Coherent two-dimensional infrared spectroscopy : a study of coupled vibrations

Abstract

This thesis provides an introduction to experimental techniques used in two-dimensional (2D) infrared (IR) spectroscopy, outlines how third-order nonlinear response of a multi-level vibrational system is calculated, and provides a detailed methodology of line shape analysis in 2D spectroscopy. Specific emphasis is given to inherent sensitivity of 2D spectroscopy to correlated spectral broadening. The signatures of highly correlated transition energy fluctuations in a model system of two strongly coupled carbonyl stretching vibrations are reflected by the elongation of the cross peaks along the diagonal of the 2D spectrum. The dynamics of this correlation is monitored by the changes in the 2D line shapes and successfully modeled using a correlated spectral diffusion model. The sensitivity of 2D IR spectroscopy to interactions between multiple vibrational coordinates is also explored in conformationally complex polypeptides and proteins with well-defined secondary structures. 2D IR spectroscopy of β-hairpins and globular proteins with antiparallel (AP) β-sheet domains is studied to identify 2D markers of AP β-sheet conformation. The experiments on β-hairpins and proteins with varying percentage of β-sheet character showed that the formation of cross peaks between the two characteristic vibrational modes of AP β-sheets is a marker of AP β-sheet secondary structure. The intensity, location and line shapes of the cross peaks are qualitatively related to the size, geometry and the conformational variations in the AP β-sheet structure.