IR-induced conformational isomerization of a helical peptide in a cold ion trap.

In this work, we use laser-induced population transfer techniques to study the conformational isomerization of a helical peptide, Ac-Phe-(Ala)5-LysH(+), in a cold ion trap. In one scheme, called IR-UV hole-filling spectroscopy, a single conformation is selectively excited with an IR pump laser via a distinct NH stretch vibration. After giving the vibrationally excited ions sufficient time to isomerize and re-cool in the trap, the new conformational redistribution is detected by UV photofragment spectroscopy. While we clearly observe a redistribution of the conformer populations due to isomerization, only those conformations that initially have population participate in this redistribution-we do not form conformers that were not initially present in the trap. In a second scheme, called IR-induced population transfer spectroscopy, we determine the fractional populations of the four stable conformations of Ac-Phe-(Ala)5-LysH(+) by scanning the IR laser while selectively detecting a specific conformation using UV photofragment spectroscopy.