Accurate Molecular Mass Determination of Mycolic Acids by MALDI-TOF Mass Spectrometry

Abstract

Mycolic acids, major and specific long-chain fatty (C70-C90) acid components of the mycobacterial cell envelope, were analyzed for the first time using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry operating in a reflectron mode. The various types of purified mycolates from representative mycobacterial species were analyzed using 2,5-DHB as matrix, because less than 10 pmol of mycolates was sufficient to obtain well-resolved mass spectra composed exclusively of pseudomolecular [M + Na]+ ions consistent with the structures deduced from the chemical analytical techniques applied to these molecules. Examination of the MALDI mass spectra demonstrated that the chain lengths of the various mycolates correlated with the growth rate of mycobacterial strains. Although slow growers, such as Mycobacterium tuberculosis and Mycobacterium ulcerans, produced a series of odd carbon numbers (C74-C82) of alpha-mycolic acids, rapid growers synthesized both odd and even carbon numbers. In addition, the main chain of oxygenated mycolic acids from slow growers were four to six carbon atoms longer than the corresponding alpha-mycolic acids, whereas rapid growers elaborated oxygenated homologues possessing the same chain lengths as their alpha-mycolic acids. Furthermore, a comparative analysis of the crude fatty acid mixtures from a wild-type strain of M. tuberculosis and its isogenic mutant effected in the synthesis of oxygenated mycolates by MALDI mass spectrometry revealed structural differences between the alpha-mycolates from the two strains. Thus, this technique appeared to be a rapid and highly sensitive technique for the analysis of mycolic acids, not only by providing accurate molecular masses and new structural information, but also by both reducing sample consumption and saving time.