = 68.88(2)°, final R = 0.0427 for 2229 unique reflections with I 2σ(I); crystals [II] with Z = 2 and cell dimensions: A = 14.109(4), B = 12.209(9), C = 8.206(7) Å, α = 103.54(3), β = 104.95(2), γ = 81.96(2)°, final R = 0.0411 for 3637 unique reflections (1 2σ(I)). The structure of [I] is built up of diprotonated organic cations, water molecules and dinuclear centrosymmetric [Bi₂Cl₁₀]⁴⁻ anions held together by N-HCl, N-HO, O-HCl hydrogen bonds and Van der Waals interactions. The [Bi₂Cl₁₀]⁴⁻ complex consists of two edge-sharing octahedra in which three pairs of bonds of similar length are observed (Bi-Cl_av = 2.602(5), 2.712(4), 2.855(5) Å). The structure of [II] consists of monoprotonated cations and [BiCl_5.67I_0.33]³⁻ anions held together by a tridimensional network of hydrogen bonds. Each bismuth atom is octahedrally surrounded by six chlorine atoms, one of which is statistically substituted by a iodine atom.     

Triple-helix formation in the antiparallel binding motif of oligodeoxynucleotides containing N(9)- and N(7)-2-aminopurine deoxynucleosides

Triplex-forming oligodeoxynucleotide 15mers, designed to bind in the antiparallel triple-helical binding motif, containing single substitutions (Z) of the four isomeric αN⁷-, βN⁷-, αN⁹- and βN⁹-2-aminopurine (ap)-deoxyribonucleosides were prepared. Their association with double-stranded DNA targets containing all four natural base pairs (X-Y) opposite the aminopurine residues was determined by quantitative DNase I footprint titration in the absence of monovalent metal cations. The corresponding association constants were found to be in a rather narrow range between 1.0 × 10⁶ and 1.3 × 10⁸ M^–1. The following relative order in Z × X-Y base-triple stabilities was found: Z = αN⁷ap: T-A > A-T> C-G ~ G-C; Z = βN⁷ap: A-T > C-G > G-C > T-A; Z = αN⁹ap: A-T = G-C > T-A > C-G; and Z = βN⁹ap: G-C > A-T > C-G > T-A.     

Structure of the deoxytetranucleotide d-pApTpApT and a sequence-dependent model for poly(dA-dT)

The x-ray structure of the hydrated ammonium salt of the deoxytetranucleotide d-pApTpApT was determined by Patterson and direct methods at a resolution of 1 Å. The crystal structure contains right-handed double-helical segments formed by complementary Watson-Crick-type hydrogen bonding between the adenine and thymine bases of neighboring molecules. The minihelix contains two base pairs. The chains are antiparallel. The A-T and T-A sequences have different phosphodiester conformations. The deoxyribose-pucker and the sugar–base orientation alternate along the chain depending on the nature of the base (3′-endo for purine and 2′-endo for pyrimidine). The extended structure is stabilized by base–base, base–sugar, and hydrogen-bond interactions. The minihelix of two base pairs provides starting coordinates for model-building studies of the dA-dT polymer. A B-DNA-type polymer structure is described, which has sequence-dependent alternations of both the deoxyribose pucker and the phosphate diester bridge conformation. Such sequence-dependent DNA structures, if present locally in regions such as operator sequences, could facilitate sequence-specific interactions. The crystal study also suggests possible geometrical parameters for the replication fork.     

Conformational Changes of DNA by Photoirradiation of DNA-Bis(ZnII-Cyclen)-Azobenzene Complex

Bis(Zn^II-cyclen)-azobenzene derivative, which has two Zn^II-macrocyclic tetraamine complexes connected through azobenzene spacer, has been synthesized as a cross-linking agent for double stranded DNA in aqueous solution. The Zn^II-cyclen derivative selectively binds to A-T base pairs producing complexes between the Zn^II-cyclen moiety and the imide-deprotonated thymine with breaking A-T base pairs. The azobenzene spacer undergoes cis/trans photoisomerization in the complex between the Zn^II-cyclen derivative and the DNA duplex. The conformation of the DNA remarkably changed by photoisomerization of the azobenzene linker, when the Zn^II-cyclen derivative binds to the DNA duplex with an interstrand cross-linking manner.     

Adsorption of several actinide (Th,U) and lanthanide (La,Eu, Yb) ions by Mycobacterium smegmatis

Adsorption measurements of several actinide [thorium (Th), uranium (U)] and lanthanide [lanthanum (La), europium (Eu), ytterbium (Yb)] cations by Mycobacterium smegmatis showed that sorption kinetics followed a three-phase pattern. For 5% (w/w) bacterial suspensions at pH 1, maximum cation biosorption per gram dry biomass corresponded to 170 mol Th⁴⁺ and 187 mol UO _inf2 ^sup2+ . Adsorption of all cations studied obeyed the Brunauer-Emmett-Teller isotherm, which assumes multilayer binding at constant energy. Plots for the Scatchard model showed the existence of at least two types of cation complexation site, with strong and weak affinity and negative cooperation. Th⁴⁺ was preferentially adsorbed with respect to the other cations, although all species appeared to compete for the same sites independently of bacterial viability. Adsorption of these cations was accompanied by partial release of magnesium from the cell wall, indicating that exchange reactions occurred at magnesium (Mg)-bonding sites.     

Structural analysis of DNA-chlorophyll complexes by Fourier transform infrared difference spectroscopy

Porphyrins and metalloporphyrins are strong DNA binders. Some of these compounds have been used for radiation sensitization therapy of cancer and are targeted to interact with cellular DNA. This study was designed to examine the interaction of calf thymus DNA with chlorophyll a (CHL) in aqueous solution at physiological pH with CHL/DNA(phosphate) ratios (r) of 1/160, 1/80, 1/40, 1/20, 1/10, and 1/5. Fourier transform infrared (FTIR) difference spectroscopy was used to characterize the nature of DNA-pigment interactions and to establish correlations between spectral changes and the CHL binding mode, binding constant, sequence selectivity, DNA secondary structure, and structural variations of DNA-CHL complexes in aqueous solution. Spectroscopic results showed that CHL is an external DNA binder with no affinity for DNA intercalation. At low pigment concentration (r = 1/160, 1/80, and 1/40), there are two major binding sites for CHL on DNA duplex: 1) Mg-PO2 and 2) Mg-N7 (guanine) with an overall binding constant of K = 1.13 x 10(4) M-1. The pigment distributions are 60% with the backbone PO2 group and 20% with the G-C base pairs. The chlorophyll interaction is associated with a major reduction of B-DNA structure in favor of A-DNA. At high chlorophyll content (r = 1/10), helix opening occurs, with major spectral alterations of the G-C and A-T bases. At high chlorophyll concentration (1/5), pigment aggregation is observed, which does not favor CHL-DNA complexation.