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Molecular mechanism of fluorescence quenching of flavins in flavodoxin from Desulfovibrio vulgaris, strain Miyazaki, and riboflavin binding protein from egg white has been investigated by means of picosecond laser photolysis technique. In the case of flavodoxin, a transient absorption band characteristic of the non-fluorescent exciplex formed by electron transfer from indole to excited flavins in model systems has been observed around 600 nm at the delay time of 33 ps from exciting ps pulse pulse width, 25 ps). In the case of riboflavin binding protein, the transient absorption spectra were different from those of flavin-indole exciplex and rather similar to the spectra of the model system of flavin-phenol. These results suggest that tryptophan residue exists near the isoalloxazine nucleus in flavodoxin, and in riboflavin binding protein, tyrosine residue exists near the flavin. Direct measurements of the ultrafast process of the electron transfer in flavoproteins as developed here could provide useful information for elucidating protein dynamics, associated with redox reaction, in the picosecond time region.

Stevioside, a constituent of Stevia rebaudiana, is commonly used as a non-caloric sugar substitute in Japan. The genetic toxicities of stevioside and its aglycone, steviol, were examined with seven mutagenicity tests using bacteria (reverse mutation assay, forward mutation assay, umu test and rec assay), cultured mammalian cells (chromosomal aberration test and gene mutation assay) and mice (micronucleus test). Stevioside was not mutagenic in any of the assays examined. The aglycone, steviol, however, produced dose-related positive responses in some mutagenicity tests, i.e. the forward mutation assay using Salmonella typhimurium TM677, the chromosomal aberration test using Chinese hamster lung fibroblast cell line (CHL) and the gene mutation assay using CHL. Metabolic activation systems containing 9000 g supernatant fraction (S9) of liver homogenates prepared from polychlorinated biphenyl or phenobarbital plus 5,6-benzoflavone-pretreated rats were required for mutagenesis and clastogenesis. Steviol was weakly positive in the umu test using S.typhimurium TA1535/pSK1002 either with or without the metabolic activation system. Steviol, even in the presence of the S9 activation system, was negative in other assays, i.e. the reverse mutation assays using S.typhimurium TA97, TA98, TA100, TA102, TA104, TA1535, TA1537 and Escherichia coli WP2 uvrA/pKM101 and the rec-assay using Bacillus subtilis. Steviol was negative in the mouse micronucleus test. The genotoxic risk of steviol to humans is discussed.

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The viscosities and densities at 15, 25, and 35 °C, and activity coefficients at 25 °C for the aqueous solutions of α- and γ-cyclodextrins were measured, and the results were interpreted in terms of solute-solvent and solute-solute interactions. The concentration and temperature dependences of viscosities and apparent molar volumes are almost similar to those of linear saccharides, i.e. maltose and maltotriose, indicating that α- and γ-cyclodextrins are structure forming solutes. However, the concentration dependences of activity coefficients of cyclodextrins are significantly different from those of maltose and maltotriose. The results were tentatively assigned to the formation of dimer in aqueous solutions.

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Activity coefficients of isomeric monosaccharides, d-glucose, d-mannose, and d-galactose were determined by isopiestic comparison method at 25 °C. Excess partial molar entropy of water (\barSwex) of each monosaccharide solution was negative and the absolute value increased with concentration of solute. From the magnitudes of negative \barSwex, it was concluded that the structure making ability among three isomers lies in the order d-glucose>d-mannose>d-galactose.