A new cycloamphilectene metabolite from the Vanuatu sponge Axinella sp.

A new diterpene, N-formyl-7-amino-11-cycloamphilectene (1), was isolated from the apolar extract of the Vanuatu sponge Axinella sp. The structure and relative stereochemistry were established by spectroscopic and single-crystal X-ray studies.

Cinchoninium L-tartrate tetrahydrate.

The title compound, (3R,4S,8R,9S)-cinchoninium (2R,3R)-tartrate tetrahydrate, C19H23N2O+*C4H5O6-*4H2O, is a hydrated salt of cinchonine. In the cinchoninium cation, the geometry around the quinuclidinic N atom is typical of a protonated N atom, and the bond lengths and angles in the tartrate moiety clearly indicate the mono-ionized form. The relative orientation of the quinoline and quinuclidine systems is that most frequently observed in structures of cinchona salts and corresponds to one of the energy minima calculated for this type of molecule in the gas phase. An extended network of intermolecular hydrogen bonds spreads parallel to the bc plane separating apolar layers.

3,6-Bis(dimethylamino)-10-propylacridinium iodide.

In the title compound, C(20)H(26)N(3)(+) x I(-), the acridinium moiety shows mirror symmetry about the central C-N vector. The fused tricyclic system is only approximately planar and the geometry is affected by the presence of both dimethylamino groups and the propyl substitution at the central N atom. The propyl chain adopts an extended trans conformation and the plane through the chain C atoms is perpendicular to the mean plane through the rings. The I(-) ion is involved in short-range hydrogen-bonding interactions with two centrosymmetrically related cations via three activated acridinium C atoms. Stacks of acridinium cations propagate through the crystal along the c direction. The ring overlap is partial, but the dimethylamino groups also participate in the stacking.

Solid-state photodimerization of cholest-4-en-3-one.

Crystalline cholest-4-en-3-one undergoes solid-state dimerization by UV radiation to give two ring A - ring A connected dimers. No dimerization occurs in solution. The first dimer, characterized by a cyclobutane ring, is formed by connection of C-2 and C-3 of a moiety with C-5' and C-6' of another moiety, respectively. The latter dimer has a six-membered ketal ring formed by connection of C-2 with C-5' and of O, linked to C-3, with C-3'. The structures have been determined by spectroscopic means. X-ray analysis of title compound evidences the proximity of the axial H-2 of a molecule to the C-4' of a molecule in the upper layer. The transfer of the hydrogen and the connection between C-2 and C-5' might be the driving force of dimerization.

Crystal structure of a new alpha-cyclodextrin hydrate form. Molecular geometry and packing features: disordered solvent contribution.

The crystallographic study of a new hydrated form of alpha-cyclodextrin (cyclohexaamylose) is reported. C36H60O30 . 11H2O; space group P2(1)2(1)2(1) with cell constants a = 13.839(3), b = 15.398(3), c = 24.209(7) A; final discrepancy index R = 0.057 for the 5182 observed reflections and 632 refined parameters. Besides four ordered water molecules placed outside alpha-cyclodextrins, the structure shows regions of severely disordered solvent mainly confined in the oligosaccharide cavities. The contribution of the observed disorder has been computed via Fourier inversions of the residual electron density and incorporated into the structure factors in further refinements of the ordered part. The alpha-cyclodextrin molecule assumes a relaxed round shape stabilised by a ring sequence of all the six possible O2 ... O3 intramolecular hydrogen bonds. The four ordered water molecules take part in an extensive network of hydrogen bonds (infinite chains and loops) without modifying the scheme of intramolecular H-bonds or the (-)gauche conformation of O-6-H hydroxyl groups. The structure shows a new molecular arrangement, for an "empty" hydrated alpha-cyclodextrin, like that "brick-type" observed for alpha-CD in the iodoanilide trihydrate complex crystallising in an isomorphous cell.

Bovine seminal ribonuclease: structure at 1.9 A resolution.

The crystal structure of bovine seminal ribonuclease, a homodimeric enzyme closely related to pancreatic ribonuclease, has been refined at a nominal resolution of 1.9 A employing data collected on an electronic area detector. The final model consists of two chains containing 1990 non-H atoms, seven sulfate anions and 113 water molecules per asymmetric unit. The unit-cell parameters are a = 36.5 (1), b = 66.7 (1) and c = 107.5 (2) A, space group P22(1)2(1). The R factor is 0.177 for 16 492 reflections in the resolution range 6.0-1.9 A and the deviations from ideal values of bond lengths and bond angles are 0.020 A and 3.7 degrees, respectively. The molecule is formed by two pancreatic like chains, which have their N-terminal segments interchanged so that each active site is formed by residues from both subunits. The two chains are related by a non-crystallographic twofold symmetry and are covalently linked by two consecutive disulfide bridges, which form an unusual sixteen-membered ring across the dimer interface. The deviations from the molecular symmetry, the hydration shell and the sulfate-binding sites are also discussed in relation to the known structure of the pancreatic enzyme.

Conformation of aminosuccinyl dipeptides Ac-L-X-L-Asu-NMe from empirical energy calculations.

The aminosuccinyl (Asu) residue is formed as intermediate in some peptide and protein reactions. Potential energy calculations, using the parameters of the empirical conformational energy program for peptides (ECEPP), were performed on blocked X-Asu dipeptides, where X = Gly, Ala, Ser, Val and Pro. Results indicate that intra-residue interactions are dominant, and the low-energy dipeptide conformations correspond to the low-energy single-residue minima. Comparisons were made with previous results from energy calculations on blocked Asu-X dipeptides (X = Gly, Ala, Ser and Val).

Structure of N-acetyl-L-leucyl-L-prolinamide monohydrate.

C13H23N3O3.H2O, Mr = 287.36, orthorhombic, P2(1)2(1)2(1), a = 6.3863 (8), b = 14.346 (2), c = 17.247 (2) A, V = 1580.1 (6) A3, Z = 4, Dx = 1.208 g cm-3, lambda(Cu K alpha) = 1.54178 A, mu = 7.04 cm-1, F(000) = 624, room temperature, final R = 0.037 for 1693 reflections with I greater than or equal to 2.5 sigma (I) and 257 variables. The peptide linkage is in a trans conformation. The pyrrolidine ring adopts a puckered C gamma-endo conformation and the leucine side chain is in a twisted [g-(tg-)] conformation. The crystal packing is stabilized by a three-dimensional network of hydrogen bonds which involve all the donor groups. The water molecule plays an important part in the crystal packing, being engaged in three hydrogen bonds with different N-acetylamide molecules.

Conformation of aminosuccinyl dipeptides Ac-Asu-X-NHMe from empirical energy calculations.

Conformational energy calculations were carried out on the N-acetyl-N'-methylamides of aminosuccinyl (Asu) peptides. Computations were performed using a procedure analogous to that of the ECEPP/2 program, on N-Ac-L-Asu-NMe, N-Ac-L-Asu-L-X-NHMe, where X = Gly, Ala, Ser, Val and N-Ac-D-Asu-L-Ala-NHMe. With the exception of N-Ac-L-Asu-L-Val-NHMe, the lowest-energy forms of all the dipeptides correspond to a beta-bend conformation of type II' or II for a L,L and D,L sequence, respectively. When X = Val, the folded conformation is destabilized, and more extended conformations are preferred. The puckering of the cyclic imide has a small but meaningful influence on the relative energies of the minima. The calculations were shown to be in agreement with available experimental data.