Role of the Triplet State and Protein Dynamics in the Formation and Stability of the Tryptophan Radical in an Apoazurin Mutant.

The protein, azurin, has enabled the study of the tryptophan radical. Upon UV excitation of tyrosine-deficient apoazurin and in the presence of a Co(III) electron acceptor, the neutral radical (W48•) is formed. The lifetime of W48• in apoazurin is 41 s, which is shorter than the lifetime of several hours in Zn-substituted azurin. Molecular dynamics simulations revealed enhanced fluctuations of apoazurin which likely destabilize W48•. The photophysics of W48 was investigated to probe the precursor state for ET. The phosphorescence intensity was eliminated in the presence of an electron acceptor while the fluorescence was unchanged; this quenching of the phosphorescence is attributed to ET. The kinetics associated with W48• were examined with a model that incorporates intersystem crossing, ET, deprotonation, and decay of the cation radical. The estimated rate constants for ET (6 × 10 s) and deprotonation (3 × 10 s) are in agreement with a photoinduced mechanism where W48• is derived from the triplet state. The triplet as the precursor state for ET was supported by photolysis of apoazurin with 280 nm in the absence and presence of triplet-absorbing 405 nm light. Absorption bands from the neutral radical were observed only in the presence of blue light.