Oxygen quenching and fluorescence depolarization of tyrosine residues in proteins

We examined the dynamics of oxygen quenching and depolarization of tyrosine fluorescence in small peptides and proteins lacking tryptophan. The oxygen-quenching constants and the apparent correlation times for fluorescence depolarization were found to be sensitive to the conformational state of the proteins. For small peptides and random coil proteins, the oxygen bimolecular quenching constants indicated complete accessibility of the tyrosine residues to collisions with oxygen. For folded proteins, the quenching constants were about 2-fold smaller, indicating only limited shielding of the tyrosine residues from oxygen by the protein matrix. We also used the steady state anisotropies, measured under conditions of oxygen quenching, to estimate the motional freedom of the tyrosine residues. For random coil proteins, such as a tyrosine copolymer and histones at low pH, the data clearly indicated that depolarization occurs due to subnanosecond segmental motions of the tyrosine residues which are independent of overall protein rotation. For some folded proteins, including bovine pancreatic trypsin inhibitor, the data are consistent with, but do not unambiguously demonstrate, motional freedom of the residues. In these cases, energy transfer among tyrosine residues may also contribute to the observed depolarization. Overall, these results indicate that the rate and extent of tyrosine rotation in proteins depend upon the conformation of the protein and the specific protein under observation.