IR/UV spectra and quantum chemical calculations of Trp-Ser: stacking interactions between backbone and indole side-chain.

We present infrared-UV double resonance spectra and quantum chemical calculations of the natural di-peptide H-Trp-Ser-OH. Two conformers are present in the supersonic expansions. They have a compact folded structure with two hydrogen bonds and with the serin residue stacked above the indole ring and an unusual NH(Trp)...O=C interaction in the lowest energy conformer. Conformational assignments are based on comparison with calculated (B97-D/TZV2P) structures and vibrational frequencies. Inclusion of dispersion in the quantum chemical calculations is mandatory for an accurate description of the conformer energies. We provide comparisons between methods often used in biochemistry and the dispersion-corrected double hybrid functional (B2PLYP-D) as a reference. Only concerted experimental and theoretical studies can unravel the conformational complexity already present in a dipeptide.