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We have investigated the kinetics of light-induced defect (dangling bond) creation and annealing processes in a-Si:H containing a large amount of hydrogen at 300 K and 77 K using the ESR technique. We have obtained direct evidence for the light-induced annealing of dangling bonds at 300 K. A model, in which nonradiative recombination of electrons and holes at hydrogen-related dangling bonds is taken into account, is presented to interpret the experimental results.

The origin of two distinct electron paramagnetic resonance (EPR) spectra observed in Cr-doped Sn02 crystals, i.e. , (EPR)l and (EPR)», was investigated using the superposition model specifically developed for rutile-type crystals. Analytical results suggest that these spectra are due to the substitutional Cr'+ ions having a unique structure with respect to its local surrounding oxygen atoms. The (EPR)l spectrum showed that the four nearest-neighbor oxygen atoms on the (110) plane have moved from normal sites toward the Cr + ion by 0.072 A (3.5%) as a result of a reduction in the metal ionic radius. The (EPR)» spectrum showed the Cr + ion to be coupled with the vacancy of the two nearest-neighbor oxygen atoms on the [110]axis. In this case, the ratio of spin-Hamiltonian parameters takes the form E/D = -cos2a from the superposition model, where a is the angle between the [001] axis and the cation-ligand bonding axis in (110) plane. The theoretical value E/D = -0.205 was obtained from this simple relation, and was in good agreement with the experimental values.

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Pseudomonas stutzeri l-rhamnose isomerase (l-RhI) is capable of catalyzing the isomerization between various aldoses and ketoses, showing high catalytic activity with broad substrate-specificity compared with Escherichia coli l-RhI. In a previous study, the crystal structure of P. stutzeri l-RhI revealed an active site comparable with that of E. coli l-RhI and d-xylose isomerases (d-XIs) with structurally conserved amino acids, but also with a different residue seemingly responsible for the specificity of P. stutzeri l-RhI, though the residue itself does not interact with the bound substrate. This residue, Ser329, corresponds to Phe336 in E. coli l-RhI and Lys294 in Actinoplanes missouriensis d-XI. To elucidate the role of Ser329 in P. stutzeri l-RhI, we constructed mutants, S329F (E. coli l-RhI type), S329K (A. missouriensis d-XI type), S329L and S329A. Analyses of the catalytic activity and crystal structure of the mutants revealed a hydroxyl group of Ser329 to be crucial for catalytic activity via interaction with a water molecule. In addition, in complexes with substrate, the mutants S329F and S329L exhibited significant electron density in the C-terminal region not observed in the wild-type P. stutzeri l-RhI. The C-terminal region of P. stutzeri l-RhI has flexibility and shows a flip-flop movement at the inter-molecular surface of the dimeric form.