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Studies of binding of substrates and inhibitors of the enzyme D-xylose isomerase show, from X-ray diffraction data at 1.6-1.9 A resolution, that there are a variety of binding modes. These vary in the manner in which the substrate or its analogue extend, on binding, across the carboxy end of the (betaalpha)(8)-barrel structure. These binding sites are His54 and the metal ion (magnesium or manganese) that is held in place by Glul81, Asp245, Glu217 and Asp287. Possible catalytic groups have been identified in proposed mechanisms and their role in the binding of ligands is illustrated.

Helicases are ATP-driven enzymes essential for DNA unwinding. The broad host range plasmid RSFI010 harbours a gene (repA) encoding for one of the smallest known oligomeric helicases, RepA, a homo-hexamer with 30 kDa subunits. Electron micrographs indicate that the overall shape of RepA resembles a hexagon with globular monomers at the corners, diameter 140 A, and a central channel. Below pH 6, the molecules aggregate into tubular structures. The enzyme has been purified and crystallized using the hanging-drop vapour-diffusion method with polyethyleneglycol monomethylether as precipitating agent. The crystals exhibit the monoclinic space group P2(1) with unit-cell parameters a = 105.8, b = 180.3, c = 115.4 A, beta = 95.2 degrees, and diffract to 3.5 A resolution using rotating-anode Cu Kalpha radiation. Assuming two 180 kDa molecules per asymmetric unit, the volume per unit weight is V(m) = 3.06 A Da(-1), equivalent to a solvent content of 60%. A self-rotation search indicates that the sixfold axis of the hexamer is parallel to the ac plane and inclined at about 2 degrees to the c axis. The two hexamers are oriented head-to-head with point-group symmetry D(6).

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A close to planar conformation is established for 2-(4-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine (M-H-P) by crystal structure determination. For 2-(4-hexoxy-2-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine (H-MeO-P), hydrophobic interaction between the long hexoxy groups enforces rotation of the dialkoxyphenyl ring at C6 and the C2 phenyl ring relative to the plane of the triazine ring. Time-resolved fluorescence measurements demonstrate the existence of a short-lived Franck-Condon (FC) and a longer-lived TICT (twisted intramolecular charge transfer) state, emitting at longer wavelengths, for the O-methyl derivative 2-(4-hexoxy-2-methoxyphenyl)-4,6-bis(2,4,6-trimethylphenyl)-1,3,5-triazine (H-MeO-Ms). The decay of the protontransferred (PT) fluorescence for crystalline powders of 2-(2-hydroxyphenyl)-4-(dimethylamino)-6-aryl-1,3,5triazines is shown, by the same method, to be retarded within the series phenyl f tolyl f xylyl, well in line with the gradation of quantum yields determined by steady-state methods. The high quantum yields of the PT fluorescence for triazines with only one 2-hydroxyaryl moiety on the triazine ring are reduced by 2 orders of magnitude each upon introduction of a second and a third aryl group. Most of the 2-hydroxyaryl-1,3,5triazines with additional aryl moieties at C4 and C6 show PT fluorescence, as long as the 2-hydroxyaryl and the triazine ring are more or less coplanar and the intramolecular hydrogen bond remains intact. In some cases, however, this fluorescence is very weak, or can be detected only in copolymers, in mixtures of the stabilizer with the polymer or in crystalline powders.

Chloromuconate cycloisomerase (E.C. 5.5.1.7) is an enzyme involved in the 2,4-dichlorophenoxyacetate degradation pathway of Alcaligenes eutrophus JMP134 (pJP4). The crystal structure of this protein was determined at 3 A resolution by molecularreplacement techniques using atomic coordinates from the reported crystal structure of the homologous muconate cycloisomerase (E.C. 5.5.1.1) from Pseudomonas putida as the search model (42% identical positions in the sequences). Structure refinement by simulated-annealing and restrained least-squares techniques converged at R = 0.195. In the crystals studied, space group 14, the protein is present as two octamers per unit cell with two subunits per asymmetric unit. Each subunit consists of two globular domains, one of which forms an a/fl-barrel. Com-parison of this structure with that of muconate cycloisomerase reveals the reasons for the altered substrate specificity of chloromuconate cyclo-