Assessment of protein rotational diffusion by 13C off-resonance rotating frame spin-lattice relaxation: effect of backbone and side-chain internal motion.

The 13C off-resonance rotating frame spin-lattice relaxation technique is applicable to the study of protein rotational diffusion behavior in a variety of experimental situations. The original formalism of James and co-workers (1978) (J. Am. Chem. Soc. Vol. 100, pp. 3590-3594) incorporated random isotropic reorientational motion of a rigid spherical rotor with no provision for backbone or side-chain carbonyl group internal motion. Here we demonstrate that the failure to include such internal motion may lead to erroneous rotational correlation time determinations for overall reorientational motion. The effect becomes severe for protein molecular masses in excess of 100 kD. Inclusion of both backbone and side-chain carbonyl carbon internal motion, using reasonable parameters derived from the literature [R. Levy and M. Karplus (1979), Chemical Physics Letters, Vol. 65, pp. 4-11; G. Careri, P. Fasella, and E. Gratton (1975), Critical Reviews in Biochemistry, Vol. 3, pp. 141-164; G. Lipari, A. Szabo, and R. LEvy (1982), Nature, Vol. 300, pp. 197-198], plus corrections for anisotropic tumbling [C. F. Morgan, T. Schleich, G. H. Caines, and D. Michael (1990), Biopolymers, Vol. 29, pp. 469-480] and microscopic viscosity [S. H. Koenig (1980), ACS Symposium Series, Vol. 127, pp. 157-176], leads to reliable values for the correlation time describing overall protein reorientation up to molecular masses of approximately 1000 kD.