Photoionization of aromatic amino acid at different pH values

The photoioization of aromatic amino acid in solutions of different pH values were investigated by 248 nm laser flash photolysis. The results showed that the photoionizations of tyrosine and tryptophan were affected by pH value of solution, but phenylalanine was not. The ionization was in favor of monophotonic process at high pH value and of biphotonic process at low pH value. The mechanisms of pH effect on photoionization of tyrosine and tryptophan wee the deprotonation of phenolic hydroxyl group and the deprotonation of > NH on the indole ring respectively. Our results explain the discrepancy of values for the yield of hydrated electron obtained by other investigators.