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

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Abstract
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, 10688] and ab initio calculations performed at the DFT B3LYP 6-31G(d,p) level. Apart from an NHNH2 interaction, the peptide backbone of one conformer of each dipeptide is unfolded. The second conformer of Gly-Trp shows a COOHOC 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.