Conformers of the peptides glycine-tryptophan, tryptophan-glycine and tryptophan-glycine-glycine as revealed by double resonance laser spectroscopy

The peptides Trp-Gly, Gly-Trp and Trp-Gly-Gly were investigated by UV-UV and IR-UV hole burning spectroscopy. Solid samples of the three peptides were vaporised into an argon jet by laser desorption. The IR-UV spectra of different conformers of the peptides were assigned by comparison with the IR-UV spectra of tryptophan [Snoek et al., Phys. Chem. Chem. Phys., 2001, 3, 1819], the free peptide bond in N-acetyl tryptophan methyl amide [Dian et al., J Chem. Phys., 2002, 117, 10 688] and ab initio calculations performed at the DFT B3LYP 6-31G(d,p) level. Apart from an NH...NH2 interaction, the peptide backbone of one conformer of each dipeptide is unfolded. The second conformer of Gly-Trp shows a COOH...O=C hydrogen bond and the second conformer of Trp-Gly-Gly a hydrogen bond between the peptide backbone and the NH group of the indole ring.