No abstract

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker's yeast was demonstrated.The world-wide trend to "natural" flavor and aroma has significantly increased interest in biogenetical pathways leading to volatiles in natural systems. For chiral compounds the exploration of potential biosynthetic routes is even more important, because chemical syntheses are often difficult and expensive; in many cases however sensory qualities of enantiomers are different (1-3). In our current studies of chiral volatiles in plant and microbial systems we use different analytical approaches, (a) Capillary gas chromatographic separations of diastereoisomeric derivatives are used to determine the configurations of chiral constituents at trace levels, (b) Chemically synthesized (labeled) precursors are added to fruit tissues and microorganisms. Their transformation into chiral constituents is investigated by means of capillary gas chromatography/mass spectrometry; the stereochemical course of these metabolizations is followed, (c) Enzymes catalyzing the stereospecific conversion of precursors to chiral compounds are isolated and characterized; commercially available enzymes are investigated as model systems to elucidate the stereochemical course of biogenetical pathways. The combination of these methods revealed some new aspects of the biosynthesis of chiral compounds in natural systems.