Abstract
Cytochrome oxidase exhibits phosphorescence from trytophan in aqueous solution in the absence of oxygen. The lifetime for the resting reduced enzyme suspended in Tween-20 is around 30 ms at pH 8. The lifetime is longest between pH 7 and 8 and decreases with lowering of pH. Oxygen quenches the phosphorescence with a Stern-Volmer quenching constant of ∼5 × 10 7 M −1 s −1 at 5°C whereas cytochrome c has no effect. We interpret these results to indicate that room temperature tryptophan phosphorescence arises from trytophan(s) in structured region(s) remote from the hemes and that the protein does not impose a significant barrier for the diffusion of oxygen.
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