title: Polarization-angle-scanning two-dimensional spectroscopy: application to dipeptide structure determination.
abstract: Coherent two-dimensional optical spectroscopy based on a heterodyne-detected stimulated photon echo measurement technique requires four ultrashort pulses whose pulse-to-pulse delay times, wavevectors, and frequencies are experimentally controllable variables. In addition, the polarization directions of the four radiations can also be arbitrarily adjusted. We show that the polarization-angle-scanning two-dimensional spectroscopy can be of effective use to selectively suppress either all the diagonal peaks or a cross-peak in a given two-dimensional spectrum. Theoretical relationships between the transition dipole vectors of a given pair of coupled modes or quantum transitions and the polarization angle configuration making the corresponding cross-peak vanish are established. Here, to shed light into the underlying principles of the polarization-angle-scanning two-dimensional spectroscopy, we considered the amide I vibrations of various isotope-labeled dipeptide conformers and show that one can selectively suppress a cross-peak by properly controlling the polarization angle of a chosen beam among them. Once the relative directions of the amide I transition dipole vectors are determined using the polarization-angle-scanning technique theoretically proposed here, they can serve as a set of constraints for determining structures of model peptides. The present work demonstrates that the polarization-controlled two-dimensional vibrational or electronic spectroscopy can provide invaluable information on intricate details of molecular structures.
authors.affiliation: Department of Chemistry and Research Institute for Natural Sciences, Korea University, Seoul 136-701, Korea.
authors.forename: Jun-Ho
authors.initials: JH
authors.lastname: Choi
authors.forename: Minhaeng
authors.initials: M
authors.lastname: Cho
mesh_terms: D000577:Amides / Q000737:chemistry; D004151:Dipeptides / Q000737:chemistry*; D011487:Protein Conformation; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared
chemical_list: D000577:Amides; D004151:Dipeptides