Abstract
The longitudinal ^13^C spin relaxation times T~1~ and the ^13^C{^1^H} nuclear Overhauser enhancement were measured in a concentrated aqueous solution of the basic pancreatic trypsin inhibitor. The correlation time for overall rotational motions of the basic pancreatic trypsin inhibitor molecules was found to be τ~R~ ≈ 2 × 10^−8^ s. In connection with previous ^1^H n.m.r. studies of intramolecular motions of the aromatics, we were particularly interested in the correlation times τ~G~ for intramolecular segmental motions of the aromatic rings. The present experiments revealed no manifestation of intramolecular motions of the aromatics, indicating that τ~G~ ≫ 2 × 10^−8^ s for the aromatic ring carbon atoms. On the other hand, rapid segmental motions were evidenced for the peripheral carbon atoms of aliphatic amino acid sidechains. Comparison of the ^1^H and ^13^C n.m.r. data on the basic pancreatic trypsin inhibitor indicates that the time scale of high resolution ^1^H n.m.r. at high fields may in many instances be more appropriate for studies of the molecular dynamics in globular proteins than the time scale of spin relaxation measurements.