Redox-Active Tyrosine Residues in Pentapeptides

Tyrosyl radicals are important in long-range electron transfer in several enzymes, but the protein environmental factors that control midpoint potential and electron-transfer rate are not well understood. To develop a more detailed understanding of the effect of protein sequence on their photophysical properties, we have studied the spectroscopic properties of tyrosyl radicals at 85 K. Tyrosyl radical was generated by UV-photolysis of pentapeptides in polycrystalline samples. The sequence of the pentapeptides was chosen to mimic peptide sequences found in redox-active tyrosine containing enzymes, ribonucleotide reductase and photosystem II. From EPR studies, we report that the EPR line shape of the tyrosyl radical depends on peptide sequence. We also present the first evidence for a component of the tyrosyl radical EPR signal, which decays on the seconds time scale at 85 K. We suggest that this transient results from a spontaneous, small conformational rearrangement in the radical. From FT-IR studies, we show that amide I vibrational bands (1680−1620 cm-1) and peptide bond skeletal vibrations (1230−1090 cm-1) are observed in the photolysis spectra of tyrosine-containing pentapeptides. From these data, we conclude that oxidation of the tyrosine aromatic ring perturbs the electronic structure of the peptide bond in tyrosine-containing oligopeptides. We also report sequence-dependent alterations in these bands. These results support the previous suggestion (J. Am. Chem. Soc. 2002, 124, 5496) that spin delocalization can occur from the tyrosine aromatic ring into the peptide bond. We hypothesize that these sequence-dependent effects are mediated either by electrostatics or by changes in conformer preference in the peptides. Our findings suggest that primary structure influences the functional properties of redox-active tyrosines in enzymes.