By sulfur substitution of the N-5 atom in flavins and flavocoenzymes a flavin analog is obtained, Sthiaflavin, which is found to be isoelectronic and isosteric with natural flavin in the fully reduced and half-reduced states, but not in the oxidized state. Among the three ‘redox shuttles’ characterizing the flavin system, viz. upper le-, lower leand 2eshuttle, only the second one is retained in...