Crystal and molecular structure of a new Z-DNA crystal form: d[CGT(2-NH2-A)CG] and its platinated derivative.

The three-dimensional structure of d[CGTA'CG], where A' = [2-NH2-A], was determined to atomic (1.35 A) resolution by single isomorphous replacement. The d[CGTA'CG] hexamer crystallizes in space group P3221, and is not isomorphous with other DNA hexanucleotides. Despite completely different crystal packing, the essential characteristics of the Z-DNA conformation are maintained. The structure was determined by single isomorphous replacement using a triammine platinum fragment. Thus, this study also demonstrates, for the first time, the feasibility of the use of this reagent for the direct phasing of DNA crystal structures.

9,11-Secogorgost-5-en-9-one-3 beta,11-diol, a marine steroid from the sea whip Pseudopterogorgia hummelinkii.

The title steroid [(22R,23R,24R)-22,23-methylene-23,24-dimethyl-9,11- secocholest-5-en-9-one-3 beta,11-diol, C30H50O3], was isolated from Pseudopterogorgia hummelinkii, a Caribbean gorgonian. The cyclopropane ring in the side chain of this molecule, a feature very unusual in terrestrial steroids, has been found in several other marine steroids. The molecular structure is potentially very flexible because of the oxidative cleavage of ring C, but the two independent molecules in the crystal have quite similar overall conformations. The observed conformational differences correlate with dissimilar participation of the hydroxyl and carbonyl groups of each molecule in hydrogen bonding, which is entirely intermolecular. The crystal structure was solved by direct methods, but only with great difficulty.

Approaches to isozyme-specific inhibitors. 16. A novel methyl-C5' covalent adduct of L-ethionine and beta,gamma-imido-ATP as a potent multisubstrate inhibitor of rat methionine adenosyltransferases.

N6,N6-Dibenzoyl-2',3'-O-isopropylideneadenosine, which is readily synthesized by one-pot 5'-O-trimethylsilylation, N6-benzoylation, and desilylation, was converted to the corresponding 5'-aldehyde. This was treated with CH2 = CHMgBr to afford, after debenzoylation, a 1:3 mixture of the 5'S and 5'R epimers, respectively, of 5'-C-vinyl-2',3'-O-isopropylideneadenosine. The configurations were established by single-crystal X-ray diffraction analysis of the 5'R epimer. Hydroboration of the 5'-O-tetrahydropyranyl derivative of the mixed epimeric 5'-C-vinyl nucleosides readily furnished 5'(S,R)-C-(2-hydroxyethyl)-2',3'-O-isopropylideneadenosine. Treatment of the 5'(S,R)-C-(2-O-tosyl) derivative of this with disodium L-homocysteinate permitted facile introduction of the L-ethionine system. By means of methods developed earlier in the synthesis of homologous methionine-ATP adducts, the alpha-amino acid group was protected, a beta,gamma-imidotriphosphoryl group was introduced at O5', and blocking groups were removed to give the title adduct as a 2:3 mixture of its two 5' epimers. It was a powerful inhibitor [KM(ATP)/Ki = 520 and 340] of the M-2 (normal tissue) and M-T (hepatoma tissue) forms, respectively, of the title enzyme and displayed predominantly competitive kinetics with the two substrates L-methionine and MgATP. It inhibited M-2 and M-T slightly less effectively than its homologue possessing one less CH2 between sulfur and C5' and gave kinetic evidence of an increased tendency to form L-methionine-enzyme-adduct and MgATP-enzyme-adduct complexes.