The Structure of Deprotonated Tri-Alanine and Its a3− Fragment Anion by IR Spectroscopy

We present the first infrared spectra of a mass-selected deprotonated peptide anion (AlaAlaAla) and its decarboxylated fragment anion formed by collision induced dissociation. Spectra are obtained by IRMPD spectroscopy using an FTICR mass spectrometer in combination with the free electron laser FELIX. Spectra have been recorded over the 800–1800 cm−1 spectral range and compared with density functional theory calculated spectra at the B3LYP/6-31++G(d,p) level for different isomeric structures. These experiments suggest a carboxylate anion for [M − H]− and an amide deprotonated (amidate) structure for the a3 fragment anion [M − H − CO2]−. The frequency for the amidate carbonyl stretch occurring around 1555 ± 5 cm−1 has been confirmed by additional spectroscopic studies of the conjugated base of N-methylacetamide, which serves as a simple model system for the deprotonated amide linkage in a peptide anion