FAST-ATOM-BOMBARDMENT MASS-SPECTRA OF THE 13(2)-EPIMERS OF 13(2)-HYDROXYCHLOROPHYLL-A

Fast-atom bombardment (FAB) mass spectra have been measured for resolv ed 13(2)-hydroxychlorophyll a epimers. The fragmentation of each epime r was studied by tandem mass spectrometry using linked scans at consta nt B/E. The FAB spectra showed a very intense radical cation (M+.) and EI-type fragmentations. The most abundant fragment ions were formed b y the losses of the phytyl, the 13(2)-hydroxy, and the 13(2)-methoxyca rbonyl groups in various combinations. The fragment ions [M - OH]+, [M - phytadiene]+., and [M - phytadiene - OH]+ were found to be characte ristic of both 13(2)-hydroxychlorophyll a epimers. Nevertheless, it wa s shown that the loss of the 13(2)-hydroxy group occurs more easily fr om the 13(2)(S)-epimer than from the 13(2)(R)-epimer. This fragmentati on difference offers a new possibility for the determination of the st ereochemistry at the 13(2)-chiral centre of purified 13(2)-hydroxychlo rophyll preparations.