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π‐ELECTRON PHOTOCHEMISTRY OF DL‐PHENYLALANINE IN OXYGEN SATURATED AND OXYGEN FREE SOLUTIONS *
Author(s) -
Kenyon D. H.,
Blois M. S.
Publication year - 1965
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1965.tb09747.x
Subject(s) - phenylalanine , chemistry , oxygen , photochemistry , aqueous solution , quantum yield , benzoic acid , irradiation , organic chemistry , amino acid , biochemistry , physics , quantum mechanics , fluorescence , nuclear physics
— The photochemistry of Dhphenylakinine in dilute aqueous solutions has been investigated in detail by means of quantitative paper chromatography using C 14 ‐labelled material. Solutions were irradiated at the exit slit of a grating monochromator using a high pressure xenon arc as light source. The illuminating beam contained wavelengths from 2475Å to 2675 Å at the half‐power points with maximum intensity at 2575 Å By using six different chromatographic solvent systems, the following photoproducts have been identified: tyrosine, 3, 4‐dihydroxyphenylalanine (DOPA), aspartic acid, benzoic acid, phenyllactic acid and a high molecular weight melanic polymer. An additonal photo‐product derived from two of the side chain carbons of phenylalanine and having an R f value of 0.13 in n ‐butanol: acetic acid: water, 4:1:5 (v/v/v), has been detected in both oxygen saturated and oxygen free solutions, but has not yet been identified. All of these substances are stable solids at room temperature. Volatile or gaseous products could not be detected directly by the methods used in this investigation. Indirect evidence has been obtained that decarboxy‐lation is the dominant photochemical process in phenylalanine solutions irradiated both in the presence and absence of oxygen. The occurrence of phenyllactic acid in irradiated phenylalanine solutions shows indirectly that ammonia is liberated, but the quantum yield is about an order of magnitude smaller than that for carbon dioxide liberation. The kinetics of photoproduct accumulation and of phenylalanine loss have been determined under well‐defined conditions of incident light intensity, temperature, pH, dissolved oxygen, and phenylalanine concentration. These data were used to estimate quantum yields for the photoproducts and for phenylalanine loss after a specified dose of radiation at 2575Å under the given conditions. The results show that dissolved molecular oxygen greatly enhances the yields of tyrosine, DOPA, and the melanic polymer. Dissolved oxygen is not required for the synthesis of aspartic acid or phenyllactic acid. In the case of phenyllactic acid, water provides the necessary oxygen. Evidence is presented which suggests that the optical absorbance changes occurring during ultraviolet irradiation of phenylalanine are due in part to the formation of intermediates in melanin synthesis which cannot be detected by paper chromatography because they are too oxygen labile and occur in very low concentrations.