A novel Z-structure for poly d(GC).poly d(GC)

A novel zig-zag (Z) structure is proposed for poly d(GC).poly d(GC). The proposed model closely resembles the crystal structure of d(CG)3.

X-ray crystal and molecular structure of a hydrated lithium picrate complex of benzo-15-crown-5; An example of non-chelation of the crown

Complexation of alkali and alkaline earth metal ions with crown ethers is well known (1) and chemical and crystallographic studies have been carried out for number of complexes (2,3). The interaction of the metal with the crown ether depends on the nature of the cation and particularly on the basicity of the anion (4) , In this paper we report the crystal and molecular structure of a lithium picrate complex of benzo-15-crown-5, the first x-ray crystallographic study of a lithlum-crown system.

Structure of 2-(2,6-dimethoxyphenyl)-2,5- dimethylbicyclo[3.2.1]octane-6,8-dione

C18H2204, orthorhombic, P212~21, a = 7.343 (4), b = 11.251 (4), c = 19.357 (4)A, Z = 4, Dr, ' = 1.20, D e = 1.254 g cm -3, F(000) = 648, p(MoKa) = 0.94 cm -~. X-ray intensity data were collected on a Nonius CAD-4 diffractometer and the structure was solved by direct methods. Full-matrix least-squares refinement gave R = 0.052 (R w = 0.045) for 1053 observed reflections. The stereochemical configuration at C(2) has been shown to be 2-exo-methyl-2-endo-(2,6-dimethoxyphenyl), i.e. (3) in contrast to the structure (2) assigned earlier based on its ~H NMR data.

Sequence-dependent conformation of an A-DNA double helix: The crystal structure of the octamer d(G-G-T-A-T-A-C-C)

The crystal structures of the synthetic self-complementary octamer d(G-G-T-A-T-A-C-C) and its 5-bromouracil-containing analogue have been refined to R values of 20% and 14% at resolutions of 1·8 and 2·25 Å, respectively. The molecules adopt an A-DNA type double-helical conformation, which is minimally affected by crystal forces. A detailed analysis of the structure shows a considerable influence of the nucleotide sequence on the base-pair stacking patterns. In particular, the electrostatic stacking interactions between adjacent guanine and thymine bases produce symmetric bending of the double helix and a major-groove widening. The sugar-phosphate backbone appears to be only slightly affected by the base sequence. The local variations in the base-pair orientation are brought about by correlated adjustments in the backbone torsion angles and the glycosidic orientation. Sequence-dependent conformational variations of the type observed here may contribute to the specificity of certain protein-DNA interactions.

Structure and Conformation of an Antidepressant Drug, Nitroxazepine Hydrochloride Monohydrate

CIsH20N3Oa+.C1-.H2 O, M r = 395, orthorhombic, Pn21a, a = 7.710 (4), b = 11.455 (3), c -- 21.199 (3)/k, Z = 4, V = 1872.4/k 3, D m = 1.38, D C = 1.403 g cm -3, F(000) = 832, g(Cu Kct) = 20.94 cm -l. Intensities for 1641 reflections were measured on a Nonius CAD-4 diffractometer; of these, 1470 were significant. The structure was solved by direct methods and refined to an R index of 0.045 using a blockdiagonal least-squares procedure. The angle between the least-squares planes through the benzene rings is 125.0 (5) ° and the side chain is folded similarly to one of the independent molecules of imipramine hydrochloride.

The structure of monopotassium phosphoenolpyruvate

CaH406P-.K +, M r = 206.10, is orthorhombic, space group Pbca (from systematic absences), a = 14.538(4), b = 13.364(5), c = 6.880 (6)A, U = 1383.9 A 3, D x = 2.07 Mg m -a, Z = 8, ~.(Mo Ka) = 0.7107/~, p(MO Ka) = 1.015 mm -1. The final R value is 0.042 for a total of 1397 reflections. The high energy P-O(13) and the enolic C(1)-O(13) bonds are 1.612 and 1.374 A respectively. The enolpyruvate moiety is essentially planar. The orientation of the phosphate with respect to the pyruvate group in PEP.K is distinctly different from that in the PEP-cyclohexylammonium salt, the torsion angle C (2)-C (1)-O(13)- P being -209.1 in the former and -90 ° in the latter. The K + ion binds simultaneously to both the phosphate and carboxyl ends of the same PEP molecule. The ester O(13) is also a binding site for the cation. The K + ion is coplanar with the pyruvate moiety and binds to 0(22) and O(13) almost along their lone-pair directions. The carbonyl 0(22) prefers to bind to the K + ion rather than take part in the formation of hydrogen bonds usually observed in carboxylic acid structures.

Crystal and molecular structure of the quinoxaline antibiotic analog TANDEM (des-N-tetramethyltriostin A)

The crystal structure of TANDEM (des-N-tetramethyltriostin A), a synthetic analogue of the quinoxaline antibiotic triostin A, has been determined independently at -135 and 7 'C and refined to R values of 0.088 and 0.147, respectively. The molecule has approximate 2-fold symmetry, with the quinoxaline chromophores and the disulfide cross-bridge projecting from opposite sides of the peptide ring. The quinoxaline groups are nearly parallel to each other and separated by about 6.5 A. The peptide backbone resembles a distorted antiparallel 13 ribbon joined by intramolecular hydrogen bonds N-H(LVal)--O(L-Ala). At low temperatures, the TANDEM molecule is surrounded by a regular first- and second-order hydration sphere containing 14 independent water molecules. At room temperature, only the first-order hydration shell is maintained. Calculations of the interplanar separation of the quinoxaline groups as a function of their orientation with respect to the peptide ring support the viability of TANDEM to intercalate bifunctionally into DNA.

Structure of a peptide oxazolone: 2-(1'-benzyloxycarbonylamino-1'-methylethyl)-4,4-dimethyl-5-oxazolone

C16H20N204, monoclinic, P21, a = 6.270 (1),b= 11.119(3),c= ll.640(4)A, fl= 100.7 (2)°,Dm = 1-27 (flotation), Dc = 1-26 Mg m -3, Z = 2. The structure has been refined to a final R value of 0.041 for 1584 independent counter-measured reflections. The oxazolone ring in the molecule is nearly planar. The exocyclic O atom is 0.065 A out of the plane defined by the other four atoms in the ring belonging to the lactone group. The difference in length between the two adjacent C-O bonds in the ring is small, but significant. The crystal structure is stabilized by van der Waals interactions and a N--H... N hydrogen bond.

Structure of (4S)-2,4-dimethyl-1,2-dihydropyrazino[2,1-b]quinazoline-3(4H),6-dione

C13HI3N302, orthorhombic, P2~2121, a = 17.443 (5), b = 11.650 (4), c = 5.784 (1)/~, Z = 4, d m = 1.456, d c = 1.429 Mg m -3, F(000) = 512, g(Cu Ka) = 0.843 mm-L The R index is 0.040 for 1358 significant reflections. The structure is stabilized by C-H...O interactions. The N-methylated eis peptide group which forms part of a six-membered ring is non-planar. The torsion angle about the peptide bond is -6.1 (4) ° and the peptide bond length is 1.337 (3) A.

Structure and dynamics of two-dimensional sheared granular flows

The structure and dynamics of the two-dimensional linear shear flow of inelastic disks at high area fractions are analyzed. The event-driven simulation technique is used in the hard-particle limit, where the particles interact through instantaneous collisions. The structure (relative arrangement of particles) is analyzed using the bond-orientational order parameter. It is found that the shear flow reduces the order in the system, and the order parameter in a shear flow is lower than that in a collection of elastic hard disks at equilibrium. The distribution of relative velocities between colliding particles is analyzed. The relative velocity distribution undergoes a transition from a Gaussian distribution for nearly elastic particles, to an exponential distribution at low coefficients of restitution. However, the single-particle distribution function is close to a Gaussian in the dense limit, indicating that correlations between colliding particles have a strong influence on the relative velocity distribution. This results in a much lower dissipation rate than that predicted using the molecular chaos assumption, where the velocities of colliding particles are considered to be uncorrelated.

Structure of 4-Hydroxymonophenylbutazone

4-Butyl-4-hydroxy-l-phenyl-3,5-pyrazolidinedione, ClaH16N20 a, Mr=248.3, monoclinic, P21/n, a = 22.357 (5), b = 5.014 (2), c = 11.350 (4)/~,, //=91.88(3) °, V=1272(1)A 3, Z=4, D,,=1.296(3), D x = 1.297 Mg m -3, 2(Cu Ka) = 1.5418/~, a = 0.777 mm -~, F(000) = 528, T= 293 K. Final R - 0.059 for 1668 observed reflections. The hetero nitrogen which carries the six-membered ring is planar in the structure while the other unsubstituted one is pyramidal. The five- and six-membered rings are almost coplanar. The crystal is made up of infinite columns of hydrogen-bonded molecules.

Crystal and molecular structure of isogarcinol

The controversy with regard to the structures of the closely related polyisoprenylated phenolic compounds, garcinol, isogarcinol, camboginol, cambogin, xanthochymol and isoxanthochymol is cleared by X-ray crystallographic analysis of the naturally occurring isogarcinol. The unusual UV spectral characteristics of the chromophore of isogarcinol are discussed.

Structure of the disodium salt of glucose 1-phosphate hydrate, 2Na+.C6H11O9P2-.3.5H2O

Mr= 367.2, monoclinic, C2, a = 8.429 (1),b= 10.184(2), c= 16.570(2)A, /~= 99.18 (1) °, U= 1404.2 A 3, z = 4, D m = 1.73, D x = 1.74 Mg m -3,Cu K~, 2 = 1.5418 A, g = 2.99 mm -1, F(000) = 764,T= 300K, final R for 1524 observed reflections is0.069. The endocyclic C-O bonds in the glucose ring are nearly equal with C(5)-O(5)= 1.445 (10) and C(1)-O(5)= 1.424(10). The pyranose sugar ring adopts a 4C 1 chair conformation. The conformation about the exocyclic C(5)-C(6) bond is gauche-gauche, in contrast to gauche-trans observed in the structure of the dipotassium salt of glucose 1-phosphate. The phosphate ester bond, P-O(1), is 1.641 (6)A, slightly longer than the 'high-energy' P-,.O bond in the monopotassium salt of phosphoenolpyruvate [1.612 (6)A]. Two sodium ions are six coordinated while the third has only five neighbours.

beta.-Turn conformation of N-acetyl-L-prolylglycyl-L-phenylalanine. Crystal structure and solution studies

Pro-Gly segments in peptides and proteins are prone to adopt the 0-turn conformation. This paper reports experimental data for the presence of this conformation in a linear tripeptide N-acetyl-L-prolylglycyl-L-phenylalanineb oth in the solid state and in solution. X-ray diffraction data on the tripeptide crystal show that it exists in the type I1 0-turn conformation. CD and proton NMR data show that this conformation persists in trifluoroethanol and methanol solutions in equilibrium with the nonhydrogen-bonded structures. Isomerization around the acetyl-prolyl bond is seen to take place in dimethyl sulfoxide solutions of the tripeptide.