Full-wave multiple scattering and depolarization of electromagnetic fields from rough surfaces

Using the full wave approach, the single and the double scattered electromagnetic fields from deterministic one dimensionally rough surfaces are computed. Full wave expressions for the single and the double scattered far fields are given in terms of multidimensional integrals. These integrals are evaluated using a supercomputer*. Applying the steepest descent approximation to the double scattered fields expression, the dimension of the integrals are reduced from four to two in the case of one dimensionally rough surfaces. It is shown that double scatter in the backward direction is significant only for near normal incidence when the rough surface is highly conducting and its mean square slope is very large. Even for one dimensionally rough surfaces, depolarization occurs when the reference plane of incidence is not parallel to the local planes of incidence and scatter. A geometrical optics approximation is used to interpret the results of the doubly scattered fields for normal incidence near backscatter. The physical interpretation of the results could shed light on the observed fluctuations in the enhanced backscatter phenomenon as the angle of incidence increases from near normal to grazing angles. For surfaces with large mean square slopes, it is found that the double scattered fields are large in the near specular direction. The results show that multiple scattering strongly depends upon the slope of the rough surface, the angle of incidence, and the conductivity of the rough surface. The analysis is extended to the two dimensional deterministic rough surfaces. Full wave expression for double scatter from the two dimensional surfaces is given in terms of four dimensional integrals. This work provides physical insight to problems of scattering from random rough surfaces. Two approaches for multiple scatter cross sections from random rough surfaces are presented in this work. In the first approach the double scatter cross section expression reduces to a product of two single scatter cross sections. This is because the stationary phase points on the surface which contribute mostly to the double scatter are assumed to be far and statistically independent. In the second approach the surface is regarded to be a random distribution of realistic models of rough surface scatterers with different mean square height and slope, width and mean depth. The full wave results of single and multiple scatter from random rough surfaces are consistent with experimental results. ftn*IBM/3090 Cornell National Supercomputer Facility, Cornell Theory Center, Cornell University

Demethylation and degradation of phenylmethylethers by the sulfide-methylating homoacetogenic bacterium strain TMBS 4

Biochemical studies on anaerobic phenylmethylether cleavage by homoacetogenic bacteria have been hampered so far by the complexity of the reaction chain involving methyl transfer to acetyl-CoA synthase and subsequent methyl group carbonylation to acetyl-CoA. Strain TMBS 4 differs from other demethylating homoacetogenic bacteria in using sulfide as a methyl acceptor, thereby forming methanethiol and dimethylsulfide. Growing and resting cells of strain TMBS 4 used alternatitively CO 2 as a precursor of the methyl acceptor CO for homoacetogenic acetate formation. Demethylation was inhibited by propyl iodide and reactivated by light, indicating involvement of a corrinoid-dependent ethyltransferase. Strain TMBS 4 contained ca. 750 nmol g dry mass - 1 of a corrinoid tentatively identified as 5-hydroxybenzimidazolyl cobamide. A photometric assay for measuring the demethylation activity in cell extracts was developed based on the formation of a yellow complex of Ti 3 § with 5-hydroxyvanillate produced from syringate by demethylation. In cell extracts, the methyltransfer reaction from methoxylated aromatic compounds to sulfide or methanethiol depended on reductive activation by Ti 3 +. ATP and Mg 2 + together greatly stimulated this reductive activation without being necessary for the demethylation reaction itself. The specific activity of the transmethylating enzyme system increased proportionally with protein concentration up to 3 mg ml- 1 reaching a constant level of 20 nmol min -a mg -1 at protein concentrations > 10 mg ml- 1. The specific rate of activation increased in a non-linear manner with protein concentration. Strain TMBS 4 degraded gallate, the product of sequential demethylations, to 3 acetate through the phloroglucinol pathway as found earlier with Pelobacter acidigallici

Magnetosome Formation in Prokaryotes

Magnetotactic bacteria were discovered almost 30 years ago, and for many years and many different reasons, the number of researchers working in this field was few and progress was slow. Recently, however, thanks to the isolation of new strains and the development of new techniques for manipulating these strains, researchers from several laboratories have made significant progress in elucidating the molecular, biochemical, chemical and genetic bases of magnetosome formation and understanding how these unique intracellular organelles function. We focus here on this progress