Photolysis of aromatic amino acids in aqueous solution by nanosecond 248 and 193 nm laser light

Abstract

An investigation of the photochemical properties of three aromatic amino acids, tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe) was carried out using 248 and 193 nm laser irradiation and high performance liquid chromatography analysis. The quantum yields of formation of hydrated electrons (Φ e − ) and decomposition of substrate (Φ d) were determined at room temperature in aqueous solutions at pH 6–7. From the dependences of Φ e − vs. the laser intensity I, we found that the mechanism of photoionization of the aromatic amino acids is monophotonic (single-step) for λ exc = 193 nm throughout and mainly biphotonic (two-step) for λ exc = 248 nm. In the latter case a minor single-step process contributes; the values for I→O are Φ e − = 0.05 (Trp), 0.03 (Tyr) and 0.01 (Phe), compared with those of 0.24 (Trp), 0.36 (Tyr) and 0.17 (Phe) at I = 1.2 × 10 11 W m −2. The Φ d, values under argon and in the presence of electron scavengers (O 2 and N 2O) are interpreted on the basis of photoionization as the dominant process for decomposition and a proposed reaction scheme. The efficiencies for decomposition per ionization event range from 0.3 for Trp under argon to about 2 for Phe under N 2O for λ exc = 193 nm. A comparison between our new and previous experimental results for Φ d in the presence of oxygen using 254, 266 and 213 nm irradiation was made. In addition, the threshold energy for photochemical induced splitting of water (dissociation) was found to be in the region 6.4–6.7 eV.