Trifurcated (Four-Center) Hydrogen Bond in Solid State Crystal Structure of 5′-Amino-5′-deoxyadenosine p-Toluenesulfonate

Abstract

The crystal structure of 5′-amino-5′-deoxyadenosine (5′-Am.dA) p-toluenesulfonate has been determined by X-ray crystallographic methods. It belongs to the orthorhombic space group P2₁2₁2₁ with a=7.754(3)Å, b=8.065(l)Å and c=32.481(2)Å. This nucleoside shows a syn conformation about the glycosyl bond and C2′-endo-C3′-exo puckering for the ribose sugar. The orientation of N5′ atom is gauche-trans about the exocyclic C4′-C5′ bond. The amino nitrogen N5′ forms a trifurcated hydrogen bond with N3, O9T and 04′ atoms. Adenine bases form A.A.A triplets through hydrogen bonding between N6, N7 and N1 atoms of symmetry related nucleoside molecules.     

Disordered Single Crystal Evidence for a Quadruple Helix Formed by Guanosine 5′-Monophosphate

Abstract

The disodium salt of guanosine 5′-monophosphate (5′-GMP) has been crystallized earlier in an orthorhombic array. We have obtained a new crystal form of 5′-GMP at pH 8 which reveals a clear helical nature, with guanine bases stacked perpendicular to the helix axis. Although the X-ray pictures show partial disorder, they can be indexed on a hexagonal net with a = b = 28.6 Å,c = 9.8 Å, V= 6942ų(1Å = 0.1 nm). The probable space group is P6₄, and past experience with ca. 600 ų per base in oligonucleotide crystals suggests that the cell contains 12 GMP molecules. The crystal packing parameters and the intensity distribution agree with a model of three hydrogen-bonded guanine tetrads in the unit cell, stacked so as to build a quadruple helix similar to that proposed earlier from fiber studies (Zimmerman, S.B., J. Mol. Biol. 106, 663–672 (1976)).     

Stereochemistry of 2′-5′ Linked Nucleic Acids: Crystal and Molecular Structure of Ammonium Adenylyl-2′, 5′-Adenosine Tetrahydrate: A Core Fragment of 2′-5′ Oligo A′s Produced by Interferon Induced Adenylate Synthetased

Abstract

The preponderance of 3′-5′ phosphodiester links in nucleic acids is well known. Albeit less prevalent, the 2′-5′ links are specifically utilised in the formation of ‘lariat’ in group II introns and in the msDNA-RNA junction in myxobacterium. As a sequel to our earlier study on cytidylyl-2′,5′-adenosine we have now obtained the crystal structure of adenylyl-2′,5′-adenosine (A2′p5′A) at atomic resolution. This dinucleoside monophosphate crystallises in the orthorhombic space group P2₁2₁2₁ with a= 7.956(3)Å, b = 12.212(3)Å and c = 36.654 (3) Å. CuKα intensity data were collected on a diffractometer. The structure was sloved by direct methods and refined by full matrix least squares methods to R = 10.8 %. The 2′ terminal adenine is in the commonly observed anti (χ₂ =?161°) conformation and the 5′ terminal base has a syn (χ₁ = 55°) conformation more often seen in purine nucleotides. A noteworthy feature of A2′p5′ A is the intranucleotide hydrogen bond between N3 and 05′ atoms of the 5′ adenine base. The two furanose rings in A2′ p5′ A show different conformations-C2′ endo, C3′ endo puckering for the 5′ and 2′ ends respectively. In this structure too there is a stacking of the purine base on the ribose 04′ just as in other 2′-5′ dinucleoside structures, a feature characteristically seen in the left handed ZDNA. In having syn, anti conformation about the glycosyl bonds, C2′ endo, C3′ endo mixed sugar puckering and N3–05′ intramolecular hydrogen bond A2′p5′ A resembles its 3′-5′ analogue and several other 2′-5′ dinucleoside monophosphate structures solved so far. Striking similarities between the 2′-5′ dinucleoside monophosphate structures suggest that the conformation of the 5′-end nucleoside dictates the conformation of the 2′ end nucleoside. Also, the 2′-5′ dimers do not favour formation of miniature classical double helical structures like the 3′-5′ dimers. It is conceivable, 2–5(A) could be using the stereochemical features of A2′p5′ A which accounts for its higher activity.     

Crystal Structure and Molecular Conformation of 4-Thiouracil-2′-Trifluorothioacetamide -3′, 5′-Diacetyl-β-D-Riboside

Abstract

4-thiouracil-2′-trifluorothioacetamide-3′, 5′-diacetyl-β-D-riboside is one of the modified thiouracil analogs synthesized in our institute. The determination of the crystal and molecular structure of this compound was carried out with a view to study the conformation of the molecule in the solid state as well as to investigate the conformations of the trifluoroacetamide and the acetyl substituents of the ribose and their effects on the conformation of the ribose ring. Crystals of 4-thiouracil-2′-trifluorothioacetamide-3′,5′- diacetyl-β-D-riboside are orthorhombic, space group P2₁ 2₁ 2₁, with cell dimensions a= 15.351 (2), b= 15.535 (1), c= 8.307 (1) Å, V=1981.0 (7) ų, Z=4, D_m= 1.53, D_c=1.527 g/c.c. and μ=30.1cm -¹. The structure was determined using CuKα (λ, =1.5418 Å) at a temperature T of 297K, with 2333 reflections, which were collected on a Enraf-Nonius CAD-4 diffactometer, out of which 2249 (I ≥20) were considered observed. The structure was determined by direct methods using MULTAN and refined by full matrix least squares method to a final reliability factor of 0.054 and a weighted R factor of 0.079. The nucleoside is in the anti conformation [X_CN =51.4 (5)°], the ribose has the unusual C (2′) endo -C (1′) exo (²T₁), and a g+ conformation [ψ=47.5 (4)] across C(4′)-C(5′) bond. The pseudorotation angle P is 152.8 (4) ° and the amplitude of pucker τ_m of 42.7 (3)°. The average C-F bond distance is 1.308 Å. There is no base pairing and the typical base-base hydrogen bonded interactions are not present in this structure. On the other hand, a hydrogen bonded dimer is formed involving C(3′) - H(3′)… O (2) and N(3) -H (N3) … O (Al) hydrogen bonds joining the base, ribose ring and the acetyl group. The trend towards longer exocyclic bonds at the acetyl centers in compounds with strongly electronegative aglycones, is also exhibited in this compound, with C(3′)-O(3′) and C(5′)-0(5′) being much longer than C(1′)-O(4′). The acetyl groups also take part in C-H…O hydrogen bonding with the acetyl oxygen atom OA2.     

Sodium binding to d-glucuronate residues: Crystal structure of sodium d-glucuronate monohydrate

Three-dimensional X-ray diffraction data were used to determine the crystal structure of sodium β-d-glucuronate monohydrate, a model system for investigating the factors involved in the binding of sodium ions to d-glucuronate residues of glycosaminoglycans. Crystals of the salt are monoclinic, space group P2₁, with a = 9.206(3) Å, b = 7.007(2) Å, c = 7.378(3) Å, β = 96.84(3)°, and Z = 2. Intensity data for 858 reflections were measured with an automated diffractometer. A trial structure, obtained by direct methods, was refined by least squares to R = 0.035. An outstanding feature of the crystal packing is the interaction of d-glucuronate anions with sodium ions. The sodium ion is coordinated to three symmetry-related $\text{d}$-glucuronate anions and to one water molecule. The d-glucuronate anion binds sodium cations through the three following sites: one that involves a carboxyl oxygen atom combined with ring oxygen O-5; one that includes a single carboxyl oxygen atom, and one composed of the O-3–O-4 pair of hydroxyl groups.     

Conformational Preferences of Heterochiral Peptides. Crystal Structures of Heterochiral Peptides Boc-(D) Val-(D) Ala-Leu-Ala-OMe and Boc-Val-Ala-Leu-(D) Ala-OMe-Enhanced Stability of β-sheet Through C-H…O Hydrogen Bonds

Abstract

The crystal structures of Boc-(D) Val-(D) Ala-Leu-Ala-OMe (vaLA) and Boc-Val-Ala-Leu-(D) Ala-OMe (VALa) have been determined. vaLA crystallises in space group P2₁2₁2₁ with a = 9.401 (4), b = 17.253 (5), c = 36.276 (9)Å, V = 5884 (3) ų, Z = 8, R = 0.086. VALa crystallises in space group P2₁ with a = 9.683 (9), b = 17.355 (7), c = 18.187 (9) Å, β = 95.84 (8)°, V = 3040(4) ų, Z = 4, R = 0.125. There are two molecules in the asymmetric unit in antiparallel β-sheet arrangement in both the structures. Several of the C^α hydrogens are in hydrogen bonding contact with the carbonyl oxygen in the adjacent strand.

An analysis of the observed conformational feature of D-chiral amino acid residues in oligopeptides, using coordinates of 123 crystal structures selected from the 1998 release of CSD has been carried out. This shows that all the residues except D-isoleucine prefer both extended and α_L conformation though the frequence of occurence may not be equal. In addition to this, D-leucine, valine, proline and phenylalanine have assumed α_R conformations in solid state. D-leucine has a strong preference for helical conformation in linear peptides whereas they prefer an extended conformation in cyclic peptides.     

Metal-phenoxyalkanoic acid interactions. Part 15. The crystal structures of diaquabis(phenoxyacetato)cadmium(II) and catena-μ-[aquabis(phenoxyacetato-O)lead(II)-bis(phenoxyacetato)lead(II)]

The crystal structures of the cadmium(II) and lead(II) complexes of phenoxyacetic acid (PAH) have been determined by single crystal X-ray diffraction techniques. The cadmium complex, [Cd(PA)₂(H₂O)₂] (1), space group C2, with Z = 2 in a cell of dimensions, a = 11.801(2), b = 5.484(1), c = 13.431(3) Å, β = 100.87(2)°, possesses a distorted trapezoidal bipyramidal coordination around the metal atom, involving two water oxygens [2.210(5) Å] and four carboxyl oxygens from two symmetrical bidentate phenoxyacetate ligands [2.363(4), 2.365(4) Å] with Cd lying on the crystallographic two- fold axis. The lead complex, [Pb₂(PA)₄(H₂O)]_n(2) is triclinic, space group P$\text{1}$, Z = 2, with a cell of dimensions, a = 10.135(4), b = 10.675(3), c = 19.285(9) Å, α = 114.66(3), β = 91.94(3) and γ = 114.99(3)°. (2) is a two-dimensional polymer with a repeating dimer sub-unit. The first lead [Pb(1)] has an irregular MO₈ coordination [2.34?2.96(2) Å: mean, 2.63(2) Å] involving the water molecule, two oxygens from an asymmetric bidentate carboxylate group, two from a bidentate chelate [O(ether), O(carboxylate)] group and three from bridging oxygens, one of which also provides a polymer link to another symmetry generated lead. The second lead [Pb(2)] is irregular seven-coordinate [PbO, 2.48?2.73(2) Å: mean, 2.61(2) Å] with three bonds from the bridging groups, two from an unsymmetrical bidentate carboxylate (O, O′) group and one from a second carboxyl group which also bridges two Pb(2) centres in the polymer.     

Visualization of Drug-Nucleic Acid Interactions at Atomic Resolution. X. Structure of a N,N-Dimethylproflavine: Deoxycytidylyl(3′-S′)Deoxyguanosine Crystalline Complex

Abstract

N,N-dimethylproflavine forms a crystalline complex with deoxycytidylyl(3′-5′)deoxyguanosine (d-CpG), space group P2₁,2₁2, with a = 21.37 Å, b = 34.05 Å, c = 13.63 Å. The structure has been solved to atomic resolution and refined by Fourier and least squares methods to a residual of 0.18 on 2,032 observed reflections. The structure consists of two N,N- dimethylproflavine molecules, two deoxycytidylyl (3′-5′)deoxyguanosine molecules and 16 water molecules, a total of 128 nonhydrogen atoms. As with other structures of this type, N,N-dimethylproflavine molecules intercalate between base-paired d-CpG dimers. In addition, dimethylproflavine molecules stack on either side of the intercalated duplex, being related by a unit cell translation along the c axis.

Both sugar-phosphate chains demonstrate the mixed sugar puckering geometry: C3′ endo (3′-5′) C2′ endo. This same intercalative geometry has been seen in two other complexes containing N,N-dimethylproflavine and iodoCpG, described in the accompanying paper. Taken together, these studies indicate a common intercalative geometry present in both RNA- and DNA- model systems. Again, N,N-dimethylproflavine behaves as a simple intercalator, intercalating asymmetrically between guanine-cytosine base-pairs. The free amino- group on the intercalated dimethylproflavine molecule does not hydrogen bond directly to the phosphate oxygen. Other aspects of the structure will be presented.     

Purification, crystallization and preliminary crystallographic investigations of selenium-containing phycocyanin from selenium-rich algae (Spirulina platensis)

The selenium-containing phycocyanin from the selenium-rich algae (Spirulina platensis) has been crystallized in two crystal forms by the hanging-drop vapor diffusion techniques. A chromatographic procedure of gel filtration and anion exchange was used for purification. Form I crystal with space group P2₁ and cell parameters a =108.0 Å , b = 117.0 Å , c = 184.0 Å , β = 90.2° and 12(αβ ) units in the asymmetric unit was obtained by using (NH₄)₂SO₄ as precipitant. These crystals diffract up to 2.8 Å . Form II crystal obtained by using PEG4000 as precipitant belongs to space group P63 with unit cell constants a = 155.0 Å , c = 40.3 Å , γ =120.0° and one(αβ ) unit in the asymmetric unit. The crystals diffract beyond 2.9 Å . The possible stacking forms of phycocyanin molecules in the first crystal form were discussed.     

Molecular structure of the cis-syn photodimer of d(TpT) (cyanoethyl ester)

The three-dimensional structure was determined by x-ray crystallography for d(T[p](CE)T), a uv photoproduct of the cyanoethyl (CE) derivative of d(TpT), having the cis-syn cyclobutane (CB) geometry and the S-configuration at the chiral phosphorus atom. The crystals of C₂₃H₃₀N₅O₁₂P · 2H₂O belong to the orthorhombic space group P2₁2₁2₁ (Z = 4), with cell dimensions a = 11.596 Å, b = 14.834 Å, and c = 15.946 Å, containing two water molecules per asymmetric unit. The CB ring is puckered with a dihedral angle of 151°. The two pyrimidine bases are rotated by –29° from the position of direct overlap of their corresponding atoms. This represents a major distortion of DNA, since in DNA adjacent thymines are rotated by +36°. The pyrimidine rings are puckered with Cremer–Pople parameters for T[p] and in parentheses [p]T: Q: 0.24 Å (0.31 Å); θ: 123° (120°); ?: 141° (86°). These represent half-chairs designated as ⁶H₁ (T[p]) and ₆H⁵ ([p]T). The CB and pyrimidine ring conformations are interrelated, and we postulate that they execute a coupled interconversion in solution. The T[p] segment has the syn glycosyl conformation, a ²T₃ sugar pucker, and gauche^? conformation at C4′-C5′; the [p]T segment is anti, ³T₄, trans. The C5′-O5′ torsion of the [p]T unit is –124.5°, and the C3′-O3′ torsion of the T[p] unit is –152.9°. Bond angles and bond lengths involving the phosphorus atom are similar to those of other phosphotriesters. The P-O3′ and P-05′ torsion angles are –138.1° and 58.6°, respectively. Several intermolecular (but no intramolecular) hydrogen bonds are found in the crystal.     

Crystal structures of two cyclic pseudopentapeptides containing ψ[CH2S] and ψ[CH2SO] backbone surrogates

The solid state conformations of cyclo[Gly–Proψ[CH₂S]Gly–D –Phe–Pro] and cyclo[Gly–Proψ[CH₂–(S)–SO]Gly–D –Phe–Pro] have been characterized by X-ray diffraction analysis. Crystals of the sulfide trihydrate are orthorhombic, P2₁2₁2₁, with a = 10.156(3) Å, b = 11.704(3) Å, c = 21.913(4) Å, and Z = 4. Crystals of the sulfoxide are monoclinic, P2₁, with a = 10.662(1) Å, b = 8.552(3) Å, c = 12.947(2) Å, β = 94.28(2), and Z = 2. Unlike their all-amide parent, which adopts an all-trans backbone conformation and a type II β-turn encompassing Gly-Pro-Gly-D -Phe, both of these peptides contain a cis Gly¹-Pro² bond and form a novel turn structure, i.e., a type II′ β-turn consisting of Gly–D –Phe–Pro–Gly. The turn structure in each of these peptides is stabilized by an intramolecular H bond between the carbonyl oxygen of Gly¹ and the amide proton of D -Phe⁴. In the cyclic sulfoxide, the sulfinyl group is not involved in H bonding despite its strong potential as a hydrogen-bond acceptor. The crystal structure made it possible to establish the absolute configuration of the sulfinyl group in this peptide. The two crystal structures also helped identify a type II′ β-turn in the DMSO-d₆ solution conformers of these peptides. © 1993 John Wiley & Sons, Inc.     

Structure of guanylyl-3',5'-cytidine monophosphate. II. Description of the molecular and crystal structure of the calcium derivative in space group P2(1).

The structural features of calcium guanosine-3′,5′-cytidine monophosphate (GpC) have been elucidated by X-ray diffraction analysis. The molecule was crystallized in space group P2₁ with cell constants of a = 21.224 Å, b = 34.207 Å, c = 9.327 Å, and β = 90.527°, Z = 8. The hydration of the crystal is 21% by weight with 72 water molecules in the unit cell. The four GpC molecules in the asymmetric unit occur as two Watson-Crick hydrogen-bonded dimers related by a pseudo-C face centering. Each dimer consists of two independent GpC molecules whose bases are hydrogen bonded to each other in the traditional Watson-Crick fashion. Each dimer possesses a pseudo twofold axis broken by a calcium ion and associated solvent. The four molecules are conformationally similar to helical RNA, but are not identical to it or to each other. Instead, values of conformational angles reflect the intrinsic flexibility of the molecule within the range of basic helical conformations. All eight bases are anti, sugars are all C3′-endo, and the C4′-C5′ bond rotations are gauche-gauche. The R factor is 12.6% for 2918 observed reflections at 1.2-Å resolution.     

The crystal structure of the photosynthetic inhibitors DCMU and DTPU

X-ray diffraction studies have been carried out on a single crystal of the photosynthetic inhibitors N-(3,4-dichlorophenyl)-N′-dimethylurea (DCMU) and its newly synthesized spin-labeled analog N-(3,4-dichlorophenyl)-N′-(3,3,5,5-tetramethylpiperidine-4-oxyl)-urea (DTPU). The synthesis of DTPU as well as its crystallographic data are reported. The crystal system of both compounds is monoclinic with a space group P2₁/c. The cell constants of DCMU are a = 7.759(1), b = 14.737(3), c = 9.233(2) Å, β = 100.99(6)°; of DTPU they are a = 6.976(1), b = 11.998(2), c = 23.585(3) Å, β = 91.38(5)°. Comparison of conformational parameters of DCMU and DTPU reveal differences in the dihedral angle between the aromatic ring and the ureido plane. The measured volumes of DCMU and DTPU are 259.1 and 493.3 ų, respectively. These figures suggest the size of the binding site of the inhibitors in the photosynthetic membrane.     

Potential multifunctional anti-cancer metal complexes. I. Synthesis and x-ray structural studies of some dinuclear rhodium(II) carboxylate complexes with diamine-substituted acridine ligands in terminal coordination positions

The preparations are reported of [Rh(RCO₂)₂L]₂ [where R = CH₃, C₂H₅, and CH₃OCH₂; L = 6-chloro-2-methoxy-9-[2(NR′₂)ethyl]aminoacridine (R′ = H, CH₃)]. X-ray structural studies have been carried out on two of the compounds [ R = C₂H₅, R′ = H, (1); R = CH₃, R′ = CH₃, (2)]. Compound 1 is monoclinic, space group C2/c, with a = 20.864(11), b = 15.736(4), c = 14.402(4) Å, β = 93.14(4)°, V = 4721 ų, and Z = 4; 2 is monoclinic, space group P2₁/n, a = 8.861(2), b = 23.089(10), c = 12.014(2) Å, β = 105.84(2)°, V = 2365 ų, and Z = 2. Both compounds comprise the standard dinuclear rhodium(II) carboxylate unit with the substituted acridine ligands coordinated to rhodium in the axial positions, via the NH₂ group nitrogen in 1 and the N(CH₃)₂ nitrogen in 2.The dimethyl substitution on the tertiary amine group in 2, and an associated conformational change in the diamine chain, result in an increased separation of the acridine ligand from the metal centre. There is a pronounced acridine base stacking in 1 but not in 2.     

Comparative structural studies of the first row early transition metal(III) chloride tetrahydrofuran solvates

Two compounds of empirical formula MCl₃- (THF)₃, M = V and Cr, have been characterized by single crystal X-ray studies. The VCl₃(THF)₃ molecule, which has a mer octahedral stereochemistry, crystallizes in the monoclinic space group P2₁/c with a= 8.847(2),b= 12.861(5),c= 15.134(3) Å, β = 91.94(2)°, V = 1721(1) ų and Z = 4. The V-Ci(1) and V-CI(2) distances have a mean value of 2.330 [3] Å while V-CI(3) = 2.297(2) Å, The VO(1) and VO(2) distances have a mean value of 2.061[8] Å while V-O(3) = 2.102(3) Å cis ClVCl angles average 92.0[5]° and cis OVO angles average 86.2[2]° . The isostmctural complex, CrCl₃(THF)₃, has a crystal structure made up of discrete octahedral mer-CrCl₃(THF)₃ molecules with the following unit cell dimensions (space group P2₁/c): a = 8.715(1), b= 12.786(3), c = 15.122(3) Å, β = 92.15(1)°, V = 1684(1) ų and Z = 4. The CrCl(1) and CrCl(2) distances have a mean value of 2.310131 Å while CrCl(3) = 2.283(2) Å. The CrO(1) and CrO(2) distances have a mean value of 2.0101171 Å while CrO(3) = 2.077(4) Å. cis ClCrCl angles average 90.9[4]° and cis OCrO angles average 86.1 [2]°. The structures of these two octahedral complexes and those previously reported for ScCl₃(THF)₃ and TiCl₃(THF)₃ are compared and certain general trends are discussed.     

Visualization of Drug-Nucleic Acid Interactions At Atomic Resolution. VIII. Structures of Two Ethidium/Dinucleoside Monophosphate Crystalline Complexes Containing Ethidium: Cytidylyl(3′-5′) Guanosine

Abstract

This paper describes two complexes containing ethidium and the dinucleoside monophosphate, cytidylyl(3′-5′)guanosine (CpG). Both crystals are monoclinic, space group P2₁, with unit cell dimensions as follows: modification 1: a = 13.64 Å, b = 32.16 Å, c - 14.93 Å, β = 114.8° and modification 2: a = 13.79 Å, b = 31.94 Å, c = 15.66 Å, β = 117.5°. Each structure has been solved to atomic resolution and refined by Fourier and least squares methods; the first has been refined to a residual of 0.187 on 1,903 reflections, while the second has been refined to a residual of 0.187 on 1,001 reflections. The asymmetric unit in both structures contains two ethidium molecules and two CpG molecules; the first structure has 30 water molecules (a total of 158 non-hydrogen atoms), while the second structure has 19 water molecules (a total of 147 non-hydrogen atoms). Both structures demonstrate intercalation of ethidium between base-paired CpG dimers. In addition, ethidium molecules stack on either side of the intercalated duplex, being related by a unit cell translation along the a axis.

The basic feature of the sugar-phosphate chains accompanying ethidium intercalation in both structures is: C3′ endo (3′-5′) C2′ endo. This mixed sugar-puckering pattern has been observed in all previous studies of ethidium intercalation and is a feature common to other drug-nucleic acid structural studies carried out in our laboratory. We discuss this further in this paper and in the accompanying papers.     

Structure of a Novel Drug-Nucleic Acid Crystalline Complex: 1,10-Phenanthroline-Platinum(II) Ethylenediamine—5′-Phosphoryl-Thymidylyl(3′-5′) Deoxyadenosine

Abstract

1,10-Phenanthroline-platinum (II) ethylenediamine (PEPt) forms a 1:2 crystalline complex with 5′-phosphorylthymidylyl (3′-5′) deoxyadenosine (d-pTpA). Crystals are monoclinic, P2₁, with a - 10.204 Å, b =24.743 Å, c = 21.064 Å, β = 94.6°. The structure has been determined by Patterson and Fourier methods, and refined by least squares to a residual of 0.128 on 2,367 observed reflections.

PEPt molecules form sandwich-like stacks with adenine-thymine hydrogen-bonded pairs along the a axis. Intercalation in the classic sense is not observed in this structure. Instead, d-pTpA molecules form an open chain structure in which adenine-thymine residues hydrogen- bond together with the reversed Hoogsteen type base-pairing configuration. Deoxyadenosine residues exist in the syn conformation and are C3′ endo and C1′ exo. Thymidine residues are in the high anti conformation with C2′ endo puckers. The structure is heavily hydrated, forming a channel-like water network along the a axis. Other features of the structure are described.     

Crystal and molecular structure of the (2,2′-diaminobiphenyl)(R,R-trans-1,2-diaminocyclohexane)platinum(II) complex

2,2′-Diaminobiphenyl-R,R-trans-1,2-diaminocyclohexaneplatinum(II) Chloride Trihydrate, (R,R-chxn)(dabp)Pt]Cl₂·3H₂O, crystallizes in the space group _p2₁2₁2₁ (D₂⁴, No. 19) with a = 6.219(4) Å, b = 17.633(2) Å, c = 21.523(3) Å, V = 2,360.4(8) ų, ?_calcd = 1.739 g cm^?3, ?_measd = 1.74 g cm^?3, and Z = 4. Diffraction data were collected with a Picker FACS-1 four-circle diffractometer. The structure was solved by the heavy atom method and refined by least-square calculations to residuals R = 0.0586 and weighted R = 0.0668. The 2,2′-diaminobiphenyl ligand exhibits complete stereospecificity in its coordination to platinum(II) ion with λ chiral conformation.     

Preparation,properties and crystal structures of nickel(II) complexes with acyclic schiff bases derived from 2,6-diformyl-4-chlorophenol and 1,5-diamino-3-thiapentane or 3,3′-diamino-N-methyl-dipropylamine

Nickel(II) complexes with the compartmental Schiff bases derived from 2,6-diformyl-4-chlorophenol and 1,5-diamino-3-thiapentane (H₂L¹) or 3,3′-diamino-N-methyl-dipropylamine (H₂L²) were synthesized, and the crystal structures of [Ni(L¹)- (py)₂] and [Ni(L²)(dmf)]·H₂0 were determined by X-ray crystallography.Ni(L¹)(py)₂ is monoclinic, space group C2/c, with a= 18.457(6), b = 11.116(7), c= 16.098(6) Å, and β = 115.79(5)°; D_c = 1.49 g cm⁻³ for Z = 4. The structure was refined to the final R of 6.9%. The molecule has C₂ symmetry. The nickel atom is six-coordinated octahedral. Selected bond lengths are: NiO 2.04(1) Å, NiN (L¹) 2.08(1) Å, NiN(py) 2.17(1) Å.[Ni(L²)(dmf)]·H₂O is monoclinic, space group P2₁/n, with a = 17.329(6), b = 13.322(7), c = 12.476(7) Å and β = 95.43(5)°; D_c = 1.45 g cm⁻³ for Z = 4. The structure was refined to the final R of 5.1%. The nickel atom is bonded in the octahedral geometry to the bianionic pentadentate ligand L² and to one molecule of dimethylformamide. Selected bond lengths are: NiO (charged) 2.063(3) Å (mean value), NiO (neutral) 2.120(3) Å, NiN (planar) 2.050(3) Å (mean value), NiN (tetrahedral) 2.177(3) Å.     

Structural and mass spectral studies on some multiply bonded diosmium tetracarboxylate compounds

The compounds Os₂(O₂CCH₃)₄Cl₂,1 and Os(O₂CC₂H₅)₄Cl₂, 2, have been structurally characterized. Both compounds crystallize in space group P2₁/n. For 1 the unit cell parameters are a = 6.546(1) Å, b = 8.950(1) Å, c = 12.533(1) Å, β = 90.17(1)° and Z = 2; for 2 they are a = 6.792(2) Å, b = 10.519(3) Å, c = 13.372(4) Å, β = 89.27(3)° and Z = 2. Both molecules have approximate D_4th symmetry with important dimensions as follows: for 1, Os≡Os 2.314(1) Å, OsCl 2.448(2) Å; for 2, Os≡Os 2.316(2) Å, OsCl 2.430(5) Å. The mass spectra of these compounds as well as that of Os₂(O₂CC₃H₇)₄Cl₂,3, are reported and discussed.