Infrared spectroscopy and structure of photochemically protonated biomolecules in the gas phase: a noradrenaline analogue, lysine and alanyl alanine.

A novel photochemical technique combined with mass spectrometry and resonant infrared multiphoton dissociation spectroscopy (R-IRMPD) has been used to record infrared vibrational spectra of the free protonated noradrenaline analogue, 2-amino-1-phenylethanol (APE-H(+)), the amino acid, lysine (Lys-H(+)), and the dipeptide, alanyl alanine (Ala-Ala-H(+)) in the gas phase. Coupling their spectra, obtained in the OH, NH and CH stretch regions, with ab initio calculations has allowed assignment of their preferred protonation sites and conformations. This simple technique will have wide applicability in future investigations of protonated biomolecular structure and conformation.