SIMILARITIES AND DIFFERENCES BETWEEN PHENOXYL AND TYROSINE PHENOXYL RADICAL STRUCTURES, VIBRATIONAL FREQUENCIES, AND SPIN-DENSITIES

Tyrosine phenoxyl radical (TyrO(.)) has been detected recently in a nu mber of proteins by comparing experimentally observed electron paramag netic resonance, UV resonance Raman, or Fourier transform IR vibration al spectra with the corresponding spectra for the organic phenoxyl rad ical (PhO(.)). Density-functional calculations are described to illust rate the strengths and limitations of the phenoxyl radical model for t he structures, electronic spin densities, vibrational frequencies, and vibrational modes of TyrO(.). Both the PhO(.) and TyrO(.) radicals di splay substantial C=O double bond character, whereas distances within the carbon ring are intermediate between distances observed for the co rresponding bonds of phenol and p-benzoquinone. The striking structura l similarity between the two radicals appears despite the proximity of the CO2H and NH2 groups located gauche to the phenoxyl side chain of TyrO(.) in the amino acid radical's most stable calculated gas-phase c onformation. Electronic spin densities calculated for the atoms of bot h PhO(.) and TyrO(.) agree well with experimentally derived spin densi ty ratios and display a pattern characteristic of odd-alternant hydroc arbons. Calculated spin densities for the two radicals differ from eac h other by less than 0.03, implying that the unpaired electron of TyrO (.) resides entirely on its phenoxyl side chain. Calculated, harmonic vibrational frequencies for both PhO(.) and TyrO(.) are within -3.3% t o +3.9% of experimentally determined frequencies. Most vibrational fre quencies and modes involving motions within the ring planes of PhO(.) and TyrO(.) are also very similar to each other. The largest frequency shifts upon replacing the hydrogen of PhO(.) with the peptide chain o f TyrO(.) can be attributed to two effects: (1) the different bonding and mass of the peptide chain compared to the hydrogen it replaces in PhO(.) and (2) interactions between the TyrO(.) peptide chain and its phenoxyl side chain. TyrO(.) modes with the largest mixing between pep tide chain and phenoxyl side chain motions are identified, as they, ar e likely to be most sensitive to TyrO(.) conformation and offer the be st potential for studying subtle conformational differences between Ty rO(.) radicals in different proteins. Calculated isotopic frequency sh ifts for TyrO(.)-d(7) and TyrO(.)-C-13(6) are also reported to aid in mode assignments. Furthermore, the (COeta)-O-zeta stretching mode is t he only mode of TyrO(.)-O-18(eta) and TyrO(.)-C--13(zeta) calculated t o appear above 1350 cm(-1) that displays a substantial isotopic freque ncy shift (-24 and -37 cm(-1), respectively). Thus, the (COeta)-O-zeta stretching mode may be identified by either O-18(eta) or C-13(zeta) i sotopic substitution experiment.