A new method of identifying the site of tyrosyl radicals in proteins.

Protein-bound tyrosyl radicals catalyze many important enzymatic reactions. They can also initiate oxidative damage to cells. Here we report a new method of computer simulation of tyrosyl radical electron paramagnetic resonance spectra. The method enables the determination of the rotational conformation of the phenoxyl ring in a radical with unprecedented accuracy (approximately 2 degrees ). When coupled with a new online database, all tyrosine residues in a protein can be screened for that particular conformation. For the first time we show relationships between the spin density on atom C1 (rho(C1)) and the principal g-factors measured by electron paramagnetic resonance spectroscopy (rho(C1) on g(x) is shown to be linear). The new method enables the accurate determination of rho(C1) in all known tyrosyl radicals, evaluates the likelihood of a hydrogen bond, and determines the possibility of a rho(C1) distribution in the radicals. This information, together with the accurately determined rotational conformation, is frequently sufficient to allow for an unambiguous identification of the site of radical formation. The possibility of a similar relationship between rho(C) and g(x) in other radicals, e.g., tryptophanyl, is discussed.