Density functional theory calculations and exploration of a possible mechanism of N2 reduction by nitrogenase

Density functional theory (DFT) calculations have been performed on the nitrogenase cofactor, FeMoco. Issues that have been addressed concern the nature of M-M interactions and the identity and origin of the central light atom, revealed in a recent crystallographic study of the FeMo protein of nitrogenase (Einsle, O.; et al. Science 2002, 297, 871). Introduction of Se in place of the S atoms in the cofactor and energy minimization results in an optimized structure very similar to that in the native enzyme. The nearly identical, short, lengths of the Fe-Fe distances in the Se and S analogues are interpreted in terms of M-M weak bonding interactions. DFT calculations with O or N as the central atoms in the FeMoco marginally support the assignment of the central atom as N rather than O. The assumption was made that the central atom is the N atom, and steps of a catalytic cycle were calculated starting with either of two possible states for the cofactor and maintaining the same charge throughout (by addition of equal numbers of H(+) and e(-)) between steps. The states were [(Cl)Fe(II)(6)Fe(III)Mo(IV)S(9)(H(+))(3)N(3-)(Gl)(Im)](2-), [I-N-3H](2-), and [(Cl)Fe(II)(4)Fe(III)(3)Mo(IV)S(9)(H(+))(3)N(3-)(Gl)(Im)], [I-N-3H](0) (Gl = deprotonated glycol; Im = imidazole). These are the triply protonated ENDOR/ESEEM [I-N](5-) and M?ssbauer [I-N](3-) models, respectively. The proposed mechanism explores the possibilities that (a) redox-induced distortions facilitate insertion of N(2) and derivative substrates into the Fe(6) central unit of the cofactor, (b) the central atom in the cofactor is an exchangeable nitrogen, and (c) the individual steps are related by H(+)/e(-) additions (and reduction of substrate) or aquation/dehydration (and distortion of the Fe(6) center). The Delta E's associated with the individual steps of the proposed mechanism are small and either positive or negative. The largest positive Delta E is +121 kJ/mol. The largest negative Delta E of -333 kJ/mol is for the FeMoco with a N(3-) in the center (the isolated form) and an intermediate in the proposed mechanism.     

Planarization of Tetracoordinate Carbon Atom. Synthesis of 13-oxa-14-oxo-pentacyclo [5.5.2.1.04,15010,15]pentadecane,a bridged ‘tetraquinacane’ preliminary communication

Structural aspects of planoid deformation of tetracoordinate carbon atom, particularly those in annulenes with central carbon atom are discussed. [12] Annulene 11 with central carbon atom is proposed as compound with strong planoid deformation around the internal carbon centre. The synthesis of a saturated precursor 15 is described.     

Isomerisierung der Bis(triphenylphosphin)platin(0)-Komplexe von Bicyclo[4.2.1]non-1-en und Bicyclo[4.2.1]non-1(8)-en

Isomerization of the Bis(triphenylphosphine)platin(0) Complexes of Bicyclo[4.2.1]non-1-ene and Bicyclo[4.2.1]non-1(8)-ene Isomrisation of the Bredt-Olefins 3 and 7 as the bis (triphenylphosphine)-Pt(0) complexes 1 and 8 , respectively, has been observed. The bridgehead-olefin 3 as well as the bicyclic olefins 4 , 5 and 6 have also been isomerized with catalytical amounts of (ethylene)bis(triphenylphosphine)platinum(0) ( 2 ).     

Semidirect MP2 gradient evaluation on workstation computers: The MPGRAD program

A semidirect implementation of the closed-shell MP2 gradient for efficient use on workstation computers is presented. The approach is based on the algorithm proposed by Frisch and coworkers but includes several modifications to reduce disk storage requirements and exploits nonabelian point group symmetry. The performance of the resulting program MPGRAD (BIOSYM Corp., San Diego, CA) is demonstrated in applications to the molecules [AlSi(CH₃)₃]₄ and ferrocene. The largest calculation involved 492 basis functions and was carried out on IBM RS/6000 workstations with memory sizes of 32 and 128 Mb. The ratio of CPU to wallclock time exceeds 90% in all typical applications. © 1993 John Wiley & Sons, Inc.