STUDIES ON THE PHOTO DEGRADATION OF TRYPTOPHAN

— The photodegradation of tryptophan in aqueous solution increases steadily with decrease in wavelength over the U.V. range 370–230 mμ, but the action spectrum does not parallel the absorption spectrum over these wavelengths. Thus appreciable decomposition occurs at wavelengths greater than 300 mp, even though at the concentrations tested the indole ring does not absorb energy in this region, and a photosensitive auto-oxidation mechanism is suggested to account for this. The quantum efficiency for the destruction tryptophan at 265 mμ is 0 01 at pH 4, 5 and 7, but increases to 0.02 at pH 12. For the acetyl, ester and amide derivatives it is 0.03 in neutral solution. The yellowing of tryptophan solutions during irradiation is accompanied by the appearance of an absorption maximum at about 305 mμ in the differential absorption spectrum. The increase in optical density at this wavelength bears a direct relationship to the decrease in optical density at 280 mμ: the degree of yellowing is related to the degree of destruction of tryptophan. It is less pronounced in neutral solution than at higher or lower pH values. The yellowing of N-substituted tryptophan derivatives at 265 mμ is appreciably less than that of tryptophan, whereas derivatives with a free amino group are indistinguishable from the unsubstituted amino acid in this respect.