Structural determination of neutral gas-phase tripeptides Gly-Gly-Gly and Ala-Ala-Ala by IRMPD-VUV spectroscopy

This paper reports results from experimental and theoretical investigations of conformer preferences of Gly-Gly-Gly (Gly 3 ) and Ala-Ala-Ala (Ala 3 ) peptides in the gas phase. The experimental studies employed IRMPD-VUV spectroscopy carried out with the IR light of the free electron laser FELIX in the spectral regions of 1000 - 1850 cm − 1 and 3250 - 3700 cm − 1 . To interpret the experimental IR spectra, the theoretical calculations were performed, including a conformer search, DFT calculations, single-point CBS-4M energy calculations, and a transition state analysis. The obtained results suggest that both peptides have a similar lowest-electronic energy conformer which we denote as 5 N − /7 eq / g +( t ). This conformer is the predominantly populated Gly 3 species under our experimental conditions. However, for the Ala 3 case, this conformer is not observed experimentally, but rather the 5 N /7 eq / g -( t ) and 5 N /7 eq / t ( t ) conformers are concluded to be the most prominent species. For both peptides, the observed conformer populations are rationalized in terms of laser desorption temperature and inter-conformer relaxations happening in the supersonic jet.