Binding sites of the polyamines putrescine, cadaverine, spermidine and spermine on A- and B-DNA located by simulated annealing

Molecular dynamics simulations with simulated annealing are performed on polyamine-DNA systems in order to determine the binding sites of putrescine, cadaverine, spermidine and spermine on A- and B-DNA. The simulations either contain no additional counterions or sufficient Na+ ions, together with the charge on the polyamine, to provide 73% neutralisation of the charges on the DNA phosphates. The stabilisation energies of the complexes indicate that all four polyamines should stabilise A-DNA in preference to B-DNA, which is in agreement with experiment in the case of spermine and spermidine, but not in the case of putrescine or cadaverine. The major groove is the preferred binding site on A-DNA of all the polyamines. Putrescine and cadaverine tend to bind to the sugar-phosphate backbone of B-DNA, whereas spermidine and spermine occupy more varied sites, including binding along the backbone and bridging both the major and minor grooves.     

Binding Sites of the Polyamines Putrescine,Cadaverine, Spermidine and Spermine on A- and B-DNA Located by Simulated Annealing

Abstract

Molecular dynamics simulations with simulated annealing are performed on polyamine-DNA systems in order to determine the binding sites of putrescine, cadaverine, spermidine and spermine on A- and B-DNA. The simulations either contain no additional counterions or sufficient Na⁺ ions, together with the charge on the polyamine, to provide 73% neutralisation of the charges on the DNA phosphates. The stabilisation energies of the complexes indicate that all four polyamines should stabilise A-DNA in preference to B-DNA, which is in agreement with experiment in the case of spermine and spermidine, but not in the case of putrescine or cadaverine. The major groove is the preferred binding site on A-DNA of all the polyamines. Putrescine and cadaverine tend to bind to the sugar-phosphate backbone of B-DNA, whereas spermidine and spermine occupy more varied sites, including binding along the backbone and bridging both the major and minor grooves.     

Structural analysis of DNA interactions with biogenic polyamines and cobalt(III)hexamine studied by Fourier transform infrared and capillary electrophoresis

Biogenic polyamines, such as putrescine, spermidine, and spermine are small organic polycations involved in numerous diverse biological processes. These compounds play an important role in nucleic acid function due to their binding to DNA and RNA. It has been shown that biogenic polyamines cause DNA condensation and aggregation similar to that of inorganic cobalt(III)hexamine cation, which has the ability to induce DNA conformational changes. However, the nature of the polyamine.DNA binding at the molecular level is not clearly established and is the subject of much controversy. In the present study the effects of spermine, spermidine, putrescine, and cobalt(III)hexamine on the solution structure of calf-thymus DNA were investigated using affinity capillary electrophoresis, Fourier transform infrared, and circular dichroism spectroscopic methods. At low polycation concentrations, putrescine binds preferentially through the minor and major grooves of double strand DNA, whereas spermine, spermidine, and cobalt(III)hexamine bind to the major groove. At high polycation concentrations, putrescine interaction with the bases is weak, whereas strong base binding occurred for spermidine in the major and minor grooves of DNA duplex. However, major groove binding is preferred by spermine and cobalt(III)hexamine cations. Electrostatic attractions between polycation and the backbone phosphate group were also observed. No major alterations of B-DNA were observed for biogenic polyamines, whereas cobalt(III)hexamine induced a partial B --> A transition. DNA condensation was also observed for cobalt(III)hexamine cation, whereas organic polyamines induced duplex stabilization. The binding constants calculated for biogenic polyamines are K(Spm) = 2.3 x 10(5) M(-1), K(Spd) = 1.4 x 10(5) M(-1), and K(Put) = 1.02 x 10(5) M(-1). Two binding constants have been found for cobalt(III)hexamine with K(1) = 1.8 x 10(5) M(-1) and K(2) = 9.2 x 10(4) M(-1). The Hill coefficients indicate a positive cooperativity binding for biogenic polyamines and a negative cooperativity for cobalt(III)hexamine.     

The Interactions of Ruthenium Hexaammine with Z-DNA: Crystal Structure of a RU(NH3)+3 6 Salt of d(CGCGCG) at 1.2 Å Resolution

Abstract

Acrystalofd(CGCGCG)in the Z-DNA lattice was soaked with ruthenium(III) hexaammine and its structure refined at 1.2 Å resolution. Three unique metal complexes were found adsorbed to each hexamer duplex. In addition, two symmetry-related binding sites were located, yielding a total of five ruthenium complexes bound to each d(CGCGCG) duplex. One unique site and its symmetry related site are nearly identical to the binding site of cobalt(III) hexaammine on Z-DNA. At that position, the metal complex bridges the convex surfaces of two adjacent Z-DNA strands by hydrogen bonds to the N7 and 06 functional groups of the guanine bases. The remaining three ruthenium(III) hexaammine binding sites are not present in the cobalt(III) hexaammine Z-DNA structure. Of these, two are related by symmetry and span the gap between the convex outer surface of one Z-DNA strand and the helical groove crevice of a neighboring strand. The third ruthenium site has no symmetry mate and involves interactions with only the deep groove. In this interaction, the metal complex hydrogen bonds to both the phosphate backbone and to a set of primary shell water molecules that extend the hydrogen bonding potential of the deep groove crevice out to the surface of the molecule. Solution studies comparing the circular dichroism spectra of low salt poly(dG-dC) · poly(dG-dC) samples in the presence of ruthenium(III) and cobalt(III) hexaammine show that the ruthenium complex does stabilize Z-DNA in solution, but not as effectively as the cobalt analogue. This suggests that some of the interactions available for the larger ruthenium complex may not be important for stabilization of the left-handed DNA conformation.     

The interactions of ruthenium hexaammine with Z-DNA: crystal structure of a Ru(NH3)6+3 salt of d(CGCGCG) at 1.2 A resolution

A crystal of d(CGCGCG) in the Z-DNA lattice was soaked with ruthenium(III) hexaammine and its structure refined at 1.2 A resolution. Three unique metal complexes were found absorbed to each hexamer duplex. In addition, two symmetry-related binding sites were located, yielding a total of five ruthenium complexes bound to each d(CGCGCG) duplex. One unique site and its symmetry related site are nearly identical to the binding site of cobalt(III) hexaammine on Z-DNA. At that position, the metal complex bridges the convex surfaces of two adjacent Z-DNA strands by hydrogen bonds to the N7 and O6 functional groups of the guanine bases. The remaining three ruthenium three ruthenium(III) hexaammine binding sites are not present in the cobalt(III) hexaammine Z-DNA structure. Of these, two are related by symmetry and span the gap between the convex outer surface of one Z-DNA strand and the helical groove crevice of a neighboring strand. The third ruthenium site has no symmetry mate and involves interactions with only the deep groove. In this interaction, the metal complex hydrogen bonds to both the phosphate backbone and to a set of primary shell water molecules that extend the hydrogen bonding potential of the deep groove crevice out to the surface of the molecule. Solution studies comparing the circular dichroism spectra of low salt poly(dG-dC).poly(dG-dC) samples in the presence of ruthenium(III) and cobalt(III) hexammine show that the ruthenium complex does stabilize Z-DNA in solution, but not as effectively as the cobalt analogue. This suggests that some of the interactions available for the larger ruthenium complex may not be important for stabilization of the left-handed DNA conformation.     

Polyamine-induced B-DNA to Z-DNA conformational transition of a plasmid DNA with (dG-dC)n insert.

We investigated the ability of natural polyamines putrescine, spermidine, and spermine to provoke a left-handed Z-DNA conformation in a recombinant plasmid (pDHg16) with a 23-base pair insert of (dG-dC)n.(dG-dC)n sequences. Using a monoclonal anti-Z-DNA antibody (Z22) and an enzyme-linked immunosorbent assay protocol, we found that spermidine and spermine were capable of converting pDHg16 to the Z-DNA form. The concentrations of spermidine and spermine at the midpoint of the B-DNA to Z-DNA transition were 280 and 5 microM, respectively, in buffer containing 50 mM NaCl, 1 mM sodium cacodylate, and 0.15 mM EDTA, pH 7.4. A plot of ln[Na+] versus ln [spermine4+], where [Na+] is the bulk NaCl concentration and [spermine4+] is the spermine concentration at the midpoint of the B-DNA to Z-DNA transition, gave a straight line with a slope of 1.2. Structural specificity was clearly evident in the efficacy of three spermidine homologs to induce the Z-DNA conformation in pDHg16. Putrescine and acetylspermidines had no effect on the conformation of the plasmid DNA up to a 3 mM concentration. Control experiments with the parental plasmid (pDPL6) showed no binding of the plasmid DNA with Z22. These results indicate that spermidine and spermine are capable of provoking the left-handed Z-DNA conformation in small blocks of (dG-dC)n sequences embedded in a right-handed B-DNA matrix. Since blocks of (dG-dC)n sequences are found in certain native DNAs, conformational alterations of these regions to the Z-DNA form in the presence of polyamines may have important gene regulatory effects.     

Interactions of anti-poly[d(G-br5C)] IgG with synthetic, viral and cellular Z DNAs

Enzymatically synthesized poly[d(G-br5C)] was used to prepare specific polyclonal and monoclonal anti-Z DNA IgGs. The binding specificities of these antibodies were characterized using left-handed polynucleotides with the sequences d(G-x5C)n and d(A-x5C)n.d(G-T)n (mean = aza, methyl, bromo, or iodo). Polyclonal anti-poly[d(G-br5C)] IgG binds the convex surface of the Z helix as evidenced by the strong requirement for a methyl or halogen group at the C5 position of cytosine. Little or no anti-poly[d(G-br5C)] IgG binding occurs to left-handed DNAs carrying a phosphorothioate substitution in the dGpdC bond or an N-5 aza substitution in the cytosine ring. Anti-poly[d(G-br5C)] IgG can stabilize transient Z DNA structures in both polymer families, thereby displacing the equilibrium in solution between the right-and left-handed DNA conformations. Anti-poly[d(G-br5C)] IgG binding sites are found in all tested covalently closed circular natural DNAs (Form I) at their extracted negative superhelical densities, but not in any of the corresponding relaxed Form II or linear Form III DNAs. Binding of anti-poly[d(G-br5-C)] IgG leads to a reduction in the electrophoretic mobility of Form I DNA (e.g. SV40, phi X174, or pBR322) and to the formation of dimers comprised of the bivalent antibody and two supercoiled Form I DNA molecules. The dimers are converted to monomers by DTT treatment. The formation of IgG-DNA complexes is dependent on external conditions (ionic strength, temperature), the properties of the DNA (torsional stress, sequence), and the immunoglobulin (specificity, valency, and concentration). Higher order oligomeric species, indicative of two or more left-handed segments per DNA molecule are formed in reactions of anti-poly[d(G-br5C)] IgG with M13 RF I DNA but not with SV40, pBR322, or phi X174 DNAs. However, oligomers of the latter are generated with other anti-Z DNA IgGs having a broader spectrum of anti-Z DNA reactivity. Conditions which destabilize natural Z sequences in deproteinized supercoiled genomes are: monovalent salt concentrations at or above the 'physiological' range, high temperature, and topological relaxation with DNA gyrase (in the absence of ATP) or with type I topoisomerases. DNA gyrase (plus ATP) catalyses an increase in DNA negative superhelical density which leads to greater anti-Z DNA IgG binding, indicating the formation of additional left-handed regions. Polytene chromosomes of insect larvae bind anti-poly[d(G-br5C)] IgG specifically and stably at Z DNA sites. The distribution of this IgG binding differs in certain regions from that displayed by anti-Z DNA IgG probes with other sequence specificities.(ABSTRACT TRUNCATED AT 400 WORDS)     

Insight into the molecular recognition of spermine by DNA quadruplexes from an NMR study of the association of spermine with the thrombin‐binding aptamer

The preferred residence sites and the conformation of DNA‐bound polyamines are central to understanding the regulatory roles of polyamines. To this end, we have used a series of selective ¹³C‐edited and selective total correlation spectroscopy‐edited one‐dimensional (1D) nuclear Overhauser effect spectroscopy NMR experiments to determine a number of intramolecular ¹H nuclear Overhauser effect (NOE) connectivities in ¹³C‐labelled spermine bound to the thrombin‐binding aptamer. The results provide evidence that the aptamer‐bound spermine adopts a conformation that optimizes electrostatic and hydrogen bond contacts with the aptamer backbone. The distance between the nitrogen atoms of the central aminobutyl is reduced by an increase in the population of gauche conformers at the C6–C7 bonds, which results in either a curved or S‐shaped spermine conformation. Molecular modelling contributes insight toward the mode of spermine binding of these spermine structures within the narrow grooves of DNA quadruplexes. In each case, the N5 ammonium group makes hydrogen bonds with two nearby phosphates across the narrow groove. Our results have implications for the understanding of chromatin structure and the rational design of quadruplex‐binding drugs. Copyright © 2013 John Wiley & Sons, Ltd.     

Location of spermine and other polyamines on DNA as revealed by photoaffinity cleavage with polyaminobenzenediazonium salts

Although polyamines interact strongly with nucleic acids, X-ray and NMR studies have not revealed much structural information about spermine-DNA complexes. Therefore, it was of interest to look at the binding of polyamines to 32P-labeled DNA restriction fragments by sequencing gel electrophoresis of the photoaffinity cleavage products induced by polyaminobenzendiazonium salts. The shift of cleavage patterns observed on opposite strands as well as competition experiments with distamycin shows polyamines to be located in the minor groove of B-DNA and to depend on the nucleic acid polymorphism, jumping to the major groove in the A-form. The sequence selectivities of various polycations (spermine, putrescine, and cobalt (III) hexaammine) are similar and slightly favor A,T-rich regions. Taken together, these results show that polycations which are not point charges are guided by the electronegative potential along the nucleic acid and suggest fast crawling of the polyamine within the minor groove, due to individual NH2+ jumping between multiple equidistant and isoenergetic bidentate hydrogen-bonding sites. Such a picture could be the clue to the unexpected NMR and to the frequently silent X-ray behavior of polyamines when bound to DNA.     

Probing the surface of Z-DNA with anti-nucleoside antibodies

Antibodies specific for cytidine (C) and guanosine (G) were used to probe the surface of two Z-DNA conformers. When tested by ELISA, anti-G reacted with poly(dG-dC).poly(dG-dC) treated with bromine water [Br-poly(dG-dC).poly(dG-dC)] but anti-C did not. A weak reaction with anti-C was detected by dot immunobinding. In contrast, anti-C reacted strongly with poly(dG-dC).poly(dG-dC) treated with N-acetoxy-2-(acetylamino)fluorene [AAF-poly(dG-dC).poly(dG-dC)]; anti-G reacted weakly, despite the fact that most G residues had not been substituted with AAF. Neither antinucleoside bound to the B conformation of poly(dG-dC).poly(dG-dC). In competition experiments, GMP was the most efficient competitor of the reaction of anti-G with Br-poly(dG-dC).poly(dG-dC); AMP and TMP were 100-fold less efficient, and CMP did not compete to a significant extent. In contrast, the reaction of anti-Z with Br-poly(dG-dC).poly(dG-dC) was not inhibited by nucleotides. Of five possible sites recognized on guanosine by anti-G antibodies (N1, C6, O6, N7, and C8), AMP and TMP share three or their equivalent and CMP only one. The binding of anti-C to AAF-poly(dG-dC).poly(dG-dC) was inhibited best by CMP; AMP was 8 times less efficient; GMP and TMP were about 35-fold less efficient than CMP. Thus, although the amino group on the C4 position of CMP appears to be immunodominant, the capacity of GMP and TMP to inhibit the reaction indicates that other sites are also recognized in AAF-poly(dG-dC).poly(dG-dC), e.g., the exposed C5 position.(ABSTRACT TRUNCATED AT 250 WORDS)     

A Z-DNA binding protein isolated from D. radiodurans

A DNA binding protein isolated from D. radiodurans changes CD-spectrum of Z-form poly(dG-dC) X poly(dG-dC). We have found that a positive band at 268 nm is converted close to that of B-form in the presence of the protein. Concomitantly, a negative band at 295 nm shown by Z-form poly(dG-dC) X poly (dG-dC) was weakened by the protein but not by albumin. Such changes in the CD-spectra were not induced by the protein and by albumin when they were mixed with Z- or B-form poly(dG-me5dC) X poly(dG-me5dC) or with B-form poly(dG-dC) X poly(dG-dC). The protein formed a complex preferentially with Z-form poly(dG-dC) X poly(dG-dC).     

Characterization of anti-Z-DNA antibody binding sites on Z-DNA by nuclear magnetic resonance spectroscopy

Two monoclonal anti-Z-DNA antibodies, Z22 and Z44, were shown to bind to the oligonucleotides, d(CG)2 and d(CG)3, and to interact with different parts of the helix. 1H nuclear magnetic resonance spectroscopy showed that Fab fragments stabilize an ordered structure in the tetranucleotide d(CG)2. Nuclear Overhauser effects measured in the presence of Z22 Fab indicate a syn conformation of guanine residues of d(CG)2. Intermolecular transfer of saturation between the Fabs and bound d(CG)3 was detected by a saturation of the protein spectrum and observation of changes in the DNA spectrum. Antibodies with deuterated aromatic amino acids were prepared to eliminate the protein aromatic resonances and thereby allow a more detailed analysis of the transfer to the DNA base protons. The greatest transfer with Z44 was to the dC-5 protons although all of the base protons interact with this antibody. Little, if any, transfer to the DNA base protons was observed with Z22. These results are consistent with a Z44 binding site on the convex surface of the Z-helix (analogous to the major groove of B-DNA) and a Z22 binding site on the sugar-phosphate backbone.     

Vibrational and electronic circular dichroism study of the interactions of cationic porphyrins with (dG-dC)10 and (dA-dT)10

The interactions of two different porphyrins, without axial ligands-5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin-Cu(II) tetrachloride (Cu(II)TMPyP) and with bulky meso substituents-5,10,15,20-tetrakis(N,N,N-trimethylanilinium-4-yl)porphyrin tetrachloride (TMAP), with (dG-dC)10 and (dA-dT)10 were studied by combination of vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectroscopy at different [oligonucleotide]/[porphyrin] ratios, where [oligonucleotide] and [porphyrin] are the concentrations of oligonucleotide per base-pair and porphyrin, respectively. The combination of VCD and ECD spectroscopy enables us to identify the types of interactions, and to specify the sites of interactions: The intercalative binding mode of Cu(II)TMPyP with (dG-dC)(10), which has been well described, was characterized by a new VCD "marker" and it was shown that the interaction of Cu(II)TMPyP with (dA-dT)10 via external binding to the phosphate backbone and major groove binding caused transition from the B to the non-B conformer. TMAP interacted with the major groove of (dG-dC)10, was semi-intercalated into (dA-dT)10, and caused significant variation in the structure of both oligonucleotides at the higher concentration of porphyrin. The spectroscopic techniques used in this study revealed that porphyrin binding with AT sequences caused substantial variation of the DNA structure. It was shown that VCD spectroscopy is an effective tool for the conformational studies of nucleic acid-porphyrin complexes in solution.     

Molecular dynamics of spermine-DNA interactions: sequence specificity and DNA bending for a simple ligand.

We used molecular dynamics to model interactions between the physiologically important polyamine spermine and two B-DNA oligomers, the homopolymer (dG)10-(dC)10 and the heteropolymer (dGdC)5-(dGdC)5. Water and counterions were included in the simulation. Starting coordinates for spermine-DNA complexes were structures obtained by molecular mechanics modeling of spermine with the two oligomers; in these models, spermine binding induced a bend in the heteropolymer but not in the homopolymer. During approximately 40 psec of molecular dynamics simulation, spermine moves away from the floor of the major groove and interacts nospecifically with d(G)10-d(C)10. In contrast, a spermine-induced bend in the helix of (dGdC)5-(dGdC)5 is maintained throughout the simulation and spermine remains closely associated with the major groove. These results provide further evidence that the binding of spermine to nucleic acids can be sequence specific and that bending of alternating purine-pyrimidine sequences may be a physiologically important result of spermine binding.     

Locating monovalent cations in the grooves of B-DNA

Here we demonstrate that monovalent cations can localize around B-DNA in geometrically regular, sequence-specific sites in oligonucleotide crystals. Positions of monovalent ions were determined from high-resolution X-ray diffraction of DNA crystals grown in the presence of thallium(I) cations (Tl(+)). Tl(+) has previously been shown to be a useful K(+) mimic. Tl(+) positions determined by refinement of model to data are consistent with positions determined using isomorphous F(Tl) - F(K) difference Fouriers and anomalous difference Fouriers. None of the observed Tl(+) sites surrounding CGCGAATTCGCG are fully occupied by Tl(+) ions. The most highly occupied sites, located within the G-tract major groove, have estimated occupancies ranging from 20% to 35%. The occupancies of the minor groove sites are estimated to be around 10%. The Tl(+) positions in general are not in direct proximity to phosphate groups. The A-tract major groove appears devoid of localized cations. The majority of the observed Tl(+) ions interact with a single duplex and so are not engaged in lattice interactions or crystal packing. The locations of the cation sites are dictated by coordination geometry, electronegative potential, avoidance of electropositive amino groups, and cation-pi interactions. It appears that partially dehydrated monovalent cations, hydrated divalent cations, and polyamines compete for a common binding region on the floor of the G-tract major groove.     

Conformations of poly(dG-dC).poly(dG-dC) modified by the O-acetyl derivative of the carcinogen 4-hydroxyaminoquinoline 1-oxide.

Poly(dG-dC).poly(dG-dC) has been modified by reaction with 4-acetoxyaminoquinoline 1-oxide (Ac-4 HAQO), the ultimate carcinogen of 4-nitroquinoline 1-oxide. The circular dichroism (CD) spectra of the modified and unmodified polymers have been compared under various experimental conditions. The CD spectra were recorded in 1 mM phosphate, 50% (v/v) ethanol, 3.8 M LiCl and 95% (v/v) ethanol, conditions in which poly(dG-dC).poly(dG-dC) adopts the B-, Z-, C- and A-form respectively. In 1 mM phosphate buffer, poly(dG-dC).poly(dG-dC) modified by Ac-4 HAQO seems not to contain regions in the Z-form. Z-form induction could be progressively obtained by the addition of ethanol as follows: in the buffer with about 30% ethanol the modified polymer started to adopt the Z structure, while 40% of ethanol in the buffer was necessary for the unmodified polymer. In the 50% ethanol-1 mM phosphate buffer mixture (v/v), poly(dG-dC).poly(dG-dC) was entirely in the Z-form while poly(dG-dC).poly(dG-dC) modified by Ac-4 HAQO remained partially in the B-form. Enzymatic digestions with the nuclease S1 which is specific of the single-stranded DNA were carried out in order to support the modified poly(dG-dC).poly(dG-dC) CD study conclusions. The role played by the two major adducts on the conformational characteristics of modified polymer is discussed.     

Influence of spermine on DNA conformation in a molecular dynamics trajectory of d(CGCGAATTCGCG)(2): major groove binding by one spermine molecule delays the A-->B transition

The effect of spermine on the A-DNA to B-DNA transition in d(CGCGAATTCGCG)(2) has been investigated by five A-start molecular dynamics simulations, using the Cornell et al. potential. In the absence of spermine an A-->B transition is initiated immediately and the DNA becomes equidistant from the A- and B-forms at 200ps. In three DNA-spermine simulations, when a spermine is located across the major groove of A-DNA in one of three different initial locations, the time taken to reach equidistance from the A- and B-forms is delayed until 800, 950 or 1000ps. In each case the A-form appears to be temporarily stabilized by spermine's electrostatic interactions with phosphates on both sides of the major groove. The onset of the A-->B transition can be correlated with the spermine losing contact with phosphates on one side of the groove and with A-like --> B-like sugar pucker transitions in the vicinity of the spermine bridge. However in the fifth trajectory, in which the spermine initially threads from the major groove via the backbone into the minor groove, the B-->A transition occurs rapidly once again and the DNA is equidistant between the A- and B-forms within 300ps. This indicates that the mere presence of spermine is insufficient to delay the transition and that major groove binding stabilizes A-DNA.     

Interaction of topotecan,DNA topoisomerase I inhibitor,with double-stranded polydeoxyribonucleotides. III. Binding at the minor groove

Interaction of topotecan (TPT) with calf thymus DNA, coliphage T4 DNA, and poly(dG-dC). poly(dG-dC) was studied by optical (linear flow dichroism, UV-vis spectroscopy) and quantum chemical methods. The linear dichroism (LD) signal of TPT bound to DNA was shown to have positive sign in the range 260-295 nm. This means that the plane of quinoline fragment (rings A and B) of TPT molecule form an angle lower 54 degrees with the long axis of DNA, and hence TPT molecule can not intercalate between DNA base pairs. TPT was established to bind to calf thymus DNA as readily as to coliphage T4 DNA whose all cytosines in the major groove were glycosylated at the 5th position. Consequently, the DNA major groove does not participate in TPT binding. TPT molecule was shown to compete with distamycin for binding sites in the minor groove of DNA and poly(dG-dC). poly(dG-dC). Thus, it was demonstrated for the first time that TPT binds to DNA at its minor groove.     

Fourier transform Raman study of the structural specificities on the interaction between DNA and biogenic polyamines

Biogenic polyamines putrescine, spermidine, and spermine are essential molecules for proliferation in all living organisms. Direct interaction of polyamines with nucleic acids has been proposed in the past based on a series of experimental evidences, such as precipitation, thermal denaturation, or protection. However, binding between polyamines and nucleic acids is not clearly explained. Several interaction models have also been proposed, although they do not always agree with one another. In the present work, we make use of the Raman spectroscopy to extend our knowledge about polyamine-DNA interaction. Raman spectra of highly polymerized calf-thymus DNA at different polyamine concentrations, ranging from 1 to 50 mM, have been studied for putrescine, spermidine, and spermine. Both natural and heavy water were used as solvents. Difference Raman spectra have been computed by subtracting the sum of the separated component spectra from the experimental spectra of the complexes. The analysis of the Raman data has supported the existence of structural specificities in the interactions, at least under our experimental conditions. These specificities lead to preferential bindings through the DNA minor groove for putrescine and spermidine, whereas spermine binds by the major groove. On the other hand, spermine and spermidine present interstrand interactions, whereas putrescine presents intrastrand interactions in addition to exo-groove interactions by phosphate moieties.