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We report a Series of measurements of lhe structural and electronic properties of the molecular conductor Cs[Pd(dmit),], (dmit = isotrithionedithiolate). This material has a C21c structure with stacks of the acceptor groups arranged in sheets separated by the cations, with appreciable dimerization of the acceptor groups along the stack direction. It shows metallic properties at room temperature. We have characterized this material with measurements of conductivity. thermopower. magnetic susceptibility and polarized optical reflectivity, and we find that the electronic structure is quite isotropic in the plane of the acceptor sheets, On cooling, there is a metal-insulator transition at 56.5 K, and this opens an energy gap at the Fermi energy. This transition is associated with a complex periodic lattice distortion comprising an incommensurate modulation and also a commensurate distortion, which breaks the Ccentringsymmetry. Wepresent acaldation oftheelectronic band structure using an extended Huckel formalism. and show that, owing to the strong dimerization along the stack direction. the Fermi energy lies halfway in a band of highest occupied molecular orbital character. We consider also how the observed structural instability is able to produce an energy gap over the whole Fermi surface, and propose a model that requires coupling between the two types of modulation.

The recognition between transcription factors and their DNA binding sites is a highly dynamic process. During transcriptional regulation, transcription factors must bind to or dissociate from their cognate DNA binding sites. The winged helix DNA binding motif is one of many highly conserved DNA binding motifs identified in transcription factors. Backbone dynamics has been studied on the 15N- and 2H-enriched winged helix family member Genesis. Our data show that the overall motions of the single domain Genesis are better described by more than two autocorrelation times (taum). Our data also demonstrate that Genesis shows structure specific conformation exchange characterized by Rex. Therefore, our results indicate that the structure of Genesis is highly dynamic and that secondary structure elements in Genesis have collective motions in the nanosecond to millisecond time scale. Since the winged helix DNA binding motif is highly conserved, this unique dynamic property observed in Genesis is also likely to be conserved in other winged helix family members and important in DNA binding.

Cellobiohydrolase II hydrolyses alpha- and beta-D-cellobiosyl fluorides to alpha-cellobiose at comparable rates, according to Michaelis-Menten kinetics. The stereochemistry, absence of transfer products and strict hyperbolic kinetics of the hydrolysis of alpha-cellobiosyl fluoride suggest that the mechanism for the alpha-fluoride may be the enzymic counterpart of the SNi reaction observed in the trifluoroethanolysis of alpha-glucopyranosyl fluoride [Sinnott and Jencks (1980) J. Am. Chem. Soc. 102, 2026-2032]. The absolute factors by which this enzyme accelerates fluoride ion release are small and greater for the alpha-fluoride than for the beta, suggesting that its biological function may not be just glycoside hydrolysis. Cellobiohydrolase I hydrolyses only beta-cellobiosyl fluoride, which is, however, an approx. 1-3% contaminant in alpha-cellobiosyl fluoride as prepared and purified by conventional methods. Instrumental assays for the various components of the cellulase complex are discussed.