DESORPTION CHEMICAL-IONIZATION TANDEM MASS-SPECTROMETRY OF POLYPRENYLAND DOLICHYL PHOSPHATES

Negative-ion desorption chemical ionization (DCI) tandem mass spectrom etry was applied to the analysis of nanomole quantities of semisynthet ic polyisoprenyl phosphates, the chain length of which ranged from 7 t o 20 isoprene units. The DCI spectrum of all the compounds tested show the presence of independently generated ions [M - HPO3 - H](-), [M - H3PO2 - H](-) and [M - H3PO4 - H](-) resulting from the loss of a part of or the entire phosphate group of a polyisoprenyl-P. In tandem mass spectrometry, the [M - H3PO4 - H](-) fragment produces series of ions 68 mass units apart, indicative of the polyisoprenoid nature of a com pound. Studies with deuterated and alpha-saturated polyisoprenyl phosp hates demonstrated that fragmentations of the [M - H3PO4 - H](-) ion p roceed from both ends (alpha and omega) of a polyisoprenoid chain and may occur at either allylic (A) or vinylic (V) sites. Fragments of mas ses equal to [n x 68 - 1] and [n x 68 - 13] (where n is the number of isoprene units and 3 I n is less than the total number of isoprene res idues within a polyisoprenoid chain) comprise the alpha A and omega V series, respectively, and represent the most abundant ions in tandem m ass spectra of the [M - H3PO4 - H](-) fragment of polyprenyl phosphate s. alpha-Saturated dolichyl phosphates can be distinguished easily fro m corresponding polyprenyl phosphates not only on the basis of a 2-u s hift of the [M - H3PO4 - H](-) ion and the alpha series of fragments, but also because of the presence of an additional (A + 14) series of i ons 14 u heavier than fragments resulting from the allylic cleavages o f an alpha-saturated polyisoprenoid chain. Possible mechanisms of the collision-induced dissociation reactions of polyprenyl phosphates are discussed.