Interresidue distance measurements in proteins. Fluorescent energy transfer between tryptophans and a Ru(III)(NH3)5-histidine complex in alpha-lytic protease and lysozyme.

The mechanism by which the intrinsic fluorescence of tryptophan residues in alpha-lytic protease and lysozyme are quenched by a complex formed between the single histidine residue in each protein and Ru(III)(NH3)5 was investigated. The R0 values for alpha-lytic protease and lysozyme were 15.5 and 11.8 A, respectively. Good agreement between the efficiency of energy transfer measured experimentally and that calculated from the X-ray data, assuming the Förster dipole-dipole mechanism, demonstrates that this mechanism is appropriate. The ease with which the ruthenium-labeled enzymes can be synthesized and purified suggests that the Ru(III)(NH3)5-His complex may have general utility in structural studies of proteins in solution.