This talk will describe recent results on the single-conformation infrared and ultraviolet spectroscopy of mass-selected, cryocooled ions in the gas phase. The apparatus used to carry out these measurements consists of a nano-ESI source, linear quadrupole ion traps for mass selection and detection, and a cryocooled ion trap where the ions are collisionally cooled to temperatures of 10 K. Single-conformation IR spectra are recorded using IR-UV double resonance, with the UV wavelength used to selectively excite and photofragment a particular conformer, and the IR absorption recorded as a depletion in this photofragment ion signal. The single-conformer infrared spectra of the D-Pro and L-Pro diastereomers of the protonated pentapeptides YADPAA and YALPAA, and their corresponding Gly-containing counterparts, YAPGA and YGPAA, will be described. The spectra show striking sensitivity to the chirality of the Pro residue, and to the additional flexibility imparted to the peptide by the presence of a Gly residue in place of Ala. In [YALPGA+H]+, conformers arising from two unique conformational families are observed. We use this prototypical circumstance to apply the technique of IR-induced population transfer spectroscopy, a method first introduced on neutrals1 and recently implemented by Rizzo and co-workers2 on ions. By introducing a delay between IR excitation and UV detection, the IR-excited ions can isomerize and re-cool, leading to a transfer of population following conformational isomerization. The gains and dips in IR signal can be used to determine the fractional abundances of the ions in the cryocooled ion trap. 1. Zwier, T. S., Laser probes of conformational isomerization in flexible molecules and complexes. J. Phys. Chem. A 2006, 110, 4133-4150. 2. Seaiby, C.; Zabuga, A. V.; Svendsen, A.; Rizzo, T. R., IR-induced conformational isomerization of a helical peptide in a cold ion trap. J. Chem. Phys. 2016, 144, 014304.

Single-Conformation spectroscopy and isomerization of cryocooled peptide ions

254th National Meeting (2017)

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