STRUCTURAL DETERMINATION OF LYSOPHOSPHOLIPID REGIOISOMERS BY ELECTROSPRAY-IONIZATION TANDEM MASS-SPECTROMETRY

The facile structural determination of lysophospholipid regioisomers r epresents a long-standing problem in phospholipid chemistry. Herein we report that positive-ion electrospray ionization (ESI) tandem mass sp ectrometry of sodiated lysophospholipid regioisomers results in the pr esence of multiple diagnostic pairs of product ions which allows the r apid and direct discrimination between sn-1-acyl- and sn-2-acyllysopho spholipid regioisomers. For example, after ESI in the positive-ion mod e and subsequent collision-induced dissociation, over a 30-fold differ ence in the peak intensity ratio of product ions at mit 104 and 147 wa s manifest with sodiated sn-1-acyllysophosphatidylcholine in compariso n to sn-2-acyllysophosphatidylcholine. The observed differences in pre cursor ion dissociation rates reflect both the kinetically favored for mation of a 5-membered phosphodiester (compared to the corresponding 6 -membered phosphodiester) and the accelerated rate of an activated vs a nonactivated rearrangement. The structure of individual molecular sp ecies and classes of lysophospholipids was substantiated after ESI in the negative-ion mode by tandem mass spectrometry which facilitated id entification of lysophospholipid aliphatic constituents and polar head groups. Thus, by exploiting the remarkable sensitivity of electrospra y ionization in conjunction with the combined utilization of tandem ma ss spectrometry in both the positive- and negative-ion modes, structur al determination of the specific regioisomer, individual molecular spe cies, and class of lysophospholipids is possible from picomole amounts of material.