Structure and Dynamics of Proteins and Peptides Revealed by Two-dimensional Infrared Spectroscopy

Book excerpt: Understanding the structure and dynamics of proteins is essential to understanding their roles and functions in these physiological processes. In this thesis, I describe the implementation of an ultrafast nonlinear spectroscopic technique, two-dimensional infrared (2D IR) spectroscopy to probe the structure and dynamics of ion channels and amyloid fibers. Regarding ion channels, I describe the combination of semisynthesis, 2D IR spectroscopy and molecular dynamic (MD) simulations in addressing the longstanding question of ion permeation through the selectivity filter of a potassium ion channel. I show that ions and water alternate through the filter and that these ions cannot occupy adjacent binding sites. Furthermore, 2D IR experiments revealed a flipped state that is predicted by MD simulations but not observed in x-ray crystallography. In another aspect of this work, we show that the collapsed state of the filter is structurally different in low K+ and low pH. Moreover, our work also reveals how the large conformational motions of the protein are coupled to structural changes in the selectivity filter, as evidenced by a change in the ion occupancy. In a second research direction, I developed an optical technique to quantify photoactivatable fluorophores with fluorescence microscopy. This technique allows for the quantification of a limitless number of fluorophores, and corrects for stochastic events such as fluorescence intermittency. This work can be extended to the study of amyloids, where determining the number of proteins in a prefibrillar aggregates is necessary for understanding their roles in amyloid related diseases. Finally, using 2D IR spectroscopy we describe the effect of common solvents on the anharmonicity of small molecule chromophores. The data indicates that the carbonyl anharmonicity, and, subsequently, the Stark tuning rate, is an intrinsic property of the carbonyl vibrational probes, which have important implications on the interpretation of carbonyl vibrational frequency shifts in the condensed phase.