The dipeptide cyclic(glycyltryptophanyl) in the gas phase: A concerted action of density functional calculations, S0–S1 two-photon ionization, spectral UV/UV hole burning and laser photoelectron spectroscopy

Abstract

The S₀–S₁ spectrum of the dipeptide cyclic(glycyltryptophanyl) (cGW) has been recorded by two-photon ionization (R2PI) spectroscopy of thermally evaporated, jet-cooled molecules. The R2PI spectrum contains several vibronic transitions with relatively small spacing. Density functional theory predicts more than 8 conformers. Applying spectral UV/UV hole burning spectroscopy we found that the strongest S₀–S₁ transitions belong to a single conformer. The transition at 35 058 cm⁻¹ is attributed to the S₁ origin and the other transitions to inter-ring modes. To obtain further information on the conformer structure, resonant two-colour two-photon ionization photoelectron spectroscopy (R(1 + 1′)PI PES) has been performed via the two most intense S₀–S₁ transitions. To our knowledge these are the first PE spectra of a dipeptide. The spectra are broad with a smooth onset. The lowest ionization onset lies at 7.709 eV, but is assumed to be not the adiabatic IE. Theoretical calculations with the B3LYP/6-311++G** theory predict that in the most stable neutral conformer an N–H group of the dipeptide ring binds to the indole π system. This structure has the highest IE of all conformers investigated and shows a strong geometry change upon ionization. The fact that the IE of the experimentally observed cGW conformer is higher than that of 3-methylindole is taken as a signature of a repulsion between the two rings in the cation: This could be explained by the N–H group of the peptide ring now interacting with the positively charged indole π system. The conformational assignment by R(1 + 1′)PI PES is tentative, because all of the lowest-energetic neutral conformers are expected to behave similarly in regard to ionization energies.