Contribution of protein conformational heterogeneity to NMR lineshapes at cryogenic temperatures
While low temperature NMR holds great promise for the analysis of unstable samples and for sensitizing NMR detection, spectral broadening in frozen protein samples is a common experimental challenge. One hypothesis explaining the additional linewidth is that a variety of conformations are in rapid equilibrium at room temperature and become frozen, creating an inhomogeneous distribution at cryogenic temperatures. Here we investigate conformational heterogeneity by measuring the backbone torsion angle ({Psi}) in E. coli DHFR at 105K. Motivated by the particularly broad N chemical shift distribution in this and other examples, we modified an established NCCN {Psi} experiment to correlate the chemical shift of Ni+1 to {Psi}i. With selective 15N and 13C enrichment of Ile, only the unique I60-I61 pair was expected to be detected in 13C-15N correlation spectrum. For this unique amide we detected three different conformation basins based on dispersed chemical shifts. Backbone torsion angles {Psi} were determined for each basin 114 {+/-} 7 for the major peak, and 150 {+/-} 8 and 164 {+/-} 16{degrees} for the minor peak as contrasted with 118 for the X-ray crystal structure (and 118-130 for various previously reported structures). These studies support the hypothesis that inhomogeneous distributions of protein backbone torsion angles contribute to the lineshape broadening in low temperature NMR spectra. Significance StatementUnderstanding protein conformational flexibility is essential for insights into the molecular basis of protein function and the thermodynamics of proteins. Here we investigate the ensemble of protein backbone conformations in a frozen protein freezing, which is likely a close representation for the ensemble in rapid equilibrium at room temperature. Various conformers are spectrally resolved due to the exquisite sensitivity of NMR shifts to local conformations, and NMR methods allow us to directly probe the torsion angles corresponding to each band of chemical shifts.