Theoretical Studies of the Spectroscopic Properties of Tryptamine, Tryptophan and Tyrosine

In order to develop model parameters for simulating the spectroscopic properties of tryptophan and tyrosine in solutions and in proteins, we have carried out Hartree–Fock (HF) and configurational interaction (CI) calculations to study the electronic and spectroscopic properties of these fluorophores. Tryptamine, which is structurally related to tryptophan, has also been studied. The 3-21G basis set was used in most of the calculations and only single excitations were included in the CI calculations. Although calculations at this level of approximations have overestimated the transition energies, they have given transition moments within the range of values obtained from semi-empirical calculations and experimental measurements on indole and several of its derivatives. The calculated transition moments can be used in molecular dynamics simulations to study the fluorescence anisotropy decay (FAD) of these fluorophores. We have also estimated the natural atomic orbital charges of the ground and excited states of tryptamine, tryptophan, and tyrosine. These natural atomic orbital charges can be used in molecular dynamics simulations to study the ground- and excited-state dynamics of these fluorophores. The calculated charge redistribution because of optical excitations of tryptophan at different pHs were consistent with the experimental observations that deprotonated tryptophan had red-shifted absorption and emission spectra. Using AM1 adiabatic potential energy curves and a rotamer model employing an electron-transfer mechanism of fluorescence quenching, we have provided an interpretation of the observed pH-dependent fluorescence decay of tryptophan. The conformational and environmental sensitivity of the electronic and spectroscopic properties of these flurophores has also been examined.