J. Am. Chem. SOC.1995,117, 6083-6092
6083
Similarities and Differences between Phenoxyl and Tyrosine Phenoxyl Radical Structures, Vibrational Frequencies, and Spin Densities Yue Qin and Ralph A. Wheeler" Contribution from the Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Room 208, Norman, Oklahoma 73019 Received November 3, I994@
Abstract: Tyrosine phenoxyl radical (TyrO') has been detected recently in a number of proteins by comparing experimentally observed electron paramagnetic resonance, UV resonance Raman, or Fourier transform IR vibrational spectra with the corresponding spectra for the organic phenoxyl radical (PhO'). Density-functional calculations are described to illustrate the strengths and limitations of the phenoxyl radical model for the structures, electronic spin densities, vibrational frequencies, and vibrational modes of Tyro'. Both the PhO' and TyrO' radicals display substantial C-0 double bond character, whereas distances within the carbon ring are intermediate between distances observed for the corresponding bonds of phenol andp-benzoquinone. The striking structural similarity between the two radicals appears despite the proximity of the C02H and NHz groups located gauche to the phenoxyl side chain of T y r o in the amino acid radical's most stable calculated gas-phase conformation. Electronic spin densities calculated for the atoms of both PhO' and TyrO' agree well with experimentally derived spin density ratios and display a pattern characteristic of odd-altemant hydrocarbons. Calculated spin densities for the two radicals differ from each 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% to +3.9% of experimentally determined frequencies. Most vibrational frequencies 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 of 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 peptide chain and phenoxyl side chain motions are identified, as they are likely to be most sensitive to TyrO' conformation and offer the best potential for studying subtle conformational differences between TyrO' radicals in different proteins. Calculated isotopic frequency shifts for TyrO'-d, and T Y ~ O ' - ' ~are C ~also reported to aid in mode assignments. Furthermore, the CcOq stretching mode is the only mode of TyrO'-180V and TyrO'-l3C
