Reduction in Desorption Mass Spectrometry: Multiple Protonation on Flavins without Charge Increment.

Abstract

Three flavin-containing compounds, namely, riboflavin, riboflavin 5-phosphate (flavin mononucleotide, FMN) and flavin-adenine dinucleotide (FAD, Scheme 1), were studied using matrix-assisted laser desorption ionization (MALDI), fast atom bombardment (FAB) and electrospray ionization (ESI) mass spectrometry. It was found that the isoalloxazine ring (flavin) was reduced in the positive-ion MALDI and FAB ionization processes producing unusual ions [M2] and [M3] in addition to the anticipated [M1] , whereas ESI generated only [MH] as the molecular-related ion. The reduction mechanism is hypothesized as the protonation from the matrix with a concomitant electron transfer, 1 rather than the transfer of hydrogen atom(s), H. ESI, in which matrix is not used, did not produce [M2] nor [M3] more than the natural isotope abundance of 13 C in the sample molecules. The reduction site expands to N(1)C(10a)C(4a)N(5), a1,4-diaza-1,3-butadiene system, in the isoalloxazine ring, resembling the behavior of prosthetic groups of aerobic dehydrogenases.