RESUMEN
We report the gas phase conformational preferences of laser desorbed Ala-Ala dipeptides probed by action spectroscopy using the IRMPD-VUV method. The molecules were internally cooled through collisional cooling in a supersonic jet environment. An IR spectrum was obtained experimentally in the spectral range of 700-1850 cm-1, and subsequently interpreted with the help of quantum chemical calculations. Although theory predicts that folded structures have lower electronic energies and, thus, are more stable at low temperatures compared to their extended (ß-strand-like) counterparts, analysis of the experimental data concluded the extended conformer to be the most dominant. An explanation to this observation is discussed in this paper and rationalized in terms of collisional conformer relaxation processes occurring in the supersonic jet molecular beam.
RESUMEN
In this article, we report the results of gas-phase IR spectroscopy of neutral glycylglycine (Gly-Gly) in the 700-1850 cm-1 frequency range. A combination of laser desorption, jet-cooling, and IR multiple-photon dissociation vacuum-ultraviolet (IRMPD-VUV) action spectroscopy is employed, together with extensive quantum chemical calculations that assist in the analysis of the experimental data. As a result, we determined that the most favorable conformer in the low-temperature environment of the supersonic jet is the nearly planar structure with two C5 hydrogen-bonding interactions. Calculations clearly show that this conformer is favored because of its flexibility (considerable entropy stabilization) as well as efficient conformer relaxation processes in the jet. To gain more understanding into the relative stability of the lowest-energy Gly-Gly conformers, the relative strength of hydrogen bonding and steric interactions is analyzed using the noncovalent interactions (NCI) approach.