Separation and analysis of novel polyunsaturated mycolic acids from a psychrophilic, acid‐fast bacterium, Gordona aurantiaca

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Abstract
More than 30 molecular species of highly unsaturated mycolic acids, ranging from C60-C78 and possessing 2-7 double bonds, were obtained from a new genus of acid-fast bacteria, G. aurantiaca. They were fully separated and identified as their trimethylsilyl ether derivatives by a combination of silica gel TLC, argentation TLC and gas chromatography/mass spectrometry (GC/MS). On silica gel TLC, 2 adjacent spots, corresponding to mycolic acids possessing different structures of straight-chain and .alpha.-alkyl branch, were detected. The lower spot was separated by argentation TLC into 4 subclasses: monoenoic (including a small amount of saturated), dienoic, trienoic and tetraenoic mycolic acids ranging from C62-C74 and possessing a C16:0, C18:0 or C20:0 alkyl branch at the C-2 position. The upper spot was separated by argentation TLC into 5 subclasses: dienoic (including a small amount of monoenoic), trienoic, tetraenoic, pentaenoic and hexaenoic (heptaenoic) acids ranging from C64-C78 and possessing a C18:1 or C20:1 alkyl branch at the C-2 position. These types of mycolic acid structure differ from those reported previously in Mycobacterium and Nocardia in the numbers of both C atoms and double-bonds and the intermediate length of the .alpha.-alkyl branch. The characteristic polyenoic structure of the straight-chain alkyl unit was also confirmed by GC/MS analysis of the meromycolaldehydes obtained after pyrolysis of the methyl mycolates. The major aldehydes obtained from the lower-spot mycolic acids were C44, C46, C48, C50 and C52, centering at C50, while those from the upper-spot mycolic acids were C48, C50, C52, C54 ad C56, centering at C54. These aldehydes possess 2-4 double bonds in the lower-spot and 2-7 double bonds in the higher-spot mycolic acids, respectively. The physiological role of such highly polyunsaturated mycolic acids in psychrophilic acid-fast bacteria is discussed.