Protonation effect on the electronic spectrum of tryptophan in the gas phase

Abstract

In this work we investigate how the presence of a proton changes the electronic spectrum of tryptophan (Trp). For the S₀–S₁ transition of protonated Trp gas phase results can be compared to theoretical calculations. Trp ions have been formed by electro-spray ionization, stored in a cooled Paul trap, mass-selected and dissociated by resonant photoexcitation by ns UV laser pulses. The S₀–S₁ spectrum shows a distinct onset, a gap and a broad rise at higher energies. Interestingly the absorption of protonated Trp is in the same energy range as the S₀–S₁ transition of neutral Trp. For calculation of the relative tautomer and conformer energies for the ground and the excited states a combined density functional theory/multi-reference configuration interaction approach was used. The proton is put into different positions: The 1 (1-H⁺–Trp) and the 3 positions in indole and the N terminal (N–H⁺–Trp). Protonation at the N terminal forms the most stable tautomer. Attaching the proton to position 1 of the indole chromophore results in a very high energy. When the proton is approached in position 3 of the indole chromophore a chemical reaction takes place and a bridged isomer is formed (BI–H⁺). This isomer lies only 400 cm⁻¹ above the N–H⁺–Trp form, but the activation barrier for the protonation is expected to be high. We calculated the excited states of the most stable neutral conformer, the neutral precursor conformer for the N–H⁺–Trp tautomer, the 1-H⁺–Trp and the N–H⁺–Trp tautomers and the BI–H⁺ isomer. The comparison of all S₀–S₁ transition energies shows that the S₁ state, here the L_b state, is not sensitive to the protonation, the site of protonation or even the isomerization. According to theory, the experimentally observed protonated Trp is most probably the N–H⁺–Trp tautomer. For this tautomer the agreement of theoretical and experimental S₀–S₁ energies is excellent (3.5 nm). Its energetic position of the L_a state is blue-shifted and strongly structure-sensitive in all protonated species.