The Tryptophan Microenvironments in Apomyoglobin

Abstract 1. The fluorescence properties of the 2 tryptophans of native sperm whale apomyoglobin suggest that they are in rather nonpolar environments but are partially exposed to solvent. 2. Apomyoglobin undergoes an abrupt unfolding in the pH region between 5.5 and 4, with loss of α helix content and increased exposure of the tryptophans. Below pH 4, unfolding, loss of α helix, and exposure of the tryptophans become even more pronounced. 3. The fluorescence yield, but not the excited lifetime, is increased significantly upon titration from pH 5.5 to 8.3, without an appreciable change in the size or shape of the protein. The most likely explanation is that at pH 5.5 the fluorescence of one of the tryptophan residues is quenched by a nearby histidine residue. Increasing the pH deprotonates the histidine residue and renders it nonquenching.