χ₁ 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 positions in ¹³C-labeled, ∼50% fractionally deuterated proteins. ¹³C^β−¹H^β 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 χ₁ torsion angle dynamics. The β protons for 18 of the 25 β-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 χ₁ angles obtained deviate from crystal structure values by only 5.2° (rmsd). The results for 11 other residues are significantly better fit by a model that assumes jumps between the three canonical (χ₁ ≈ −60°, 60°, 180°) 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 χ₁ 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.