Classical trajectory simulations of photoionization dynamics of tryptophan: intramolecular energy flow, hydrogen-transfer processes and conformational transitions

One-photon and two-photon ionization dynamics of tryptophan is studied by classical trajectory simulations using the semiempirical parametric method number 3 (PM3) potential surface in "on the fly" calculations. The tryptophan conformer is assumed to be in the vibrational ground state prior to ionization. Initial conditions for the trajectories are weighted according to the Wigner distribution function computed for that state. Vertical ionization in the spirit of the classical Franck-Condon principle is assumed. For the two-photon ionization process the ionization is assumed to go resonantively through the first excited state. Most trajectories are computed, and the analysis is carried out for the first 10 ps. A range of interesting effects are observed. The main findings are as follows: (1) Multiple conformational transitions are observed in most of the trajectories within the ultrafast duration of 10 ps. (2) Hydrogen transfer from the carboxyl group to the amino group and back has been observed. A zwitterion is formed as a transient state. (3) Two new isomers are formed during the dynamics, which have apparently not been previously observed. (4) Fast energy flow between the ring modes and the amino acid backbone is observed for both one- and two-photon ionization. However, the effective vibrational temperatures only approach the same value after 90 ps. The conformation transition dynamics, the proton-transfer processes and the vibrational energy flow are discussed and analyzed.