Theoretical Vibrational Spectroscopy of Proteins

Book excerpt: Theoretical Vibrational Spectroscopy of Proteins Lu Wang Under the supervision of Professor James L. Skinner At the University of Wisconsin-Madison Vibrational spectroscopy, such as linear and two-dimensional infrared (IR) spectroscopy, is widely utilized to study the structure and dynamics of peptides and proteins. Interpretation of the experiment, or a direct assignment of the complex experimental spectra to the underlying protein structure, can be difficult. Molecular dynamics (MD) simulations offer a complementary approach to provide high-resolution structural and temporal information of proteins, although they are limited by factors such as force field accuracy and are not directly comparable to spectroscopic experiments. We have developed vibrational frequency maps for proteins that generate instantaneous site frequencies directly from MD simulations. We combine the frequency maps with established nearest-neighbor frequency shift and coupling schemes and a mixed quantum/classical framework to form a theoretical strategy for calculating protein linear and 2D IR spectra in the amide I region. This theoretical method provides a means to bridge spectroscopic experiments and molecular simulations, which allows a critical assessment of MD simulations by comparison to experiment, and enables the interpretation of experimental spectra at the molecular level. In this dissertation, we present the development of the vibrational frequency maps and provide the theoretical protocol that allows the calculation of protein vibrational spectra directly from MD simulations. We validate the theoretical method by applying it to peptides with various secondary structures in aqueous solution, and apply it to a few biologically relevant problems. For instance, we have studied the thermal unfolding transition of the villin headpiece subdomain (HP36) using IR spectra calculations. We follow the unfolding process of HP36 by monitoring its spectral changes as a function of temperature. With the help of isotope labeling, we are able to capture the feature that helix 2 of HP36 loses its secondary structure before global unfolding occurs, in agreement with experiment. In collaboration with the Zanni group and the de Pablo group at University of Wisconsin, we have also carried out studies on IAPP, a peptide closely related to type 2 diabetes. By combining theoretical modeling with extensive computer simulations and spectroscopic experiments, we have investigated the structure and dynamics of IAPP in aqueous solution, in the fibril form and in the vicinity of lipid membranes.