Species involved in H-exchange at the reduced flavin position N(5) at its various stages of ionisation. (1) represents the exchange of cationic N(5). Species (2) involves exchange of neutral reduced flavin with H+ or H2O. (3) represent the modi of exchange of (mono-) anionic reduced flavin with H+, H2O or HO-. (4) and (5) are di- and tri-anionic reduced flavins that are likely to exchange with H2O.