Flavins mediate a wide variety of chemical reactions in biology. To learn how one cofactor can be made to execute different reactions in different enzymes, we are developing solid-state NMR (SSNMR) to probe the flavin electronic structure, via the N-15 chemical shift tensor principal values (delta(ii)). We find that SSNMR has superior responsiveness to H-bonds, compared to solution NMR. H-bonding to a model of the flavodoxin active site produced an increase of 10 ppm in the delta(11) of N5, although none of the H-bonds directly engage N5, and solution NMR detected only a 4 ppm increase in the isotropic chemical shift (delta(iso))Moreover SSNMR responded differently to different H-bonding environments, as H-bonding with water caused delta(11) to decrease by 6 ppm, whereas delta(iso) increased by less than 1 ppm. Our density functional theoretical (DFT) calculations reproduce the observations, validating the use of computed electronic structures to understand how H-bonds modulate the flavin's reactivity.