Fragmentation mechanisms of oxofatty acids via high-energy collisional activation.

Upon high-energy collisional activation, oxofatty-acid ions undergo fragmentations to produce a unique pattern of product ions by which the position of the ketone is revealed. The reactions occurring in the vicinity of the ketone, which are the subject of this article, produce a spectral pattern that is not symmetrical. Although cleavages of alpha, beta, and gamma C-C bonds occur on the side proximal to the charge site, giving alpha, beta, and gamma ions, respectively, there is only a gamma' ion formed on the side distal to the charge site. The resulting lack of symmetry seemingly contradicts the concept that the reactions are independent of the charge (i.e., that they are charge remote). To eliminate any interaction between the charge and the reaction site, oxofatty acids were linked to glycyrrhetic acid, a steroid with a rigid polycyclic system. The fragmentation pattern remains the same, indicating that the effect does not depend on charge but rather on the ketone. Isotopic labeling and MS/MS/MS studies confirm that the fragmentations of C-C bonds in the vicinity of the ketone are complex, charge-remote processes. Formation of [M-H-H2O]- and [M-H-CO2]- anions and the ion that is formed by homolytic cleavage of the beta bond at the side distal to the charge, however, are charge directed.