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The X-ray structure analysis of three compounds of interest as enzyme substrates is reported. They are the hydrated forms of (I) DL-2-amino-4-arsonobutanoic acid [HO–AsO2-–CH2–CH2–CH(NH3+)–CO2H], (II) DL-2-amino-4-phosphonobutanoic acid [HO–PO2-–CH2–CH2–CH(NH3+)–CO2H] and the hydrated barium salt of (III) D-3-phosphoglycerate [HO–PO2-–O–CH2–CH(OH)–CO2-]. The structures were fully refined to R factors of 0.033, 0.053 and 0.046. For the compounds (I) and (II) the charge distribution was directly determined by locating all H atoms. The co-ordination around As and P is approximately tetrahedral, with the valency angle between the two charged O atoms enlarged to 112 degrees in compound (I), 166 degrees in compound (II) and 122 degrees in compound (III). The As-X bond distances are increased relative to P-X to accommodate the increased atomic radius. The analysis establishes that the compounds are structural analogues. Tables of co-ordinates for H atoms, anisotropic thermal parameters, bond lengths and bond angles for the three compounds have been deposited as Supplementary Publication SUP 50122 (5 pages) with the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained directly [see Biochem J. (1983) 209, 5].

The antibiotic nogalamycin, a drug with high specificity for TG and CG steps in double-stranded DNA, has been crystallized as a 1:1 complex with the hexamer d(CCCGGG). The antibiotic is inserted at the central CG step of the duplex, with the two sugars oriented in the same direction and with strong interactions with the DNA within the grooves. The amino-glucose residue makes an integral part of a well defined major groove hydration network with van der Waals contacts and several strong hydrogen bonds to the duplex. The nogalose residue resides in the minor groove, making primarily van der Waals contacts. The single site allows an accurate molecular description of the intercalation, without perturbations from end effects observed previously. The local unwinding induced by nogalamycin is completely relaxed 2 base pairs away from the intercalation site. The two strands of the DNA show a continuous deformation from the A to the B form: 1) the cytosines toward the 5' end of the nogalomycin site in each strand have c3'-endo conformations while 5 guanosines toward the 3' ends have c2'-endo conformations; 2) within each strand, the phosphate-phosphate distances increase in a continuous manner from 5.7 A (A-form) to 7.1 A (B-form).

Oligonucleotides containing stereospecifically incorporated phosphorothioate residues have been synthesised and their conformation studied using CD spectroscopy.

The nonamer r(GCUUCGGC)dBrU, where dBrU is 5-bromo-2'-deoxyuridine, contains the tetraloop sequence UUCG. It crystallizes in the presence of Rh(NH3)6Cl3. In solution the oligomer is expected to form a hairpin loop but the x-ray structure analysis, to a resolution of 1.6 A, indicates an eight-base-pair A-RNA duplex containing a central block of two G.U and two C.U pairs. Self-pairs which approximate to Watson-Crick geometry are also formed in the extended crystal structure between symmetry-related BrU residues and are part of infinite double-helical stacks. The G.U pair is a wobble base pair analogous to the G.T pair found in DNA fragments. The C.U mismatch involves one hydrogen-bonded contact between the bases and a bridging water molecule which ensures a good fit of the base pair in the RNA helix. The BrU.BrU pair is held by two hydrogen bonds in an orientation which is compatible with duplex geometry. The structure observed within the crystal has some parallels with the structure of globular RNAs, and the presence of stable, noncanonical base pairs has implications for the prediction of RNA secondary structure.

It is well known that DNA can exist in a variety of conformations which can be interconverted by relatively mild changes in conditions. The in vivo conformation of DNA is usually thought to be the B form, but there is recent evidence that other conforma-tions may be important in DNA–protein recognition. Different fragments of DNA crystallized under virtually identical conditions can form A, B or Z helices1. A fragment that adopted an A conformation in a crystal was found in the B conformation in solution2, whereas NMR spectroscopy of A-DNA films3revealed the presence of a substantial amount of disordered B-DNA. Until now, however, a DNA fragment of a given sequence has not been crystallized in more than one global conformation4,5. We report here an X-ray diffraction study of crystals of the DNA octamer dGGBrUABrUACC. In addition to a 'framework' of A-DNA, which gives discrete X-ray reflections, there are partially disordered B-DNA helices, recognized by their diffuse scattering features.

METHYLOTROPHIC bacteria are able to use methane derivatives as their sole source of carbon and metabolic energy and so can sustain growth on methane, methanol and other organic compounds which lack carbon–carbon bonds1. They are not autotrophic and, being unable to use carbon dioxide, rely ultimately on their ability to oxidise these substances. Although the primary alcohol dehydrogenases from a variety of methanol-grown bacteria have been purified and compared, and despite the increasing commerical importance of these organisms as sources of ‘single cell protein’, the essential features of these oxidative pathways remain unknown. The dehydrogenases exhibit a rather broad substrate specificity for primary alcohols and formaldehyde, and seem to contain a common and novel cofactor2–6, which may be more generally associated with the oxidation of single-carbon compounds. The structure of the cofactor is reported here.

The lithium derivatives of the amines (I) or the amide (VII) undergo Horner‐Wittig reaction with the aldehydes (II) or (VIII) to give mixtures of the erythro‐adducts (III) or (IX) and the threo‐adducts (IV) or (XI).