Structural determination of oxofatty acids by charge-remote fragmentations

A strategy is described to locate the carbonyl position in oxofatty acids by utilizing charge-remote fragmentations of various molecular ions that are desorbed by fast atom bombardment (FAB). Oxofatty acids were cationized with alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) to form [M + 2Met-H]+ or alkaline earth metal ions (Mg2+, Ca2+, Sr2+ or Ba2+) to from [M + Met-H]+ in the gas phase. The cationized acids undergo charge-remote fragmentations upon high-energy activation, giving a product-ion pattern that has a gap corresponding to the oxo position and bordered by two high-intensity peaks. One of the peaks corresponds to an ion that is formed by the cleavage of the C-C bond beta to the oxo position and proximal to the charge (beta ion), whereas the other is formed from the cleavage of the C-C bond gamma to the oxo position and distal to the charge (gamma' ion). The oxo position is easily determined by identifying the gap and the beta and gamma' ions. Furthermore, there are two competing patterns of fragments in a CAD spectrum of an oxofatty acid or ester [M + Li]+ ion. These arise because Li+ attaches to either the oxo or the carboxylic end, as was confirmed by ab initio molecular orbital calculations. The results demonstrate that control of the fragmentation can be guided by an understanding of metal-ion affinities. Collisional activation of the anionic carboxylates gives results that are similar to those for positive ions, showing that the process is not related to the charge status. Collisional activation of [M + H]+ ions does not give structural information because the charge migrates, leading to charge-mediated fragmentations.