390J. Am. Chem. SOC. with 0.37 unit of transcarboxylase in 20 p L containing KP, (250 mM, pH 7.0) and DTT (IO mM) at 25 OC. After 1 min the incubation was centrifuged through two spun-dry DE52 ( I mL wet) beds in sequence in the cold within 10 min. The 14C radioactivity and enzyme activity of the second filtrate were compared. This ratio was determined as a function of the MMCoA concentration used to label the enzyme: 0.05,0.1, and 0.25 mM. The limit, determined graphically, was 1280 cpm/unit of activity; i.e. 0.275 nmol of biotin could be carboxylated by MMCoA per unit of activity. This value is -40% greater than the highest value reported by Wood et a1.I0 on the basis of the counts of labeled biotin 1993,115, 390-396 incorporated and the highest enzyme activity that was obtained in the same assay. The difference can be attributed to loss during storage of the assay rate, which depends on the coordinated function of the two half-reactions, without loss of capacity for the MMCoA/propionyl CoA half-reaction.* Acknowledgment.Abstract: Prochiral selectivity of various hydrolytic enzymes (lipases and proteases) in organic solvents was investigated in transformations involving a 2-substituted 1,3-propanedioI or its diester. In two instances, a significant dependence of enzyme prochiral selectivity on the solvent was found: transesterification of diol 1 with vinyl butyrate catalyzed by Aspergillus oryzae protease in anhydrous solvents and hydrolysis of diester 3 catalyzed by Pseudomonas sp. lipase in hydrated organic solvents (monoester 2 was a product in both reactions). The latter process, where the p r o 4 selectivity of the enzyme varied from around 3 in some solvents to greater than 30 in others, was examined in more detail. A mechanistic model was proposed that predicted an inverse correlation between lipase's prochiral selectivity and solvent hydrophobicity, as well as particular effects of substrate structure variation and an additive on the prochiral selectivity; all these predictions were confirmed experimentally. Subtilisin Carlsberg lacked appreciable prochiral selectivity in either transesterification or hydrolysis reactions regardless of the solvent; this was rationalized by means of interactive computer modeling based on the X-ray crystal structure of this serine protease.

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A kinetic study is reported for the ene reaction between methyl acrylate as the enophile and 8-pinene, with aluminium chloride as catalyst, in a series of solvents varying in polarity. These experiments point to a transition state having pronounced zwitterionic character, in line with earlier suggestions in the literature. An unexpected and significant change of the rate constant with the initial concentrations is also found.