Phosphorescence and optically detected magnetic resonance study of the tryptophan residue in human serum albumin.

The phosphorescence and optically detected magnetic resonance (ODMR) of the lone tryptophan residue in the protein human serum albumin (HSA) were studied. This residue shows a red-shifted phosphorescence and D-E triplet zero-field splitting frequency, which suggests that it is buried in a hydrophobic region of the protein, based on trends in triplet state properties of tryptophan residues in peptides and proteins, which have recently been observed by other workers. This conclusion is in agreement with the results of previous fluorescence quenching studies of HSA with hydrophobic quenchers. Also, the ODMR line width is consistent with the line widths usually obtained from buried tryptophans, and the ODMR line shifts and broadens when the protein is heat denatured with urea, in qualitative agreement with results observed with several other proteins. Phosphorescence wavelengths, decay times, and zero-field splittings are also studied as a function of binding of halides and acetate to HSA. The tryptophan triplet lifetime is shortened when bromide and iodide bind, apparently due to the external heavy atom effect; chloride and acetate have no effect on the phosphorescence lifetime. ODMR and phosphorescence measurements suggest that the degree of solvent exposure of the tryptophan does not change appreciably upon the binding of these anions.