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

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 [M+2]+· and [M+3]+ in addition to the anticipated [M+1]+, whereas ESI generated only [M+H]+ 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 [M+2]+· nor [M+3]+ more than the natural isotope abundance of 13C 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.