Electron impact and chemical ionization mass spectral analyses of methyl esters species of free mycolic acid fraction from Rhodococcus lentifragmentus

Add time:07/28/2019    Source:sciencedirect.com

The free mycolic acid fraction from Rhodococcus lentifragmentus was derivatized to methyl esters and further fractionated into saturated (F0), monounsaturated (F1) and diunsaturated (F2) species using argentation-TLC. Methyl esters fractions F0, F1 and F2, accounting for approximately 7.4%, 53.1% and 39.5%, respectively, were analyzed by electron impact (EI) and chemical ionization (CI) mass spectrometries. According to EI-MS, peaks observed for M+-18, that were prominent compared to those representing M+ −32 and M+ −(18 + 32), indicated that the carbon chain size ranged from C36 to C48. The pyrolytic cleavage of methyl mycolates (R2CHOHCH(R1)COOCH3), following the McLafferty rearrangement released fragment ions corresponding to, (a) the α-subunit, representing the fatty acid methyl ester (R1CH2COOCH3), methyl hexadecanoate, methyl tetracanoate and methyl dodecanoate in decreasing order of relative intensity of peaks, and (b) the β-subunit, representing the meroaldehyde moiety (R2CHO). The saturated meroaldehyde species exhibited peaks representing meroaldehyde minus 18 mass units in which R2 ranged from C19H39 to C31H63. The monunsaturated species exhibited peaks representing the meroaldehyde in which R2 ranged from C19H37 to C31H61; peaks corresponding to meroaldehyde minus 18 mass units appeared only in the most abundant components, C29H57CHO, C27H53CHO, C25H49CHO and C31H61CHO, in a decreasing order of relative abundance. The diunsaturated species exhibited peaks essentially corresponding to meroaldehyde in which R2 corresponded to C31H59 and C2H55; the latter displayed a relative intensity that was about one-half compared to that of the former. Fractions F0, F1 and F2 showed a more intense pyrolytic fragmentation under CI-MS in contrast to results found under EI-MS. Therefore, peaks representing the α-subunit and the β-subunit were more prominent than the ones representing the fragmentation of the hydrocarbon chain. Moreover, the β-subunit of saturated species exhibited peaks corresponding to meroaldehyde plus hydrogen, and no dehydration of the β-subunit occurred in this case. In turn, the β-subunit of monounsaturated and diunsaturated species showed peaks representing both the meroaldehyde plus hydrogen and its dehydration product plus hydrogen. Thus, the presence of unsaturation in the meroaldehyde subunit of methyl mycolate facilitates appearance of dehydration fragment ions under chemical ionization procedure.

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