Negative ion fast atom bombardment mass spectrometry of isoprenoid diphosphates and related analogs

Abstract

Negative ion fast atom bombardment mass spectrometry (FABMS) was used to obtain mass spectra of the naturally occurring isoprenoid metabolites isopentenyl diphosphate ( 1), dimethylallyl diphosphate ( 5), geranyl diphosphate ( 3), farnesyl diphosphate ( 6), and presqualene diphosphate ( 8). Mass spectra were also recorded for several synthetic analogs, including methylene-bridged diphosphonates 9 and 10, bisdiphosphates 12–14, and nitrogen-containing analogs 15–18. The ammonium salts of these materials gave molecular ions at [M-H] − for the acid forms (M) of the diphosphate esters. Spectra of the sodium salts of isopentenyl diphosphate ( 1) and geranyl diphosphate ( 3) showed predominant peaks at m z 267 [M + Na-2H] − and 335 [M + Na-2H] −, respectively, along with several diagnostic fragment ions at m z 177 (H 3P 2O 7), 159 (HP 2O 6), and 97 (H 2PO 4) from the diphosphate moiety. The molecular ion of isopentenyl diphosphate ( 1) ( m z 245) was readily observed with 4 μg of compound. Thus, detection at the microgram level should be possible for disphosphates whose molecular ions do not overlap with peaks from the matrix. These studies indicate that negative ion FABMS will be useful for the analysis of isoprenoid diphosphate intermediates in biosynthetic and enzyme mechanism studies.