Toward total structural analysis of cardiolipins: multiple-stage linear ion-trap mass spectrometry on the [M - 2H + 3Li]+ ions.

ESI multiple-stage linear ion-trap (LIT) mass spectrometric approaches for a near-complete structural characterization of cardiolipins (CLs), including identification of the fatty acyl substituents, assignment of the fatty acid substituents on the glycerol backbone, and location of the double-bond(s) or cyclopropyl group along the fatty acid chain are described. Upon collisionally activated dissociation (CAD) on the M - 2H + 3Li ions of CL in an ion-trap (MS(2)), two sets of fragment ions (designated as (a + 136) and (b + 136) ions) analogous to those previously reported for the M - 2H + 3Na ions were observed, leading to assignment of the phosphatidyl moieties attached to 1'- or 3'-position of the central glycerol. Further dissociation of the (a + 136) (or (b + 136)) ions (MS(3)) gives rise to the (a + 136 - R(1(or 2))CO(2)Li) (or b + 136 - R(1(or 2))CO(2)Li) ion pairs that identify the fatty acid moieties and their position on the glycerol backbone. This is followed by MS(4) on the (a + 136 - R(1(or 2))CO(2)Li) (or b + 136 - R(1(or 2))CO(2)Li) ion to eliminate a tricyclic glycerophosphate ester residue (136 Da) to yield the (a - R(1(or 2))CO(2)Li) ion, which is then subjected to MS(5). The MS(5) spectrum contains the structural information that locates the double-bond(s) or cyclopropyl group of the fatty acid substituents. Finally, the subsequent MS(6) on the dilithiated fatty acid ions generated from MS(5) also yields feature ions that confirm the assignment.