Photoionization of aromatic amino acid at different pH values

The photoioization of aromatic amino acid in solutions of different pH valu es were investigated by 248 nm laser flash photolysis. The results showed t hat the photoionizations of tyrosine and tryptophan were affected by pH val ue of solution, but phenylalanine was not. The ionization was in favor of m onophotonic process at high pH value and of biphotonic process at low pH va lue. The mechanisms of pH effect on photoionization of tyrosine and tryptop han wee the deprotonation of phenolic hydroxyl group and the deprotonation of > NH on the indole ring respectively. Our results explain the discrepanc y of values for the yield of hydrated electron obtained by other investigat ors.