Nitrogen-15 nuclear magnetic resonance of flavins

Abstract

Ninety-nine percent 15N-enriched flavins were synthesized and their proton decoupled 15N resonances were observed. The enriched compounds were [1,3-15N]riboflavin, [1,3,5-15N]riboflavin, [1,3-15N]riboflavin 5''-phosphate, [1,3,5-15N]riboflavin 5''-phosphate, [1,3,5-15N]FAD, [1,3,5-15N]lumiflavin and [1,3,5-15N]lumichrome. By comparison of their spectra and from the nuclear Overhauser effect data each 15N resonance peak could be assigned to each 15N nucleus. The order of the chemical shifts well corresponds to that of the calculated .pi.-electron densities. The N-3 nucleus gives the most intense inverted peak and the N-5 nucleus a small noninverted peak. By changing pH from neutral to alkaline, the chemical shift and the intensity of signal were mostly affected in the N-3 resonance of riboflavin 5''-phosphate. The N-5 signal of FAD showed a fairly large downfield shift with the increase of temperature. These observations can be well interpreted by the chemical structure and the proposed conformation of riboflavin 5''-phosphate and FAD.