Evidence for a slow and oxygen-insensitive intra-molecular long range electron transfer from tyrosine residues to the semi-oxidized tryptophan 214 in human serum albumin: its inhibition by bound copper (II)

A slow, long range electron transfer (SLRET) in human serum albumin (HSA) is observed from an intact tyrosine (Tyr) residue to the neutral tryptophan (Trp) radical (Trp·) generated in pulse radiolysis. This radical is formed, at neutral pH, through oxidation with Br₂^·− radical anions of the single Trp 214 present. The SLRET rate constant of ~0.2 s⁻¹ determined is independent of HSA concentration and radiation dose, consistent with an intra-molecular process. This is the slowest rate constant so far reported for an intra-molecular LRET. In sharp contrast with the LRET reported for other proteins, the SLRET observed here is insensitive to oxygen, suggesting that the oxidized Trp is inaccessible to—or do not react with radiolytically generated O₂^·−. In N₂O-saturated solutions, the SLRET is inhibited by Cu²⁺ ions bound to the His 3 residue of the N-terminal group of HSA but it is partially restored in O₂-saturated solutions.