Optically detected magnetic resonance study of tyrosine residues in point-mutated bacteriophage T4 lysozyme

Two spectroscopically distinct types of tyrosine (Tyr) residues in triply point mutated bacteriophage T4 lysozyme, which contains no tryptophan (Trp), have been detected by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy. Their triplet states are characterized by similar E but different D values. The Tyr site which exhibits the lower D value and has the red-shifted phosphorescence origin is quenched by energy transfer to Trp and has D and E values comparable to previously studied Tyr residues. The blue-shifted Tyr site, which is not quenched by Trp, exhibits a larger D value that has been found previously. Calculation of energy-transfer efficiencies of Tyr-Trp pairs based on the crystal structure of the native enzyme provides a possible assignment of Tyr sites to the two different spectral types.