Covalent modification of guanine bases in double-stranded DNA. The 1.2-A Z-DNA structure of d(CGCGCG) in the presence of CuCl2

We have solved the single crystal structure to 1.2-A resolution of the Z-DNA sequence d(CGCGCG) soaked with copper(II) chloride. This structure allows us to elucidate the structural properties of copper in a model that mimics a physiologically relevant environment. A copper(II) cation was observed to form a covalent coordinate bond to N-7 of each guanine base along the hexamer duplex. The occurrence of copper bound at each site was dependent on the exposure of the bases and the packing of the hexamers in the crystal. The copper at the highest occupied site was observed to form a regular octahedral complex, with four water ligands in the equatorial plane and a fifth water along with N-7 of the purine base at the axial positions. All other copper complexes appear to be variations of this structure. By using the octahedral complex as the prototype for copper(II) binding to guanine bases in the Z-DNA crystal, model structures were built showing that duplex B-DNA can accommodate octahedral copper(II) complexes at the guanine bases as well as copper complexes bridged at adjacent guanine residues by a reactive dioxygen species. The increased susceptibility to oxidative DNA cleavage induced by copper(II) ions in solution of the bases located 5' to one or more adjacent guanine residues can thus be explained in terms of the cation and DNA structures described by these models.

Molecular structure of a DNA decamber containing an anticancer nucleoside arabinosylcytosine: conformational perturbation by arabinosylcytosine in B-DNA

Arabinosylcytosine (araC) is an important anticancer drug that has been shown to be misincorporated into DNA double helix. The incorporation of araC into DNA may have significant conformational consequences that could affect the function of DNA. In this paper, we present the high-resolution 3D structure of an araC-containing decamer d[CCAGGC(araC)TGG], as determined by X-ray diffraction analysis, and assess the possible DNA structural perturbation induced by araC. The modified decamer was crystallized in the monoclinic C2 (a = 31.97 A, b = 25.56 A, c = 34.62 A and beta = 114.50 degrees) space group, the same as that from d(CCAGGCCTGG) [Heinemann, U., & Alings, C. (1989) J. Mol. Biol. 210, 369]. The structure of the araC-containing decamer was solved by the molecular replacement method and refined by the constrained least-squares refinement procedure to obtain a final R factor of 0.187 using 2349 [greater than 2.0 sigma(F)] observed reflections to a resolution of 1.6 A. The overall conformation resembles that of the canonical decamer DNA structure, but with significant differences in regions close to the araC site. The O2' hydroxyl groups of the araC residues lie in the major groove of the helix, and they are in close contact with the C5 methyl and C6 H6 atoms of the thymine on the 3'-side. This creates a higher buckle in the araC7-G14 base pair (14 degrees), as compared to that found in the canonical decamer (9 degrees). This may slightly destabilize B-DNA. No direct intramolecular hydrogen bond is formed, in contrast to the situation when araC is incorporated into Z-DNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular structure of antitumor drug steffimycin and modelling of its binding to DNA

The molecular and crystal structure of steffimycin have been determined by single crystal X-ray diffraction to 0.9 angstrom resolution. The triclinic crystals are in the space group P1, with the unit cell dimensions of a = 8.606(3) angstrom, b = 22.168(7) angstrom, c = 8.448(2) angstrom, alpha = 97.56(3) degrees, beta = 95.97(2) degrees, gamma = 87.94(3) degrees, Z = 2. The structure was solved by direct methods and refined by the full-matrix least-squares method to a final R value of 0.065 with 3405 (Inet greater than 2.0 sigma (Inet] observed reflections using the NRCVAX software package. The crystal lattice includes 2 independent steffimycin, 3 water and one 2-methyl-2,4-pentanediol molecules. The conformation of steffimycin is grossly similar to other anthracycline antibiotics including daunorubicin. The crystal packing interactions of steffimycin suggest a preferred stacking of the aglycone chromophore of the antibiotic which resembles the intercalative interactions seen in the daunorubicin-d(CGTACG) (Wang et al., Biochemistry 26, 1152 (1987] and nogalamycin-d(CGT(pS)ACG) (Liaw et al., Biochemistry 28, 9913 (1989] complexes. The atomic coordinates data from these complexes were used to model the intercalative binding of steffimycin to DNA. The models were then stereochemically idealized by the constraint refinement program NUCLSQ. Subsequently XPLOR software package was used for energy minimization of these models in vacuo. The model building studies suggest that steffimycin has a higher CpG base sequence specificity over the TpA step, similar to that of daunorubicin and nogalamycin.

Influence of aglycone modifications on the binding of anthracycline drugs to DNA: the molecular structure of idarubicin and 4-O-demethyl-11-deoxydoxorubicin complexed to d(CGATCG)

X-ray diffraction analyses of the complexes between two anthracycline antitumor compounds, idarubicin (IDR) and 4-O-demethyl-11-deoxydoxorubicin (ddDOX), with the DNA hexamer d(CGATCG) provided the detailed three-dimensional molecular structures at 1.7 A and 1.8 A resolution, respectively. Their structures have been refined with the constrained refinement procedure to final R-factors of 0.188 (1724 reflections for IDR) and 0.179 (1247 reflections for ddDOX). The overall structures of both complexes are similar to those of the previously studied DAU- and DOX-DNA complexes. In both complexes, two IDR (and ddDOX) molecules bind to the DNA hexamer double helix with the elongated aglycone chromophore intercalated between the CpG steps at both ends of the helix. The aglycone chromophore spans the GC Watson-Crick base pairs with its amino sugar lying in the minor groove where little structural difference is seen, compared with the daunorubicin-d(CGATCG) and doxorubicin-d(CGATCG) complexes. In contrast, the missing C4 methoxy of IDR and the missing methyl group at the O4 position of ddDOX result in a different binding surface in the major groove. The O4 hydroxyl group is capable of receiving and/or donating a hydrogen bond to proteins that bind to the drug-DNA complex. The missing O11 hydroxyl group in ring B creates an empty space in the intercalation cavity between the two GC base pairs, which appears to affect the stacking interactions between the aglycone and the DNA base pairs. Those structural changes in the major groove of the drug-DNA complexes due to the modifications of the aglycone chromophore may be responsible in part for the difference in their biological activities.

Interaction between the left-handed Z-DNA and polyamine. The crystal structure of the d(CG)3 and N-(2-aminoethyl)-1,4-diamino-butane complex

The DNA fragment d(CG)3 was co-crystallized with N-(2-aminoethyl)-1,4-diaminobutane (PA(24], a chemically synthesized polyamine. The complex crystal contained one polyamine, 3 magnesium cations and one sodium cation per duplex of d(CG)3, and well diffracted the X-ray intensities up to 1.0 A resolution. The d(CG)3 took a left-handed Z-DNA conformation, and the PA(24) molecule electrostatically interacted with the phosphate groups of the d(CG)3 duplex.

Molecular dynamics investigation of the interaction between DNA and distamycin

The complex of the minor groove binding drug distamycin and the B-DNA oligomer d-(CGCAAATTTGCG) was investigated by molecular dynamics simulations. For this purpose, accurate atomic partial charges of distamycin were determined by extended quantum chemical calculations. The complex was simulated without water but with hydrated counterions. The oligomer without the drug was simulated in the same fashion and also with 1713 water molecules and sodium counterions. The simulations revealed that the binding of distamycin in the minor groove induces a stiffening of the DNA helix. The drug also prevents a transition from B-DNA to A-DNA that was found to occur rapidly (30 ps) in the segment without bound distamycin in a water-free environment but not in simulations including water. In other simulations, we investigated the relaxation processes after distamycin was moved from its preferred binding site, either radially or along the minor groove. Binding in the major groove was simulated as well and resulted in a bound configuration with the guanidinium end of distamycin close to two phosphate groups. We suggest that, in an aqueous environment, tight hydration shells covering the DNA backbone prevent such an arrangement and thus lead to distamycin's propensity for minor groove binding.

Facile formation of a crosslinked adduct between DNA and the daunorubicin derivative MAR70 mediated by formaldehyde: molecular structure of the MAR70-d(CGTnACG) covalent adduct

MAR70 is a synthetic derivative of the anticancer drug daunorubicin that contains an additional sugar, attached to the O4' of daunosamine. When MAR70 was crystallized with the DNA hexamer d(CGTnACG), where nA is 2-aminoadenine, a covalent methylene bridge was formed between the N3' of daunosamine and the N2 of 2-aminoadenine. This spontaneous reaction occurred through the crosslinking action of formaldehyde. The crosslink was demonstrated by the three-dimensional structure of the 2:1 adduct between MAR70 and d(CGTnACG) solved at 1.3-A resolution by x-ray diffraction analysis. The perfect juxtaposition of the two amino groups in the complex provides a template for efficient addition of formaldehyde. This adduct structure is compared with the analogous structure at 1.5-A resolution of the complex of MAR70-d(CGTACG), in which no formaldehyde addition was observed. In both complexes, two MAR70 molecules bind to the DNA hexamer double helix; the elongated aglycon chromophore is intercalated between the CpG steps and spans the G.C Watson-Crick base pairs. The disaccharides occupy nearly the entire minor groove of the distorted B-DNA hexamer double helix. The second sugar is in contact with the sugar-phosphate backbone and does not affect the binding interactions of the daunorubicin portion to DNA. The structure allows us to model the binding to DNA of drugs having more extensive oligosaccharides. In addition, it suggests that placing a reactive (e.g., alkylating) functional group at the N3' amino position of daunorubicin might be a fruitful route for designing anticancer drugs.

Formaldehyde cross-links daunorubicin and DNA efficiently: HPLC and X-ray diffraction studies

Formaldehyde (HCHO) cross-links the anticancer drug daunorubicin (DAU) to DNA efficiently. When DAU is mixed with DNA hexamers, d(CGCGCG) and d(CGTDCG), in the presence of HCHO, stable covalent adducts of DNA are formed, as shown by the HPLC analyses. The major adducts are identical with the materials in the respective crystals which can be readily obtained from the 1:1 mixture of DAU-d(CGCGCG) and DAU-d(CGTDCG) plus HCHO, but not from the solution without HCHO. The high-resolution (1.5 A) X-ray crystal structure of those adducts shows unambiguously that they contain a covalent methylene bridge between the N3' of daunosamine and the N2 of the guanine or 2-aminoadenine. The perfect juxtaposition of the two amino groups in the minor groove of the complex provides a template for an efficient addition of HCHO. The methylene bridge does not perturb the conformation of the drug-DNA complex, when compared to the structure of DAU-d(CGTACG). The results suggest new approaches for synthesizing a new type of potential anticancer drug by attaching a reactive (e.g., alkylating) functional group at the N3' amino position of daunorubicin/doxorubicin. The stable drug-DNA adduct may be useful as probes for other biological studies.

Comparison of two hydrated forms of sodium inosine 5'-monophosphate

The crystal and molecular structure of a decahydrated form of the sodium salt of inosine 5'-monophosphate (C10H12N4O8P-.Na+.10H2O) was solved to study the effect of hydration on the conformation of nucleic acids. Monoclinic, space group P2(1), a = 8.730 (3), b = 22.349 (7), c = 12.282 (4) A, beta = 109.68 (3) degrees, V = 2256.52 A3, Mr = 550.34, Z = 4, F(000) = 1196, Dx = 1.62 g cm-3, mu = 21.7 cm-1, lambda(Cu K alpha) = 1.54056 A, R = 0.070, wR = 0.102 for 3404 unique [Inet greater than 2 sigma (Inet)] observed reflections out of 3457 unique reflections. The two molecules (A and B) in the asymmetric unit differ in the arrangement of the first shell of hydration and in the torsion angles of the ribose and phosphate. The bond lengths and angles are similar to those of the structure of a less hydrated ('dry') form of the same nucleotide (C10H12N4O8P-.Na+.8H2O) determined previously in space group C222(1) [Rao & Sundaraligam (1969). J. Am. Chem. Soc. 91, 1210-1217]. The twofold symmetry in the 'dry' form is destroyed in the present crystal structure due to the relative displacement of the two independent molecules and reorganization of the hydration shell. Molecule A is different from (r.m.s. = 0.190 A) while molecule B is similar to (r.m.s. = 0.093 A) that of the 'dry' form. The conformation adopted is influenced mainly by the differences in the endocyclic torsion angles of the ribose.

Binding of the antitumor drug nogalamycin and its derivatives to DNA: structural comparison

The three-dimensional molecular structures of the complexes between a novel antitumor drug nogalamycin and its derivative U-58872 with a modified DNA hexamer d[m5CGT(pS)Am5CG] have been determined at 1.7- and 1.8-A resolution, respectively, by X-ray diffraction analyses. Both structures (in space group P6(1)) have been refined with constrained refinement procedure to final R factors of 0.208 (3386 reflections) and 0.196 (2143 reflections). In both complexes, two nogalamycins bind to the DNA hexamer double helix in a 2:1 ratio with the elongated aglycon chromophore intercalated between the CpG steps at both ends of the helix. The aglycon chromophore spans across the GC Watson-Crick base pairs with its nogalose lying in the minor groove and the aminoglucose lying in the major groove of the distorted B-DNA double helix. Most of the sugars remain in the C2'-endo pucker family, except three deoxycytidine residues (terminal C1, C7, and internal C5). All nucleotides are in the anti conformation. Specific hydrogen bonds are found in the complex between the drug and guanine-cytosine bases in both grooves of the helix. One hydroxyl group of the aminoglucose donates a hydrogen bond to the N7 of guanine, while the other receives a hydrogen bond from the N4 amino group of cytosine. The orientation of these two hydrogen bonds suggests that nogalamycin prefers a GC base pair with its aglycon chromophore intercalating at the 5'-side of a guanine (between NpG), or at the 3'-side of a cytosine (between CpN) with the sugars pointing toward the GC base pair. The binding of nogalamycin to DNA requires that the base pairs in DNA open up transiently to allow the bulky sugars to go through, suggesting that nogalamycin prefers GC sequences embedded in a stretch of AT sequences.

Evidence for a new Z-type left-handed DNA helix: properties of Z(WC)-DNA

The structure of Z-DNA, currently accepted as a model for all left-handed DNAs, fails to provide convincing explanations for at least four well established properties of left-handed DNA polymers in solution. However, the major discrepancies between theory and experiment are resolved by the structure presently proposed for Z[WC]-DNA, a new left-handed, zig-zag double helix with Watson-Crick-type backbone directions. Structural features of Z[WC]-DNA include the presence of an additional H-bond between each guanine N2-amino group and an adjacent phosphate oxygen, the capacity to form four-stranded, base-matched complexes that should readily precipitate from solution, and backbone progressions that are the same as B-DNA (opposite to Z-DNA). However, since Z[WC]-DNA and Z-DNA have many parameters in common, they could be difficult to distinguish in a majority of existing experiments. In view of the close relationship of the new helix to B-DNA, which allows a relatively unhindered right-to-left transition in handedness, Z[WC]-DNA is theorized to be the left-handed structure preferentially generated in vivo by the torque available in naturally occurring DNA supercoils.

Molecular structure of the complex formed between the anticancer drug cisplatin and d(pGpG): C222(1) crystal form

The three dimensional molecular structure of the adduct formed between the anticancer drug cisplatin and a DNA dinucleotide d(pGpG) has been determined by x-ray diffraction analysis at 1.37 A resolution and refined to a final R-factor of 0.11. This structure, solved by using data from a previously reported crystal form in the space group C222(1), resembles that found in the space group P2(1)2(1)2 (Sherman, et al., Science, 230, 412-417, 1985; ibid, J. Amer. Chem. Soc. 110, 7368-7381, 1988). In both structures, four crystallographically independent cis-[Pt(NH3)2(d(pGpG]] molecules aggregate into a tetrameric cluster that is stabilized by a large number of intermolecular hydrogen bonds and base-base stacking interactions. In each molecule, the platinum atom is coordinated to the N7 atoms of two guanine bases arranged in a head-to-head orientation, resulting in a large dihedral angle between the guanines. Intermolecular guanine-guanine base pairings between different intrastrand crosslinked molecules are used extensively in the crystal lattice.

Structure of 11-deoxydaunomycin bound to DNA containing a phosphorothioate

The anthracyclines form an important family of cancer chemotherapeutic agents with a strong dependence of clinical properties on minor differences in chemical structure. We describe the X-ray crystallographic solution of the three-dimensional structure of the anthracycline 11-deoxydaunomycin plus d(CGTsACG). In this complex, two drug molecules bind to each hexamer duplex. Both the drug and the DNA are covalently modified in this complex in contrast with the three previously reported DNA-anthracycline complexes. In the 11-deoxydaunomycin complex the 11 hydroxyl group is absent and a phosphate oxygen at the TpA step has been replaced by a sulfur atom leading to a phosphorothioate with absolute stereochemistry R. Surprisingly, removal of a hydroxyl group from the 11 position does not alter the relative orientation of the intercalated chromophore. However, it appears that the phosphorothioate modification influenced the crystallization and caused the 11-deoxydaunomycin-d(CGTsACG) complex to crystallize into a different lattice (space group P2) with different lattice contacts and packing forces than the non-phosphorothioated DNA-anthracycline complexes (space group P4(1)2(1)2). In the minor groove of the DNA, the unexpected position of the amino-sugar of 11-deoxydaunomycin supports the hypothesis that in solution the position of the amino sugar is dynamic.

NMR studies on the binding of antitumor drug nogalamycin to DNA hexamer d(CGTACG)

The interactions between a novel antitumor drug nogalamycin with the self-complementary DNA hexamer d(CGTACG) have been studied by 500 MHz two dimensional proton nuclear magnetic resonance spectroscopy. When two nogalamycins are mixed with the DNA hexamer duplex in a 2:1 ratio, a symmetrical complex is formed. All non-exchangeable proton resonances (except H5' & H5") of this complex have been assigned using 2D-COSY and 2D-NOESY methods at pH 7.0. The observed NOE cross peaks are fully consistent with the 1.3 A resolution x-ray crystal structure (Liaw et al., Biochemistry 28, 9913-9918, 1989) in which the elongated aglycone chromophore is intercalated between the CpG steps at both ends of the helix. The aglycone chromophore spans across the GC Watson-Crick base pairs with its nogalose lying in the minor groove and the aminoglucose lying in the major groove of the distorted B-DNA double helix. The binding conformation suggests that specific hydrogen bonds exist in the complex between the drug and guanine-cytosine bases in both grooves of the helix. When only one drug per DNA duplex is present in solution, there are three molecular species (free DNA, 1:1 complex and 2:1 complex) in slow exchange on the NMR time scale. This equilibrium is temperature dependent. At high temperature the free DNA hexamer duplex and the 1:1 complex are completely destabilized such that at 65 degrees C only free single-stranded DNA and the 2:1 complex co-exist. At 35 degrees C the equilibrium between free DNA and the 1:1 complex is relatively fast, while that between the 1:1 complex and the 2:1 complex is slow. This may be rationalized by the fact that the binding of nogalamycin to DNA requires that the base pairs in DNA open up transiently to allow the bulky sugars to go through. A separate study of the 2:1 complex at low pH showed that the terminal GC base pair is destabilized.

Cyclic diguanylic acid behaves as a host molecule for planar intercalators

Cyclic ribodiguanylic acid, c-(GpGp), is the endogenous effector regulator of cellulose synthase. Its three-dimensional structure from two different crystal forms (tetragonal and trigonal) has been determined by X-ray diffraction analysis at 1 A resolution. In both crystal forms, two independent c-(GpGp) molecules associate with each other to form a self-intercalated dimer. A hydrated cobalt ion is found to coordinate to two N7 atoms of adjacent guanines, forcing these two guanines to destack with a large dihedral angle (32 degrees), in the dimer of the tetragonal form. This metal coordination mechanism may be relevant to that of the anticancer drug cisplatin. Moreover, c-(GpGp) exhibits unusual spectral properties not seen in any other cyclic dinucleotide. It interacts with planar organic intercalator molecules in ways similar to double helical DNA. We propose a cage-like model consisting of a tetrameric c-(GpGp) aggregate in which a large cavity ('host') is generated to afford a binding site for certain planar intercalators ('guests').

Molecular structure of nicked DNA: a substrate for DNA repair enzymes

The molecular structure of a nicked dodecamer DNA double helix, made of a ternary system containing d(CGCGAAAACGCG) + d(CGCGTT) + d(TTCGCG) oligonucleotides, has been determined by x-ray diffraction analysis at 3 A resolution. The molecule adopts a B-DNA conformation, not unlike those found in intact dodecamer DNA molecules crystallized in a somewhat different crystal lattice, despite a gap due to the absence of a phosphate group in the molecule. The helix has a distinct narrow minor groove near the center of the molecule at the AAAA region. This suggests that the internal stabilizing forces due to base stacking and hydrogen-bonding interactions are sufficient to overcome the loss of connectivity associated with the disruption of the covalent backbone of DNA.

Structural comparison of anticancer drug-DNA complexes: adriamycin and daunomycin

The anticancer drugs adriamycin and daunomycin have each been crystallized with the DNA sequence d(CGATCG) and the three-dimensional structures of the complexes solved at 1.7- and 1.5-A resolution, respectively. These antitumor drugs have significantly different clinical properties, yet they differ chemically by only the additional hydroxyl at C14 of adriamycin. In these complexes the chromophore is intercalated at the CpG steps at either end of the DNA helix with the amino sugar extended into the minor groove. Solution of the structure of daunomycin bound to d(CGATCG) has made it possible to compare it with the previously reported structure of daunomycin bound to d(CGTACG). Although the two daunomycin complexes are similar, there is an interesting sequence dependence of the binding of the amino sugar to the A-T base pair outside the intercalation site. The complex of daunomycin with d(CGATCG) has tighter binding than the complex with d(CGTACG), leading us to infer a sequence preference in the binding of this anthracycline drug. The structures of daunomycin and adriamycin with d(CGATCG) are very similar. However, there are additional solvent interactions with the adriamycin C14 hydroxyl linking it to the DNA. Surprisingly, under the influence of the altered solvation, there is considerable difference in the conformation of spermine in these two complexes. The observed changes in the overall structures of the ternary complexes amplify the small chemical differences between these two antibiotics and provide a possible explanation for the significantly different clinical activities of these important drugs.

Antitumor drug nogalamycin binds DNA in both grooves simultaneously: molecular structure of nogalamycin-DNA complex

The three-dimensional molecular structures of the complexes between an interesting antitumor drug, nogalamycin, and two DNA hexamers, d[CGT(pS)ACG] and d[m5CGT(pS)Am5CG], were determined at high resolution by X-ray diffraction analyses. Two nogalamycins bind to the DNA double helix in a 2:1 ratio with the aglycon chromophore intercalated between the CpG steps at both ends of the helix. The nogalose and aminoglucose sugars lie in the minor and major grooves, respectively, of the distorted B-DNA double helix. The binding of nogalamycin to DNA requires that the base pairs in DNA open up transiently to allow the bulky sugars to go through. Specific hydrogen bonds are found in the complex between the drug and guanine bases. We suggest that nogalamycin may prefer GC sequences embedded in a stretch of AT sequences.

Binding of a Hoechst dye to d(CGCGATATCGCG) and its influence on the conformation of the DNA fragment

Hoechst dye 33258 is a planar drug molecule that binds to the minor groove of DNA, especially where there are a number of A.T base pairs. We have solved the structure of the Hoechst dye bound to the DNA dodecamer d(CGCGATATCGCG) at 2.3 A. This structure is compared to that of the same dodecamer with the minor-groove-binding drug netropsin bound to it, as well as to structures that have been solved for this Hoechst dye bound to a DNA dodecamer containing the central four base pairs with the sequence AATT. We find that the position of the Hoechst drug in this dodecamer is quite different from that found in the other dodecamer since it has an opposite orientation compared to the other two structures. The drug covers three of the four A.T base pairs and extends its piperazine ring to the first G.C base pair adjacent to the alternating AT segment. Furthermore, the drug binding has modified the structure of the DNA dodecamer. Other DNA dodecamers with alternating AT sequences show an alternation in the size of the helical twist between the ApT step (small twist) and the TpA step (large twist). In this structure the alternation is reversed with larger twists in the ApT steps than in the TpA step. In addition, there is a rotation of one of the thymine bases in the DNA dodecamer that is associated with hydrogen bonding to the Hoechst drug. This structure illustrates the considerable plasticity found in the DNA molecule when it binds to different planar molecules inserted into the minor groove.

Interactions between a symmetrical minor groove binding compound and DNA oligonucleotides: 1H and 19F NMR studies

High-resolution NMR techniques (proton and 19F) have been used to study the interactions between several DNA oligonucleotides with varying length of AT base pairs and the synthetic pyrrole-containing compound (P1-F4S-P1), which has properties similar to the DNA minor groove binding drug distamycin A. When this two-fold symmetrical DNA binding molecule is added to the self-complementary DNA oligomers, the resulting complex exhibits an NMR spectrum without any doubling of individual resonances, consistent with a two-fold symmetry of the complex. This is in contrast to all other complexes studied so far. The minimum length of an AT stretch for specific ligand binding is judged to be greater than 4 base pairs. Inter-molecular proton nuclear Overhauser effects between the ligand molecule and a DNA dodecamer d(CGCAAATTTGCG) provide evidence that P1-F4S-P1 binds DNA in the minor groove and interacts with the middle AT base pairs. The presence of a specific interaction between P1-F4S-P1 and DNA is conclusively demonstrated by 19F NMR studies, in which four previously chemically equivalent fluorine nuclei in the free molecule become two non-equivalent pairs (yielding an AB quartet pattern) upon the binding of P1-F4S-P1 to DNA duplex. A sequence-dependent binding behavior of P1-F4S-P1 is evident by comparing the 19F NMR spectra of the complexes between P1-F4S-P1 and two different but related DNA dodecamers, d(CGCAAATTTGCG) and d(CGCTTTAAAGCG). P1-F4S-P1 binds more strongly to the former dodecamer with an association constant of approximately 1 X 10(3) M-1.

Effects of the O2' hydroxyl group on Z-DNA conformation: structure of Z-RNA and (araC)-[Z-DNA]

The left-handed Z structures of two hexamers [d(CG)r(CG)d(CG) and d(CG)(araC)d(GCG)] containing ribose and arabinose residues have been solved by X-ray diffraction analysis at 1.5-A resolution. Their conformations closely resemble that of the canonical Z-DNA. The O2' hydroxyl groups of both rC and araC residues form intramolecular hydrogen bonds with N2 of the 5' guanine residue and replace the bridging water molecules in the deep groove of Z-DNA, which stabilize the guanine in the syn conformation. The araC residue can be incorporated into the Z structure readily and facilitates B to Z transition, as supported by UV absorption spectroscopic studies. In contrast, in Z-RNA the ribose of the cytidine residue is twisted in order to form the respective hydrogen bond. The potential biological roles of the modified Z-DNA containing anticancer nucleoside araC and of Z-RNA are discussed.

Polyamine interaction with Z-DNA

In order to elucidate the detailed Z-DNA interaction with polyamines and also to clarify the mutual molecular recognition between the left-handed helix and the biologically important polyamine molecule, several polyamine-Z-DNA hexamer complexes were crystallized and their crystal structures were determined by X-ray diffraction. The general interaction modes found in these crystal structures were discussed in comparison with those in the complexes between polyamine and the right-handed DNA or RNA.

The molecular structure of the left-handed Z-DNA double helix at 1.0-A atomic resolution. Geometry, conformation, and ionic interactions of d(CGCGCG)

The structure of d(CGCGCG) crystallized in the presence of magnesium and sodium ions alone is compared to that of the spermine form of the molecule. The very high resolution nature of these structure determinations allows the first true examination of an oligonucleotide structure in fine detail. The values of bond distances and angles are compared to those derived from small molecule crystal structures. In addition, the interactions of cations and polyamines with the Z-DNA helix are analyzed. In particular, multiple cationic charges appear to offer enhanced stabilization for the Z-DNA conformation. The location of spermine molecules along the edge of the deep groove and also spanning the entrance to the groove emphasizes the importance of polyamines for stabilizing this left-handed structure. On averaging, we obtained very similar structural parameters for the two different structures with standard deviations generally smaller than the deviations of the crystallographic model from ideal values. This indicates a high degree of accuracy of the two structures, which have been refined using different data and different refinement methods. The derived bond lengths and angles may thus be more representative of this polymeric DNA structure than those derived from mono- and dinucleotide structures at a similar accuracy.

Molecular structure of an A-DNA decamer d(ACCGGCCGGT)

The molecular structure of the DNA decamer d(ACCGGCCGGT) has been solved and refined by single-crystal X-ray-diffraction analysis at 0.20 nm to a final R-factor of 18.0%. The decamer crystallizes as an A-DNA double helical fragment with unit-cell dimensions of a = b = 3.923 nm and c = 7.80 nm in the space group P6(1)22. The overall conformation of this A-DNA decamer is very similar to that of the fiber model for A-DNA which has a large average base-pair tilt and hence a wide and shallow minor groove. This structure is in contrast to that of several A-DNA octamers in which the molecules all have low base-pair-tilt angles (8-12 degrees) resulting in an appearance intermediate between B-DNA and A-DNA. The average helical parameters of this decamer are typical of A-DNA with 10.9 base pairs/turn of helix, an average helical twist angle of 33.1 degrees, and a base-pair-tilt angle of 18.2 degrees. However, the CpG step in this molecule has a low local-twist angle of 24.5 degrees, similar to that seen in other A-DNA oligomers, and therefore appears to be an intrinsic stacking pattern for this step. The molecules pack in the crystal using a recurring binding motif, namely, the terminal base pair of one helix abuts the surface of the shallow minor groove of another helix. In addition, the GC base pairs have large propeller-twist angles, unlike those found most other A-DNA structures.

The propeller DNA conformation of poly(dA).poly(dT)

Physical properties of the DNA duplex, poly(dA).poly(dT) differ considerably from the alternating copolymer poly(dAT). A number of molecular models have been used to describe these structures obtained from fiber X-ray diffraction data. The recent solutions of single crystal DNA dodecamer structures with segments of oligo-A.oligo-T have revealed the presence of a high propeller twist in the AT regions which is stabilized by the formation of bifurcated (three-center) hydrogen bonds on the floor of the major groove, involving the N6 amino group of adenine hydrogen bonding to two O4 atoms of adjacent thymine residues on the opposite strand. Here we show that it is possible to incorporate the features of the single crystal analysis, specifically high propeller twist, bifurcated hydrogen bonds, and a narrow minor groove, as well as the close interstrand NMR signal between adenine HC2 and ribose HC1' of the opposite strand, into a model that is fully compatible with the diffraction data obtained from poly(dA).poly(dT).