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

Abstract

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 changes not only in the excitation energies but also in the transition dipole moments and the rotational strengths of each excited state. The inclusion of the effect of water solvation also yields different response properties for each excited state, as well as different ground-state equilibrium geometries for the gas and solvated phases. MB weights change significantly from the gas to the solvated phase, making the effect of conformational averaging strongly phase dependent. The study of all these effects highlights the importance of an accurate and reliable treatment of both ground and excited states when aiming at predicting experimental one- or two-photon spectra. However, the comparison between the MB weighted spectra and experiment for the linear spectroscopies turns out to be rather satisfactory, showing that our approach can yield at least information on the major features of the spectra.

Differences in Two‐Photon and One‐Photon Absorption Profiles Induced by Vibronic Coupling: The Case of Dioxaborine Heterocyclic Dye

“…Recently, the long-range adapted Coulomb Attenuating Method-B3LYP (CAM-B3LYP) [26] proved to be well suited to reproducing some of the features of the spectra in OPA, TPA, and circular dichroism spectra of organic chromophores, in particular for transitions to Rydberg states or charge-transfer processes. [20,21,27,28] B3LYP appears on the other hand to be equally adequate, and in some cases performs even better than CAM-B3LYP for valence excitations, as observed in recent studies of linear spectroscopy [29] and also nonlinear spectroscopy of binaphthols [30] or of tryptophan. [28] In ref.…”

“…[20,21,27,28] B3LYP appears on the other hand to be equally adequate, and in some cases performs even better than CAM-B3LYP for valence excitations, as observed in recent studies of linear spectroscopy [29] and also nonlinear spectroscopy of binaphthols [30] or of tryptophan. [28] In ref. [28] in particular, the two functionals are shown to yield OPA and electronic circular dichroism spectra in reasonable agreement with experiment in the 180-300 nm region of absorption of tryptophan in water, when B3LYP is used to model the response of the lowest-energy region of the spectra, whereas CAM-B3LYP must be employed for the higher-lying excitations.…”

“…[28] In ref. [28] in particular, the two functionals are shown to yield OPA and electronic circular dichroism spectra in reasonable agreement with experiment in the 180-300 nm region of absorption of tryptophan in water, when B3LYP is used to model the response of the lowest-energy region of the spectra, whereas CAM-B3LYP must be employed for the higher-lying excitations.…”

“…It is nowadays generally recognized that spectral features for both one-and two-photon spectra-and, more generally, for optical response properties-can be strongly affected by conformational flexibility, in particular when, as in the linear and nonlinear dichroisms exhibited by chiral structures, [20,21,27,28] the observable arises from tiny differences between direct absorption profiles and it involves higher multipolar interactions. For the case of MH101, the comparison of the spectra from the two different C 2v and C 1 S 0 structures suggests that large-ampli-tude motions along low-frequency modes do not play a major role in the phenomena investigated (see the SI for further details).…”

Conformational Study of Cannabinoid Docking to Cannabinoid Receptor 1 (CB1) via Linear and Nonlinear Circular Dichroism

“…The polarizable continuum model (PCM) [32] was used to account for solvent (meth anol) effects. OPA spectra are reported in molar absorptivity [32][33][34],…”