Spontaneous generation of stable pnictinyl radicals from "jack-in-the-box" dipnictines: a solid-state, gas-phase, and theoretical investigation of the origins of steric stabilization.

The molecular structures of the stable phosphinyl and arsinyl radicals, .PnR(2) [Pn = P (2); As (4); R = CH(SiMe(3))(2)], have been determined by gas-phase electron diffraction (GED) in conjunction with ab initio molecular orbital calculations. The X-ray crystal structures of the corresponding dipnictines, the "dimers", R(2)PnPnR(2) [Pn = P (1), As (3)], and the chloro derivatives R(2)PnCl [Pn = P (5), As (6)] have also been determined. Collectively, these structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Principally, it is the shape and flexibility of the CH(SiMe(3))(2) ligands that permit the formation of the P-P and As-As bonds in 1and 3, respectively. However, theoretical studies indicate that in the process of pnictinyl radical dimerization to form 1 and 3, both molecules accumulate substantial amounts of potential energy and are thus primed to spring apart upon release from the solid state by melting, dissolution, or evaporation. The insights gleaned from these unusual systems have permitted a deeper understanding of the functioning of sterically demanding substituents.

Cerium masquerading as a group 4 element: synthesis, structure and computational characterisation of [CeCl(N(SiMe3)2)3].

Oxidation of the three-coordinate cerium amide [Ce(N-(SiMe3)2)3] with TeCl4 in toluene solution yields purple, diamagnetic [CeCl(N(SiMe3)2)3], whose structure has been examined by X-ray crystallographic and computational methods.

Thermally Stable Heterobinuclear Bivalent Group 14 Metal Complexes Ar2 M-Sn[1,8-(NR)2 C10 H6 ] (M=Ge, Sn; Ar=2,6-(Me2 N)2 C6 H3 ; R=CH2 tBu).

The first thermally robust Ge(II) -Sn(II) compound 1 and the structurally characterized Sn(II) -Sn(II) analogue 2, which maintain their structural integrity in solution, were obtained by treating MAr2 (M=Ge, Sn; Ar=2,6-(Me2 N)2 C6 H3 ) with Sn[1,8-(NR2 )2 C10 H6 ] (R=CH2 tBu). On the basis of structural and spectroscopic data, the M-Sn bond is regarded as the interaction of a MAr2 donor with an Sn[1,8-(NR2 )2 C10 H6 ] acceptor.

The number of functional active sites per molecule of the adenosylcobalamin-dependent enzyme, ethanolamine ammonia-lyase, as determined by a kinetic method.

1. A kinetic approach to the determination of the number of functional active sites per molecule of the adenosylcobalamin-dependent enzyme, ethanolamine ammonia-lyase, is described. 2. Time courses for formation and breakdown of a cob(II)alamin intermediate during reaction of the enzyme, fully saturated with adenosylcobalamin, with L-2-aminopropanol as substrate, were followed using a stopped-flow spectrophotometer under two conditions: (a) enzyme concentration much greater than that of substrate, (b) substrate concentration much greater than that of enzyme. 3. Results were analysed in terms of a three-step mechanism involving binding of substrate (k+1 step), cob(II)alamin formation (k+2 step) and cob(II)alamin breakdown (k+3 step). the kinetic scheme was shown to be sufficient to account for the observed time courses and rate constants of 80 s-1 (k+2) and 1.5 s-1 (k+3) were determined. 4. The number of active sites per enzyme molecule (n) was calculated from the kinetic data in three ways: (a) calculation from amplitude of absorbance measurement, (b) calculation from measurements of the values of rate constants and (c) analysis by computation of the kinetic data using the computer program FACSIMILE. A value for n close to 6 was calculated by each of these methods. This value is in disagreement with the literature value of about two sites per molecule but is consistent with the I6II6 subunit structure of the enzyme. 5. Kinetic analysis of data from experiments in which the adenosylcobalamin concentration was varied while substrate and enzyme concentrations remained constant showed that all the active sites function with identical rate constants. 6. The principle and mathematical basis of the kinetic method for determining the value of n is given as an Appendix.

Coenzyme B-12-dependent reactions. Part IV. Observations on the purification of ethanolamine ammonia-lyase.

Purification of ethanolamine ammonia-lyase (EC 4.3.1.7) from a Clostridium sp. grown at the University of Sussex, U.K. and the National Institutes of Health, U.S.A., has been compared and an improved isotopic assay for the enzyme has been developed. Successful purification of this enzyme from Sussex-grown cells requires modification of the published procedure (Kaplan and Stadtman (1968) J. Biol, Chem. 243, 1787-1793) principally a 70% decrease in volume during precipitation with 0.4 M NaCl. This modification also increases the yield from N.I.H.-grown cells. Purified enzyme, resolved of inactive cobalamins, has the same high specific activity from both sources and behaves in the same way on disc gel electrophoresis. Sussex enzyme, before resolution, has less than 20% of the specific activity of unresolved N.I.H. enzyme and contains over 50% more inactive cobalamin. The bound cobalamin from both preparations has been identified as a "base-on" Co11 psi-cobalamin.