Homocoenzyme B12 and bishomocoenzyme B12: covalent structural mimics for homolyzed, enzyme-bound coenzyme B12

Efficient electrochemical syntheses of "homocoenzyme B(12)" (2, Co(beta)-(5'-deoxy-5'-adenosyl-methyl)-cob(III)alamin) and "bishomocoenzyme B(12)" (3, Co(beta)-[2-(5'-deoxy-5'-adenosyl)-ethyl]-cob(III)alamin) are reported here. These syntheses have provided crystalline samples of 2 and 3 in 94 and 77 % yield, respectively. In addition, in-depth investigations of the structures of 2 and 3 in solution were carried out and a high-resolution crystal structure of 2 was obtained. The two homologues of coenzyme B(12) (2 and 3) are suggested to function as covalent structural mimics of the hypothetical enzyme-bound "activated" (that is, "stretched" or even homolytically cleaved) states of the B(12) cofactor. From crude molecular models, the crucial distances from the corrin-bound cobalt center to the C5' atom of the (homo)adenosine moieties in 2 and 3 were estimated to be about 3.0 and 4.4 A, respectively. These values are roughly the same as those found in the two "activated" forms of coenzyme B(12) in the crystal structure of glutamate mutase. Indeed, in the crystal structure of 2, the cobalt center was observed to be at a distance of 2.99 A from the C5' atom of the homoadenosine moiety and the latter was found to be present in the unusual syn conformation. In solution, the organometallic moieties of 2 and 3 were shown to be rather flexible and to be considerably more dynamic than the equivalent group in coenzyme B(12). The homoadenosine moiety of 2 was indicated to occur in both the syn and the anti conformations.     

C−H...π(Ar)...H−C host-guest interactions in a novel air-stable cyanoborate clathrate

The host compound forms a coordinato-clathrate in a coordination assisted host lattice. The host lattice is provided by hydrogen bonding between the nitrogen atom of the cyano group and two hydrogen atoms from the phosphonium methyl group. Guest molecules ofp-xylene occupy constricted channels in the lattice. The twop-xylene molecules participate in C–H...(Ar)...H–C interactions. Results from searches of the Cambridge Structural Database for other C–H...(Ar)...H–C and C–H...(Ar) interactions are presented.     

Electrochemical Synthesis and Structure Analysis of Neocoenzyme B12 – An Epimer of Coenzyme B12 with a Remarkably Flexible Organometallic Group

In neocoenzyme B₁₂ (=(5′-deoxy-5′-adenosyl)-13-epicob(III)alamin; 5 ), an epimer of coenzyme B₁₂ ( 1 ), the organometallic group and a propanamide side chain of the vitamin-B₁₂ ligand compete for the same region in space. Interesting consequences for structure and organometallic reactivity of this isomer of 1 are to be expected. Neocoenzyme B₁₂ ( 5 ; 89% yield) and methyl-13-epicobalamin ( 6 ; 88% yield) were prepared from neovitamin B₁₂ ( 4 ) by electrochemical means (Fig. 3). The solution structure of the organometallic neovitamin-B₁₂ derivative 5 was analyzed by homonuclear and heteronuclear NMR spectroscopy. Comparison of the structures of 1 and 5 informed on the structural consequences of the epimerization at C(13) and revealed a remarkable flexibility of the organometallic group in 5 . In 5 , both sterically interacting functionalities (organometallic group and propanamide side chain at C(13)) adapt their conformations dynamically to avoid significant mutual clashes. As one consequence of this structural adaptation, the major conformations of 5 feature counterclockwise and clockwise reorientations of the organometallic ligand with respect to its crystallographically determined position in coenzyme B₁₂ ( 1 ). One of the dominant conformers of 5 exhibits an orientation of the organometallic functionality similar to that found in the crystal structure of the coenzyme-B₁₂-dependent methylmalonyl CoA mutase. The present NMR study also revealed the significant population of syn-conformers of the organometallic adenosine group, another remarkable feature of the solution structure of 5 .     

Local error estimates for moderately smooth problems: Part I – ODEs and DAEs

The paper consists of two parts. In the first part, we propose a procedure to estimate local errors of low order methods applied to solve initial value problems in ordinary differential equations (ODEs) and index 1 differential-algebraic equations (DAEs). Based on the idea of defect correction we develop local error estimates for the case when the problem data is only moderately smooth. Numerical experiments illustrate the performance of the mesh adaptation based on the error estimation developed in this paper. In the second part of the paper, we will consider the estimation of local errors in context of stochastic differential equations with small noise. AMS subject classification (2000)  65L06, 65L80, 65L50, 65L05     

Der Multiplikator von Thetareihen höheren Grades zu quadratischen Formen ungerader Ordnung

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Extremalprobleme für eine Familie schlichter Funktionen

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