Double Ion Trap Laser Spectroscopy of Alkali Metal Ion Complexes with a Partial Peptide of the Selectivity Filter in K Channels─Temperature Effect and Barrier for Conformational Conversions.

Potassium ion channels selectively permeate K, as well as Rb and Cs to some degree, while excluding Na and Li. Conformations of alkali metal complexes of Ac-Tyr-NHMe, a model peptide of the selectivity filter in a K channel, were previously found to correlate with the permeability of alkali metal ions to a K channel by cold ion trap infrared spectroscopy. With an additional temperature-controlled ion trap, we examined the conformations of the alkali metal complexes, allowing the ions to collide with a He buffer gas at different temperatures, prior to spectroscopic investigation. The conformational distribution of the K-peptide complex shows the most significant variation with temperature, which suggests that this complex has more flexibility when complexed with K and suggests lower barrier heights than other metal-peptide complexes. The variability of the conformational distribution with temperature for the ions follows the same order of ion permeability of a K channel. This work demonstrates that the additional temperature-controlled ion trap is a powerful tool to explore the conformational landscape of flexible molecular systems.