On the Photoionization Energy Threshold of Tryptophan in Aqueous Solutions

To investigate the existence and energy position of a photoionization threshold. tryptophan (Trp) has been photoionized in desecrated neutral aqueous or alcoholic solution under monochromatic light of variable frequency, in presence of N 2 O to scavenge the photoelectron. Present findings and some literature data converge to show the existence of a threshold for the one photon ionization process. This threshold is located at 4.5 ± 0.1 eV and 4.85 ± 0.1 cV for Trp in aqueous and ethanol solutions. respectively, which corresponds to a lowering with respect to the gas phase ionization potential of 3.4 and 3.0 eV. The photoionization quantum yields for Trp is found about 4 times greater at 250 nm than at Λ cx = 265 nm, where ϕe ‐ 4M =0.080±0.025. In such spectral range. at most one photoelectron out of 4–5 escaping geminate recombination would lead to Trp photodegradation in acrated solutions. These results also point out that the neutral radical Trp. which has been previously observed for Λ cx > 275 nm, i.e. below the ionization threshold energy—would not necessarily derive from Trp + deprotonation or cation‐electron dissociative recombination. Similarly, the opening of the indole ring with formylkynurenine (FK) formation which is observed under aerobic conditions and Λ cx >, 280 nm would not imply an electron attachment on O 2 but reactions such as Trp + 3 O 2 or Trp*+ 3 O 2 or else