Monitoring the folding of Trp-cage peptide by two-dimensional infrared (2DIR) spectroscopy.

Protein folding is one of the most fundamental problems in modern molecular biology. Uncovering the detailed folding mechanism requires methods that can monitor the structures at high temporal and spatial resolution. Two-dimensional infrared (2DIR) spectroscopy is a new tool for studying protein structures and dynamics with high time resolution. Using atomistic molecular dynamics simulations, we illustrate the folding process of Trp-cage along the dominant pathway on the free energy landscape by analyzing nonchiral and chiral coherent 2DIR spectra along the pathway. Isotope labeling is used to reveal residue-specific information. We show that the high resolution structural sensitivity of 2DIR can differentiate the ensemble evolution of protein and thus provides a microscopic picture of the folding process.