DETERMINATION OF STRUCTURE AND PROPERTIES OF MODIFIED CHLOROPHYLLS BY USING FAST ATOM BOMBARDMENT COMBINED WITH TANDEM MASS-SPECTROMETRY

Tandem mass spectrometry (MS/MS) was used to investigate and compare the decompositions of radical cations (M+.), radical anions (M-.), [M + H]+ ions, and [M + Cat]+ ions (Cat = alkali metal ions) of chlorophylls. Included in this study are chlorophyll a, chlorophyll b, bacteriochlorophyll a, chlorophyll a allomers, and the corresponding pheophytins. Fast atom bombardment of chlorophyll a produces abundant M+. ions, which decompose metastably and upon collisional activation to give fragment ions from losses of the phytyl chain and the β-keto group of ring V. In addition, previously unreported charge-remote fragmentations are useful for identification of branch points on the phytyl chain. Collisional activation of [M + Cat]+ ions produces fragment ions that are complementary to those of the M+ and are used to examine the intrinsic gas-phase reactivity of metal ions and chlorophylls. Peripheral metal ion attachment in chlorophyll a in the gas phase is suggested to be at C-9, and the β-keto ester group at C-10, of ring V. Examination of decompositions of chlorophyll dimers suggests that in the gas phase the interaction between monomers involves bonding of the Mg atom of one chlorophyll a molecule and the C-9 carbonyl oxygen of the other, which was also suggested for chlorophyll a dimers in solution. © 1990 American Society for Mass Spectrometry.