Location of functional groups in mycobacterial meromycolate chains; the recognition of new structural principles in mycolic acids

Mycobacterial alpha-, methoxy- and keto-mycolic acid methyll esters were separated by argentation chromatography into mycolates with no double bond, with one trans double bond or with one cis double bond. Meromycolic acids were prepared from each methyl mycolate fraction by pyrolysis, followed by silver oxide oxidation, and analysed by high-energy collision-induced dissociation/fast atom bombardment MS to reveal the exact locations of the functional groups within the meromycolate chain. The locations of cis and trans double bonds, cis and trans cyclopropane rings, methoxy and keto groups, and methyl branches within the meromycolate chain were determined from their characteristic fragment ion profiles, and the structures of the meromycolic acids, including those with three functional groups extracted from Mycobacterium tuberculosis H37Ra, Mycobacterium bovis BCG and Mycobacterium microti, were established. Meromycolic acids with one cis double bond were structurally closely related to those with one cis cyclopropane ring, whereas the meromycolic acids with one trans cyclopropane ring were closely related to the corresponding meromycolic acids with one cis cyclopropane ring. A close relationship between methoxy- and keto-meromycolic acids was also implied. The relationship between the meromycolic acids with a trans double bond and the other meromycolic acids was not clearly revealed, and they did not appear to be immediate substrates for trans cyclopropanation.