Isotopic effect of electron excitation in l-[3H]tryptophan

Isotopic effect of electron excitation in organic compounds was for the first time experimentally and theoretically analyzed. The nature of this phenomenon is due to zero-point energies (ZPE) for the ground and the excited states of the isotope substituted tryptophan. l-[3H]tryptophan (l-[3H]Trp) with a tritium-hydrogen substitution ratio of 0.2-0.45 has been produced by high-temperature solid-state catalytic isotope exchange and was used in an experimental study. A comparison of the UV spectra of l-[3H]Trp and l-[1H]Trp showed a short-wavelength shift of the absorption maximum in the tritium-substituted compound that corresponds to an energy of 130 cal/mol for the electronic transition. To account for this effect, quantum chemical calculations of the geometric and electronic structures, frequencies of normal vibrations, and transition energies have been performed using of the restricted Hartree-Fock method (RHF), density-functional theory (RB3LYP), and configuration interaction (RCIS). The shifts of all allowed electronic transitions have been shown to shift towards the short-wavelength region and to be equal to 152 cal/mol for the first singlet-singlet transition (within the 280 nm region), to 242 cal/mol for the third transition (within the 220 nm region), and to 557 cal/mol for the singlet-triplet transition. All the shifts obtained by calculations are due to zero-point energies for the ground and the excited states of [3H]Trp and [1H]Trp.