Photolysis ( λ = 254 nm) of phenylalanine in aqueous solution

Abstract

Phenylalanine (Phe) was photolysed ( λ = 254 nm) in deoxygenated and air-saturated aqueous solutions. The quantum yields of degradation (Φ( -Phe)) were found to be 5 × 10 −2 and 5.4 × 10 −2 respectively. In the absence of O 2, eight isomeric (3-alanyl)-2- exo-bicyclo[3,1,0]hex-3-en-2-ols (AHBs) wer were formed. They were isolated (four as two binary mixtures only) and their structures determined by nuclear magnetic resonance (NMR) spectroscopy. Their yield remained almost constant in the presence of O 2 ( Φ(total AHBs) = 4.2 × 10 −2 wherethe formation of three tyrosines ( Φ(total tyrosines) = 4 × 10 −3) and phenylacetaldehyde ( Φ = 5 × 10 −5 was also observed. In addition to these products, a short-lived ( t 1 2 = 2.1 product was formed which absorbed strongly at 305 nm. A good material balance between Φ(products) and Φ(-Phe) was obtained, suggesting that the unstable product must have a low quantum yield. In N 2O-containing solutions, N 2 was formed. It was concluded that the eight AHBs arise from the excited singlet state via unstable benzvalene intermediates. The N 2O experiments suggest that photoionization also occurs at this wavelength. In the presence of O 2 this may lead to the formation of tyrosines and phenylacetaldehyde.