Chi1 torsion angle dynamics in proteins from dipolar couplings.

Experiments are presented for the measurement of one-bond carbon-proton dipolar coupling values at CH and CH2 ositions in 13C-labeled, approximately 50% fractionally deuterated proteins. 13Cbeta-1Hbeta dipolar couplings have been measured for 38 of 49 possible residues in the 63-amino-acid B1 domain of peptostreptococcal protein L in two aligning media and interpreted in the context of side-chain chi1 torsion angle dynamics. The beta protons for 18 of the 25 beta-methylene-containing amino acids for which dipolar data are available can be unambiguously stereoassigned, and for those residues which are best fit to a single rotamer model the chi(1) angles obtained deviate from crystal structure values by only 5.2 degrees (rmsd). The results for 11 other residues are significantly better fit by a model that assumes jumps between the three canonical (chi1 approximately -60 degrees, 60 degrees, 180 degrees ) rotamers. Relative populations of the rotamers are determined to within +/-6% uncertainty on average and correlate with dihedral angles observed for the three molecules in the crystal asymmetric unit. Entropic penalties for quenching chi1 jumps are considered for six mobile residues thought to be involved in binding to human immunoglobulins. This study demonstrates that dipolar couplings may be used to characterize both the conformation of static residues and side-chain motion with high precision.