Matrix methods for solving protein substructures of chlorine and sulfur from anomalous data.

The weak signal obtained from the anomalous scattering (at lambda = 1.54 A) of naturally occurring elements such as sulfur, phosphorus and ordered solvent chloride ions is used to determine the atomic positions of these atoms. Two examples are discussed: the sulfur and chlorine substructure of tetragonal hen egg-white lysozyme and an oligonucleotide containing ten P atoms. The substructure of lysozyme was also solved from Cu K(alpha) radiation data collected on a standard rotating-anode generator. The results presented here are an illustration of the power of the matrix methods, which are to be implemented in next distribution of the direct methods package CRUNCH.

Chiral recognition between host and guest: a binaphthyl-18-crown-6 host with D-phenylglycinium methyl ester perchlorate guest. A difficult structure solved with CRUNCH.

The complex between (R)-4,5,7,8,10,11,13,14,16,17-decahydro-2,19- diphenyldinaphtho[2,1-q:1',2'-s][1,4,7,- 10,13,16]hexaoxa[2,5,8,11,14,17,19]cycloicosaheptene {Chemical Abstracts name: (R)-4,5,7,8,10,11,13,14,16,17-decahydro-2,19- diphenyldinaphtho[2,1-q:1',2'-s][1,4,7,-10,13,16]hexaoxacycloic osin} and D-2-phenylglycinium methyl ester perchlorate, C9H12NO2+.ClO4-.C42H40O6.-H2O, crystallizes in the orthorhombic space group P2(1)2(1)2(1) with two C9H12NO2+.C42H40O6 complexes, two ClO4- ions and two molecules of water in the asymmetric unit. Crystal data: M(r) = 924.44, a = 23.048 (7), b = 34.383 (6), c = 11.992 (6) A, V = 9503 (6) A3, Z = 8, Dx = 1.292 Mg m-3, F(000) = 3904, mu(Cu K alpha) = 1.261 mm-1, T = 175 K, R = 0.0896 for all 7784 reflections, 1208 parameters refined in three blocks with 29 restraints. Nearly twenty years elapsed between the first data collection and the solution of the structure with the direct-methods program CRUNCH. The structural details are of interest because enantiomers of this host show a high degree of discrimination between enantiomers of alpha-amino acids and their esters. The crystal structure demonstrates the influence of C-H...O and C-H...pi interactions on the unexpected orientation of the guest in the host cavity. The same orientation is found in both of the unique complexes, and the geometric details are in agreement with solution studies.

New developments in phase refinement.

A longstanding problem in X-ray crystallography is that vital information regarding the crystal phases in missing from the experimental data that are gathered in the diffraction experiment. Prior knowledge needs to be introduced in order to resolve phase ambiguities whenever the diffraction data are not sufficient to unequivocally reconstruct the crystal phases through anomalous or isomorphous differences. Very recent developments include progress in the application of direct methods to small proteins and other compounds of a similar small size (Shake 'n' Bake, SHELXD, CRUNCH and SIR96), bias-free refinement through the gamma-correction (Solomon), improvements in the determination of phase probability distributions (SHARP) and automated atomic refinement (wARP).