Photoinduced Electron Transfer from Amino Acids and Proteins to 4-Nitroquinoline 1-Oxide in Aqueous Solutions

Article abstract of DOI:10.1021/j100285a038

The reactions of the triplet state of 4-nitroquinoline 1-oxide (4NQO) with a series of amino acids and some proteins in aqueous solutions have been studied by using a laser flash technique.Only triptophan (TrpH) and tyrosine (TyrOH) among a series of amino acids quench the triplet 4NQO (^TNQO) at a diffusion-controlled rate.Lysozyme, ribonuclease, and histone, which contain TrpH and/or TyrOH residues, had rate constants comparable to those of TrpH and TyrOH.The formation of the H adduct of 4NQO (4NQOH^.), which may be produced by the reaction of 4NQO^- with water, was confirmed from the transient absorption spectra for 4NQO solutions containing these quenchers.The transient absorption spectra observed for TrpH and TyrOH solutions elucidated the formation of the deprotonated forms of TrpH⁺ and TyrOH⁺ (Trp^. and TyrO^.) together with 4NQOH^..The result demonstrates that the elctron transfer from TrpH or TyrOH to ^T4NQO occurs in the triplet quenching by TrpH or TyrOH.Since the almost same transient spectra as Trp^. and TyrO^. were observed for lysozyme and ribonuclease solutions, respectively, TrpH residues on lysozyme and TyrOH residues on ribonuclease are main quenching sites, where elctron transfer and deprotonation occur.The quantum yields of ^T4NQO, 4NQOH^., Trp^., and TyrO^. produced by the excitation of the 4NQO solution containing TrpH or TyrOH with a 355-nm light pulse were determined to be 0.46, 0.47, 0.41, respectively.The result shows that the efficiency in electron transfer from TrpH or TyrOH to ^T4NQO is ca.90percent.For the reaction of ^T4NQO with methionine, arginine, histidine, lysozyme, or ribonuclease, the efficiency in electron transfer was also estimated to be nearly equal to that for the reaction with TrpH or TyrOH.