The affinity of flavin semiquinones for certain aromatic compounds and disulfides

A potentiometric method has been used to determine the affinity of flavin-semiquinones for certain organic compounds of biological interest. Tryptophan shows little affinity for the semiquinone but serotonin shows a strong affinity for the semiquinone based upon previously reported affinities for the fully oxidized or fully reduced flavins. Tyrosine and disulfides, both of which have been implicated at the active centers of flavo-proteins, show a strong affinity for the semiquinones but almost no affinity for the fully oxidized flavins. Complexes with various other compounds have also been reported.