The geometry and spin density distribution of the tyrosyl radical: A molecular orbital study

The oxidation of the amino acid tyrosine to the tyrosyl radical is now known to be important in many electron transfer reactions in biology. Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) have previously been used to obtain proton hyperfine couplings for the radical in vivo. This study uses AM1 molecular orbital calculations to provide a detailed insight into the geometry and electronic makeup of this important radical. Molecular orbital studies are first used to obtain an optimised geometry for the tyrosyl radical. This is shown to differ significantly from the unoxidised form. The extent of the singly occupied molecular orbital is then examined and a theoretical estimate of the unpaired electron spin distribution is obtained. This is then used to calculate the anisoptopic hyperfine coupling components for comparison with experimental determinations.