Measuring the chi 1 torsion angle in protein by CH-CH cross-correlated relaxation: a new resolution-optimised experiment.

Here we introduce an experiment with high sensitivity and resolution for the measurement of CH-CH dipolar-dipolar cross-correlated relaxation rates (CCRR) in protein side-chains. The new methodology aims to the determination of structural and dynamical parameters around the torsion angle chi(1) by measuring C(alpha)H(alpha)-C(beta)H(beta) cross-correlated relaxation rates. The method is validated on the protein ubiquitin: the chi(1) angles determined from the CCRR data are compared with the chi(1) angles of a previously determined NMR structure. The agreement between the two data sets is excellent for most residues. The few discrepancies that were found between the CCR-derived chi(1) angles and the angles of the previously determined NMR structure could be explained by taking internal motion into account. The new methodology represents a very powerful tool to determine both structure and dynamics of protein side-chains in only one experiment.