Picosecond dynamics of a membrane protein revealed by 2D IR

Fast protein dynamics can be missed with techniques that have relatively slow observation times. Using 2D IR spectroscopy and isotope labeling, we have probed the rapid, picosecond dynamics of a membrane protein in its native environment. By measuring the homogeneous and inhomogeneous IR linewidths of 11 amide 1 modes (backbone carbonyl stretch), we have captured the structural distributions and dynamics of the CD3 xi protein along its transmembrane segment that are lost with slower time-scale techniques. We find that the homogeneous lifetimes and population relaxation times are the same for almost all of the residues. In contrast, the inhomogeneous linewidths vary significantly with the largest inhomogeneous distribution occurring for residues near the N terminus and the narrowest near the center. This behavior is highly consistent with a recently reported experimental model of the protein and water accessibility as observed by molecular dynamics simulations. The data support the proposed CD3 xi peptide structure, and the simulations point to the structural disorder of water and lipid head-groups as the main source of inhomogeneous broadening. Taken together, this rigorous analysis of the vibrational dynamics of a membrane peptide provides experimental insight into a time regime of motions that has so far been largely unexplored.