1H NMR and circular dichroism studies of the B and Z conformations of the self-complementary deoxyhexanucleotide d(m5C-G-C-G-m5-C-G): mechanism of the Z-B-coil transitions

The double-helical conformations of d(m5-C-G-C-G-m5-C-G) in aqueous solution were studied by circular dichroism and 1H NMR spectroscopy. In 0.1 M NaCl, only the B form is detected whereas the Z form is strongly predominant in 3 M NaCl. In the presence of 2 M NaCl, two resonance signals corresponding to the B and Z duplexes were observed for each proton below 50 degrees C, indicating a slow exchange between B and Z. However, the B-Z exchange becomes intermediate or fast in the 55-80 degrees C temperature interval. By contrast the exchange between B helix and single-stranded (or coil) forms is much faster for the same temperature conditions. The Z form is only detectable when the coil form is practically absent. With decreasing temperature the B form decreases in favor of the Z form. From proton line-width measurements under various experimental conditions, it was also shown that Z exchanges only with B, while the latter also exchanges with the single-stranded form (S): Z in equilibrium B in equilibrium S. The enthalpy value is about 8 +/- 1 kcal/mol for the B-Z transition and about 40 +/- 2 kcal/mol for the B-S dissociation (2 M NaCl solution). The activation energy is about 47 +/- 2 kcal/mol for the Z----B and 39 +/- 2 kcal/mol for the B----Z reaction. Very good agreement between the experimental results and computed data (based on the above kinetic reaction model) was found for the B, Z, and coil proportions. The B-Z transition of methylated d(C-G)n oligomers is only possible when the Watson-Crick hydrogen bonds between the CG base pairs are firmly maintained; otherwise, the transformation from B to Z would not occur, and B-S dissociation would take place instead.     

Base sequence criteria and Cartesian coordinates for stable B/Z and B/Z/B junctions in relaxed DNA.

It seems increasingly evident that if the Z form of DNA exists in the genome it must exist as short sections of alternating pyrimidine-purine sequences in the midst of very long sections of B-form DNA. We have determined the minimum length of a string of alternating CG base pairs that can go into the Z form in the middle of a long B form. Self-complimentary oligomers of the form T(M)(CG)(N)A(M) were synthesized. The conformation of the resulting duplex was determined in 6M aqueous NaCl solution by Raman scattering. We have found that 12 alternating CG base pairs is the minimum length required to form a stable Z form of DNA inside of a long B form section. Only the 4 center CG base pairs go into the Z form. These 4 CG base pairs in the Z form are flanked on each side by 4 CG base pairs in a non-Z (probably B) form as well as the ..TT.. ..AA.. sequences in the B form. We propose a model of the B/Z junction in which the double helix flips directly from the B form to the Z form so that there are no base pairs in the junction. In this model the B form is nucleated in the AT base pairs on each end and is propagated into the CG base pairs in the center. This model is supported by isotopic H/D exchange experiments that shows that the H/D exchange of the non-Z form CG base pairs is highly retarded and indicates that they remain in the B form. A Thermodynamic analysis of the concentration dependence of the melting point of the duplexes in both low and high salt, supports our model and rules out the possibility of hairpin formation. The enthalpy for the formation of a B/Z junction is determined to be about +16 kcal/junction. A comparison of these results with recent results on B/Z junctions in super-coiled DNA is given. Molecular modeling calculations permit us to obtain values for the coordinates and torsional angles of the oligomers showing both B/Z and B/Z/B junctions. The Cartesian coordinates for these oligomers as well as stereo figures of these models in color are available from the authors.     

The Z-Z junction: the boundary between two out-of-phase Z-DNA regions

The boundary between two segments of Z-DNA that differ in the phase of their syn-anti alternation about the glycosidic bond is termed a Z-Z junction. Using chemical probes and two-dimensional gel electrophoresis, we examined a Z-Z junction consisting of the sequence d[(CG)8C(CG)8] inserted into a plasmid and used energy minimization techniques to devise a three-dimensional model that is consistent with the available data. We show that both alternating CG segments undergo the B-Z transition together to form a Z-Z junction. The junction is very compact, displaying a distinctive reactivity signature at the two base pairs at the junction. In particular, the 5' cytosine of the CC dinucleotide at the junction is hyperreactive toward hydroxylamine, and the two guanines of the GG dinucleotide on the complementary strand are less reactive toward diethyl pyrocarbonate than are the surrounding Z-DNA guanines. Statistical mechanical treatment of the 2-D gel data yields a delta G for forming the Z-Z junction equal to 3.5 kcal, significantly less than the cost of a B-Z junction and approximately equal to the cost of a base out of alternation (i.e., a Z-DNA pyrimidine in the syn conformation). The computer-generated model shows little distortion of the Z helix outside of the central two base pairs, and the energy of the structure and the steric accessibility of the reactive groups are consistent with the data.     

Left-handed Z form in superhelical DNA: a theoretical study

This is a comprehensive statistical mechanical treatment of the Z form formation in purinepyrimidine stretches of different length inserted into superhelical DNA. The B-Z transition for short inserts is shown to follow the "all-or-none" principle. Over some critical value of the insert length n, the B-Z transition in the insert proceeds in two stages. The flipping of m base pairs into the Z form is followed by a gradual growth of the Z-form stretch until it occupies the whole insert. By fitting the theoretical transition curves to experimental ones the fundamental thermodynamic parameters of the B-Z transition have been determined: the B-Z junction energy Fj = 4-5kcal.mol-1 and the free energy change delta FBZ = 0.5-7.0 kcal.mol-1 under standard salt conditions. Calculations show that the B-Z transition in short purinepyrimidine inserts may be seriously affected by cruciform formation in the carrier DNA.     

Z helix-coil transition of d(C-Br8G-C-G-C-Br8G) studied by CD, 1H-NMR and IR spectroscopies

The conformation of d(C-Br8G-C-G-C-Br8G) in aqueous solution was studied by CD and 1H-NMR spectroscopy and in condensed phase by IR spectroscopy. Whether in 0.1 M or 3 M NaCl solution or in film the only double helical structure adopted by brominated d(C-G)3 oligomer is the Z form. The IR spectrum of the film presents all the characteristic absorptions of the Z conformation and in particular is indicative of a syn conformation for the central guanosine as well as for the brominated one. Imino proton resonances of d(C-Br8G-C-G-C-Br8G) demonstrating the duplex formation were observed up to 60 degrees C. It is interesting to note that the significant highfield shifts of the dC H5" exocyclic sugar protons characteristic of the non exchangeable proton spectra of d(C-G)3 containing 5-methyl dC residues in the Z form were also detected in the proton spectrum of brominated oligomer. Whereas formation of the Z helix of methylated d(C-G)3 oligomers dependent on the salt concentration was found to occur via the preliminary formation of a B helix even in 4 M NaCl solution, the Z helix of d(C-Br8G-C-G-C-Br8G) is obtained directly from the coil form. However, IR data suggest that in the Z form of d(C-Br8G-C-G-C-Br8G), the overlapping of the base planes should be slightly different in comparison with the stacking observed in d(C-G)3 crystals. The kinetic data (activation energy and lifetime) of the Z helix-coil transition of brominated d(C-G)3 are compared to those of the B helix-coil transition observed for methylated d(C-G)3 in 0.1 M NaCl solution while the thermodynamic data of these two reactions (enthalpy and midpoint temperature) are slightly different.     

Left-handed Z Form in Superhelical DNA: A Theoretical Study

Abstract

This is a comprehensive statistical mechanical treatment of the Z form formation in purine- pyrimidine stretches of different length inserted into superhelical DNA. The B-Z transition for short inserts is shown to follow the “all-or-none” principle. Over some critical value of the insert length n, the B-Z transition in the insert proceeds in two stages. The flipping of m base pairs into the Z form is followed by a gradual growth of the Z-form stretch until it occupies the whole insert. By fitting the theoretical transition curves to experimental ones the fundamental thermodynamic parameters of the B-Z transition have been determined: the B-Z junction energy Fj = 4–5kcal?mol^?1 and the free energy change ΔF_B-Z = 0.5–0.7 kcal?mol^?1 under standard salt conditions. Calculations show that the B-Z transition in short purine-pyrimidine inserts may be seriously affected by cruciform formation in the carrier DNA.     

Conformational analysis of the B and Z forms of the d(m5C-G)3 and d(br5C-G)3 hexamers in solution. A 300-MHz and 500-MHz NMR study

The B and the Z forms of the DNA hexamers d(m5C-G)3 and d(br5C-G)3 were investigated by means of NMR spectroscopy. It is demonstrated that the low-salt form of d(m5C-G)3 is a B DNA structure. The form, which becomes increasingly predominant when increasing amounts of MgCl2 and/or methanol are added to the solution, has Z DNA characteristics. It is shown that the major geometrical features of the Z form of d(m5C-G)3 in the crystal structure are maintained in solution, with the dC residues S sugar conformation, gamma + and the base in the anti orientation and the dG residues N (except the 3'-terminal residue), gamma t and syn. Neither the Z form of the methylated nor that of the brominated compound resembles the Z' form, in which the deoxy guanosine sugar rings adopt a C1'-exo conformation. Substitution of m5C by br5C causes no perceptible conformational changes in either the B or in the Z forms.     

B,Z conformations and mechanism of the Z-B-coil transitions of the self-complementary deoxy-hexanucleotide d(C-G-m5C-G-C-G) by 1H-NMR and CD spectroscopy

The helical structures of d(C-G-m5C-G-C-G) were studied in aqueous solution at various salt concentrations and temperatures by CD and 1H-NMR spectroscopy. At room temperature only the B form is observed in 0.1 M NaCl whereas the B and Z forms are simultaneously present in 1.8 M NaCl. At high salt concentration (4 M NaCl) the Z form is largely predominant (greater than 95%). The Z form proton resonances were assigned by using the polarisation transfer method (between B and Z at 1.8 M NaCl) and by proton-proton decoupling (at high salt concentration). The Z-B-Coil transitions were studied as a function of temperature with the 1.8 M NaCl solution. At high temperature (95 degrees C) only the coil form (S) is present. Below 55 degrees C the coil proportion is negligible, and the B-Z exchange is slow. The disappearance of the coil gives rise at first to the B form and on lowering the temperature the Z proportion increases to the detriment of the B form. Proton linewidth, relaxation and polarisation transfer studies confirm the conclusion in the previous report on d(m5C-G-C-G-m5C-G) (Tran-Dinh et al Biochemistry 1984 in the press) that Z exchanges only with B whereas the latter also exchanges with S,Z in equilibrium B in equilibrium S. The present data show that even at high salt concentration where only the Z form of d(C-G-m5C-G-C-G) is observed the Z-S transition also passes through the B form as an intermediate stage. The B-Z transition takes place when the Watson-Crick hydrogen bonds are firmly maintained and is greatly favoured when there are three hydrogen bonds between the base-pairs.     

Synthesis, structure and thermodynamic properties of 8-methylguanine-containing oligonucleotides: Z-DNA under physiological salt conditions.

Various oligonucleotides containing 8-methylguanine (m8G) have been synthesized and their structures and thermodynamic properties investigated. Introduction Of M8G into DNA sequences markedly stabilizes the Z conformation under low salt conditions. The hexamer d(CGC[M8G]CG)2 exhibits a CD spectrum characteristic of the Z conformation under physiological salt conditions. The NOE-restrained refinement unequivocally demonstrated that d(CGC[m8G]CG)2 adopts a Z structure with all guanines in the syn conformation. The refined NMR structure is very similar to the Z form crystal structure of d(CGCGCG)2, with a root mean square deviation of 0.6 between the two structures. The contribution of m8G to the stabilization of Z-DNA has been estimated from the mid-point NaCl concentrations for the B-Z transition of various m8G-containing oligomers. The presence of m8G in d(CGC[m8G]CG)2 stabilizes the Z conformation by at least deltaG = -0.8 kcal/mol relative to the unmodified hexamer. The Z conformation was further stabilized by increasing the number of m8Gs incorporated and destabilized by incorporating syn-A or syn-T, found respectively in the (A,T)-containing alternating and non-alternating pyrimidine-purine sequences. The results suggest that the chemically less reactive m8G base is a useful agent for studying molecular interactions of Z-DNA or other DNA structures that incorporate syn-G conformation.     

Methylation of cytosine in the 5-position alters the structural and energetic properties of the supercoil-induced Z-helix and of B-Z junctions

The structural and energetic consequences of cytosine methylation in the 5-position on the supercoil-dependent B-Z equilibrium in alternating dC-dG sequences cloned into recombinant plasmids were investigated. The helical parameters determined with the band shift method for right-handed [10.7 base pairs (bp)/turn] and left-handed (12.8 bp/turn) 5MedC-dG inserts were different from the helical repeat values for unmethylated dC-dG inserts (10.5 bp/turn in the right-handed and 11.5 bp/turn in the left-handed form). We analyzed the thermodynamic parameters delta GBZ (free energy difference per base pair between right-handed and left-handed helix structure), delta Gjx (free energy for formation of one B-Z junction), and b (helix unwinding at a junction region) for varying lengths of dC-dG inserts by two-dimensional gel electrophoresis and application of a statistical mechanics model. A comparison of plasmids fully methylated in vitro with HhaI methylase and their unmethylated counterparts revealed that delta Gjx is not significantly changed by cytosine methylation. However, this base modification results in an approximate 3-fold decrease of delta GBZ and an approximate 2-fold decrease of the unwinding b at B-Z junction regions. Analysis of a pair of related plasmids, each containing two dC-dG blocks, revealed qualitatively different transition behaviors. When the two dC-dG blocks were separated by 95 bp of a mixed sequence, they underwent independent B to Z transitions with separate nucleation events and junction formations. When the two blocks were separated by only a 4 bp GATC sequence, only one nucleation event was necessary, and the Z-helix spread across the nonalternating GATC region.(ABSTRACT TRUNCATED AT 250 WORDS)     

Allosteric conversion of Z DNA to an intercalated right-handed conformation by daunomycin

Absorbance and fluorescence methods were used to measure the binding of the anticancer drug daunomycin to poly (dGdC) under ionic conditions that initially favor the left-handed Z conformation of the polymer. Drug binding was cooperative under these conditions and may be fully accounted for by an allosteric model in which the drug binds preferentially (but not exclusively) to the right-handed B conformation and shifts the polymer from the Z to an intercalated right-handed conformation. Quantitative analysis of binding isotherms in terms of the allosteric model allowed for estimation of the equilibrium constants for the conversion of a base pair at a B-Z interface from the Z to the B conformation and for the formation of a base pair in the B conformation within a stretch of helix in the Z conformation. The free energy of the Z to B conversion of a base pair was calculated from this data and ranges from +0.03 to +0.3 kcal/mol over the NaCl range of 2.4-3.5 M. The free energy for the formation of a B-Z junction was nearly constant at +4.0 kcal/mol over the same range of NaCl concentrations. The salt dependence of the free energy of the Z to B transition indicates preferential Na+ binding to the Z form and that there is a net release of Na+ upon conversion of a base pair from the Z to the B conformation. The energetically unfavorable Z to B transition was found by this analysis to be driven by coupling to the energetically favorable interaction of daunomycin with B form DNA. In 3.5 M NaCl, for example, the free energy change for the overall reaction (Z DNA base pairs) + (daunomycin) in equilibrium with (right-handed complex) is -7.0 kcal/mol, nearly all of which is contributed by the binding of drug to B DNA. Analysis using the allosteric model also shows that the number of base pairs converted from the Z to the B conformation per bound drug molecule is salt dependent and provides evidence that drug molecules partition into regions of the polymer in the right-handed conformation.     

Construction and analysis of monomobile DNA junctions

Immobile DNA junctions are complexes of oligomeric DNA strands that interact to yield branched structures in which the branch point cannot migrate. This is achieved by minimizing the sequence symmetry in the flanking arms, so that base pairs lock at the branch site. Here, we report the design, synthesis, and analysis of two semimobile junctions, structures in which a controlled extent of branch point migratory freedom is deliberately introduced. We have constructed two minimally symmetric four-arm semimobile junctions from synthetic deoxy 17-mers. These junctions, termed "monomobile", contain a single pair of base pairs (A-T or C-G) which can migrate at the site of branching, while the rest of the junction is immobile. We have demonstrated by gel electrophoresis techniques that these junctions form and that they have the predicted 1:1:1:1 stoichiometry. We have compared these junctions with the immobile junction on which they are based, by means of hydroxyl radical protection experiments. From these data, both migratory conformers can be seen to coexist in solution. The semimobile junction with the C-G base pair has the same crossover and stacking pattern observed for the immobile junction, while the junction with the A-T base pair has the opposite pattern. We conclude that crossover and stacking patterns are a direct consequence of the base pairs which flank the junction. In addition, the data indicate that the crossover pattern biases for these junctions are much greater than are the migratory biases.     

Hairpin and duplex formation of the DNA octamer d(m5C-G-m5C-G-T-G-m5C-G) in solution. An NMR study.

The partly self-complementary DNA octamer d(m5C-G-m5C-G-T-G-m5C-G) was investigated by NMR spectroscopy in solution. It is demonstrated that this peculiar DNA fragment, under suitable conditions of concentration, salt and temperature, exclusively prefers to adopt a monomeric hairpin form with a stem of three Watson-Crick type base pairs and a loop of two residues. At high single strand concentration (8 mM DNA) and low temperature (i.e. below 295 K) the hairpin occurs in slow equilibrium with a B-dimer structure. At high ionic strength (greater than or equal to 100 mM Na+) and/or in the presence of methanol a third species appears, which is assigned to a Z-like dimer. In the B form, as well as in the Z dimer, the two central base pairs form G.T wobble pairs with the bases as major tautomers.     

Z Helix-coil Transition of d(C-Br8G-C-G-C-Br8G) Studied by CD, 1H-NMR and IR Spectroscopies

Abstract

The conformation of díC-Bi⁸G-C-G-C-Br⁸G) in aqueous solution was studied by CD and ¹H-NMR spectroscopy and in condensed phase by IR spectroscopy. Whether in 0.1 M or 3 M NaCl solution or in film the only double helical structure adopted by brominated d(C-G)₃ oligomer is the Z form. The IR spectrum of the film presents all the characteristic absorptions of the Z conformation and in particular is indicative of a syn conformation for the central guanosine as well as for the brominated one. Imino proton resonances of diC-Bi⁸G-C- G-C-Br⁸G) demonstrating the duplex formation were observed up to 60°C. It is interesting to note that the significant highfield shifts of the dC H₅″ exocyclic sugar protons characteristic of the non exchangeable proton spectra of d(C-G)₃ containing 5-methyl dC residues in the Z form were also detected in the proton spectrum of brominated oligomer. Whereas formation of the Z helix of methylated d(C-G)₃ oligomers dependent on the salt concentration was found to occur via the preliminary formation of a B helix even in 4 M NaCl solution, the Z helix of d(C-Br⁸G-C-G-C-Br⁸G) is obtained directly from the coil form. However, IR data suggest that in the Z form of dlC-Bi⁸G-C-G-C-Bi⁸G), the overlapping of the base planes should be slightly different in comparison with the stacking observed in d(C-G)₃ crystals. The kinetic data (activation energy and lifetime) of the Z helix-coil transition of brominated d(C-G)₃ are compared to those of the B helix-coil transition observed for methylated d(C-G)₃ in 0.1 M NaCl solution while the thermodynamic data of these two reactions (enthalpy and midpoint temperature) are slightly different.     

The intrinsic structure and stability of out-of-alternation base pairs in Z-DNA.

Alternating pyrimidine-purine sequences typically form Z-DNA, with the pyrimidines in the anti and purines in the syn conformations. The observation that dC and dT nucleotides can also adopt the syn conformation (i.e. the nucleotides are out-of-alternation) extends the range of sequences that can convert to this left-handed form of DNA. Here, we study the effects of placing two adjacent d(G*C) base pairs as opposed to a single d(G*C) base pair or two d(A*T) base pairs out-of-alternation by comparing the structure of d(m5CGGCm5CG)2with the previously published structures of d(m5CGGGm5CG)*d(m5CGCCm5CG) and d(m5CGATm5CG)2. A high buckle and loss of stacking interactions are observed as intrinsic properties of the out-of-alternation base pairs regardless of sequence and the context of the dinucleotide. From solution titrations, we find that the destabilizing effect of out-of-alternation d(G*C) base pairs are identical whether these base pairs are adjacent or isolated. We can therefore conclude that it is these intrinsic distortions in the structure of the base pairs and not neighboring effects that account for the inability of out-of-alternation base pairs to adopt the left-handed Z conformation.     

B,Z Conformations and Mechanism of the Z-B-Coil Transitions of the Self-Complementary Deoxy-hexanucleotide d(C-G-m5C-G-C-G) by 1H-NMR and CD Spectroscopy

Abstract

The helical structures of d(C-G-m⁵C-G-C-G) were studied in aqueous solution at various salt concentrations and temperatures by CD and ¹H-NMR spectroscopy. At room temperature only the B form is observed in 0.1 M NaCl whereas the B and Z forms are simultaneously present in 1.8 M NaCl. At high salt concentration (4 M NaCl) the Z form is largely predominant (> 95%). The Z form proton resonances were assigned by using the polarisation transfer method (between B and Z at 1.8 M NaCl) and by proton-proton decoupling (at high salt concentration).

The Z-B-Coil transitions were studied as a function of temperature with the 1.8 M NaCl solution. At high temperature (95°C) only the coil form (S) is present. Below 55°C the coil proportion is negligible, and the B-Z exchange is slow. The disappearance of the coil gives rise at first to the B form and on lowering the temperature the Z proportion increases to the detriment of the B form. Proton linewidth, relaxation and polarisation transfer studies confirm the conclusion in the previous report on d(m⁵C-G-C-G-m⁵C-G) (Tran-Dinh et al Biochemistry 1984 in the press) that Z exchanges only with B whereas the latter also exchanges with S,Z ? B ? S. The present data show that even at high salt concentration where only the Z form of d(C-G-m⁵C-G-C-G) is observed the Z-S transition also passes through the B form as an intermediate stage. The B-Z transition takes place when the Watson-Crick hydrogen bonds are firmly maintained and is greatly favoured when there are three hydrogen bonds between the base-pairs.     

The Z-conformation of poly(dA-dT).poly(dA-dT) in solution as studied by ultraviolet resonance Raman spectroscopy

Poly(dA-dT).poly(dA-dT) structures in aqueous solutions with high NaCl concentrations and in the presence of Ni2+ ions have been studied with resonance Raman spectroscopy (RRS). In low water activity the effects of added 95 mM NiCl2 in solution stabilize the syn geometry of the purines and reorganize the water distribution via local interactions of Ni-water charged complexes with the adenine N7 position. It is shown that RRS provides good marker bands for a left-handed helix: i) a purine ring breathing mode around 630 cm-1 coupled to the deoxyribose vibration in the syn geometry, ii) a 1300-1340 cm-1 region characterizing local chemical interactions of the Ni2+ ions with the adenine N7 position, iii) lines at about 1483- and 1582 cm-1 correlated to the anti/syn reorientation of the adenine residues on B-Z structure transition, iv) marker bands of the thymidine carbonyl group couplings at 1680- and 1733 cm-1 due to the disposition of the thymidine residues in the Z helix specific geometry. Hence poly(dA-dT).poly(dA-dT) can adopt a Z form in solution. The Z form observed in alternate purine-pyrimidine sequences does not require G-C base pairs.     

Influence of dA X dT and d(2aminoA) X dT base pairs on the B in equilibrium Z transition of DNA fragments. 1H-NMR study of d(C-G-C-A-m5C-G-T-G-m5C-G), d(m5C-G-C-A-m5C-G-T-G-C-G) and d(C-2aminoA-C-G-T-G)

The Helical structures of d(C-G-C-A-m5C-G-T-G-m5C-G), d(m5C-G-C-A-m5C-G-T-G-C-G) and d(C-2aminoA-C-G-T-G) were studied in aqueous solution at various salt concentrations and temperatures by 1H-NMR spectroscopy. In 0.1 M NaCl solution only the B form was evidenced for these DNA fragments whereas in 4 M NaCl both B and Z forms, in slow exchange on the NMR time scale, were observed. Under these conditions the Z form accounted for less than 60% of the decamer conformation; conversely d(C-G)3 hexamers containing methylated cytidines were predominantly in the Z form (greater than 90%) [Tran-Dinh et al. (1984) Biochemistry 23, 1362; Cavaillès et al. (1984) J. Biomol. Struct. Dyn. 1, 1347-1371]. On the other hand, d(C-2aminoA-C-G-T-G) in which the d(2aminoA) X dT base pair forms three hydrogen bonds, was found to adopt the Z conformation in 4M NaCl solution which was not the case for d(C-A-C-G-T-G) (unpublished results). The present study shows that the B in equilibrium Z transition in solution is highly sequence-dependent and that correlation exists between the stability of the duplexes (essentially governed by the number of hydrogen bonds between complementary bases) and their ability to adopt the Z conformation.     

The Raman spectra of left-handed DNA oligomers incorporating adenine-thymine base pairs+.

Raman spectra were obtained from single crystals of [d(CGCATGCG)]2 and [d(m5CGTAm5CG)]2, both of which incorporate A-T pairs into Z-DNA structures and contain C2'-endo/syn conformers of deoxyguanosine at the oligonucleotide ends. Correlation with x-ray results permits the following Raman assignments for nucleoside conformers: C3'-endo/syn G, 623 +/- 1; C2'-endo/syn G, 671 +/- 2; C2'-endo/anti C, 782 +/- 1; C2'endo/anti T, 650 +/- 5 and ca. 750; C3'-endo/syn A, 729 +/- 1 cm-1. These results show that (i) the 670 cm-1 line of syn G is highly sensitive to the change from C3'-endo to C2'-endo pucker, (ii) the 729 cm-1 line of A is affected neither by furanose pucker nor glycosidic bond orientation and (iii) the 1200-1500 cm-1 region of the Raman spectrum of the A-T double helix is greatly altered by the B-to-Z transition. Conformation sensitive Raman frequencies in the 850-1700 cm-1 region are identified for both octamer and hexamer, and the Z-to-B transition of each is monitored by spectral changes which occur upon dissolving the crystal in H2O solution.     

Salt-induced conformational transition of poly(d2NH2A-dT) studied by ultraviolet resonance Raman spectroscopy.

The conformational changes of poly(d2NH2A-dT) in aqueous solution, induced by increasing the NaCl concentration from 0.1M to 4M, have been monitored by ultraviolet resonance Raman spectroscopy, in using the 222-, 257- and 281 nm excitation wavelengths. These changes have been interpreted in comparing the polymer spectra to those of the mononucleotide compounds on one hand, and to those of other alternating purine-pyrimidine polymers on the other hand, i.e. poly(dG-dC) and poly(dA-dT) which showed a B to Z transition in going from low- to high salt concentrations. The high salt poly(d2NH2A-dT) spectra do not show any Raman marker line of the Z conformation. The spectroscopic results indicate that most of the ribose puckering goes from C2'-endo/anti to C3'-endo/anti in increasing the salt concentration. In addition the base stacking interactions, to which the resonance Raman effect is very sensitive, are not drastically changed upon salt variations. Thus the high salt structure of poly(d2NH2A-dT) remains a right-handed helix, likely under a dominant A conformation.