Computational Study of the One- and Two-Photon Absorption and Circular Dichroism of (l)-Tryptophan

A density functional theory (DFT) study of the one- and two-photon absorption and circular dichroism spectra of (l)-tryptophan in water is presented. The effects on the simulated spectra of conformational averaging, of solvent as described by the polarizable continuum model (PCM), and of the choice of exchange-correlation (XC) functional are analyzed. Conformational Maxwell−Boltzmann (MB) averaging is carried out at room temperature in the gas phase using the ten lowest-energy conformers in the gas phase, whereas in the solvent, the nine lowest zwitterionic conformers are determined in combination with a PCM continuum model and employed in the calculations. One- and two-photon absorption and circular dichroism spectra are calculated using time-dependent DFT with both the B3LYP and CAM-B3LYP XC functionals, including the 15 lowest excited electronic states in each case. The spectra are shown to be strongly influenced by all parameters of our computational models. Changing the XC functional yields large cha...