4-Thioflavins As Active Site Probes of Flavoproteins GENERAL PROPERTIES*

4-Thioflavins (oxygen at position 4 replaced by sul-fur) have been studied as potential active site probes of flavoproteins. They react readily with thiol re-agents, with large spectral changes, which should be useful for testing the accessibility of the flavin 4-posi-tion in flavoproteins. They have an oxidation-reduc-tion potential at pH 7 of-0.055 V, approximately 0.15 V higher than that of native flavins. The spectral char-acteristics in the fully reduced state show two clear absorption bands, dependent on the ionization state (pK = 4.5). The lowest energy band of the neutral dihydro-flavin has a maximum at-485 nm while that of the anion is-425 nm. This should be useful in defining the ionization state of the reduced flavin in flavoproteins. The spectral characteristics of the semiquinoid form