Oxidised, formate-bound and fluoride-bound forms of E. coli cytochrome bo give rise to an electronic absorption band near 630 nm, diagnostic of high-spin ferrric haem o, whose position is sensitive to the nature of the bound anion. In all three forms, haem o remains spin-coupled to cuB(II), resulting in distinct broad X-band EPR signals. Those of formate-bound cytochrome bo are similar to the signals seen in slow cytochrome aa 3 but cannot be induced by incubation at acid pH suggesting that the endogenous earboxylate believed to be important in slow cytochrome aa 3 is not present in cytochrome bo. The oxidised form gives rise to novel EPR signals at g = 3.74 and g = 3.08 which have not been detected in cytochrome aa 3 and may arise from a weak magnetic coupling between high-spin haem o, S = 5/2, and Cub(ii), S = ½.
