Infrared Population Transfer Spectroscopy of Cryo-Cooled Ions: Quantitative Tests of the Effects of Collisional Cooling on the Room Temperature Conformer Populations

The single-conformation spectroscopy and infrared-induced conformational isomerization of a model protonated pentapeptide [YGPAA + H]⁺ is studied under cryo-cooled conditions in the gas phase. Building on recent results (DeBlase, A. F.; J. Am. Chem. Soc. 2017, 139, 5481−5493), firm assignments are established for the presence of two conformer families with distinct infrared and ultraviolet spectra, using IR–UV depletion spectroscopy. Families (A and B) share a similar structure near the N-terminus but differ in the way that the C-terminal COOH group configures itself (cis versus trans) in forming H-bonds with the peptide backbone. Infrared population transfer (IR-PT) spectroscopy is used to study the IR-induced conformational isomerization following single-conformer infrared excitation. IR-induced isomerization is accomplished in both directions (A → B and B → A) in the hydride stretch region and is used to determine fractional abundances for the two conformer families (F_A = 0.65 ± 0.04, F_B = 0.35 ± 0.04, 2σ error bars). The time scale for collisional cooling of the room-temperature ions to T_vib = 10 K by cold helium in the octupole trap is established as 1.0 ms. Key stationary points on the isomerization potential energy surface are calculated at the DFT B3LYP/6-31+G­(d) G3DBJ level of theory. Using RRKM theory, the energy-dependent isomerization rates and populations are calculated as a function of energy. According to the model, the observed population distribution after collisional cooling is close to that of the 298 K Boltzmann distribution and is in near-quantitative agreement with experiment. On the basis of this success, inferences are drawn for the circumstances that govern the population distribution in the trap, concluding that, in ions the size of [YGPAA + H]⁺ and larger, the observed distributions will be near those at 298 K.