Wide‐angle X‐ray Solution Scattering Studies of Slow Correlated Motions in Proteins

Slow correlated motions in the submicrosecond to second range are particularly important for protein function but remain challenging to study. Wide‐angle x‐ray solution scattering (WAXS) generates information about the length scale of intramolecular motions difficult to obtain by other methods; is highly sensitive to these motions, being able to detect changes due to single amino acid replacements, ligand binding or alterations in solution conditions; and is sufficiently fast as to make practical the screening of hundreds of conditions/ligands/mutations to identify those that impact protein motions. Direct comparison of the results of WAXS and molecular dynamics (MD) is accomplished through introduction of the sigma‐r plot, a plot of the standard deviation of interatomic vector lengths as a function of that length ‐ derivable from WAXS data and MD trajectories. MD simulations generate extraordinary detail about protein trajectories; WAXS provides experimental measures of length scale of protein motion under a large number of conditions. Direct comparison of WAXS with MD will enable detailed molecular interpretations of WAXS data using the power of MD; while making possible the extrapolation of MD results to understand molecular behavior of a large number of mutants in multiple ligation states and solution conditions. Application of this approach to HIV protease provides new insights into the role of protein dynamics in its enzymatic function.