Determination of Rotational Correlation Times of Proteins in Solution from Carbon-13 Spin-Lattice Relaxation Measurements. Effect of Magnetic Field Strength and Anisotropic Rotation

We examine the effect of magnetic field strength and the use of the isotropic rigid rotor model (when rotational motion is anisotropic) on the accuracy of rotational correlation times (τ_R) of native proteins in solution determined from measurements of ¹³C spin-lattice relaxation times (T₁) of α carbons. If τ_R is about 10^-8 s, then small experimental errors in measured T₁ values at 14 kG can result in large errors in τ_R. We compare τ_R values obtained (with the use of the isotropic model) from T₁ measurements at 14.2 and at 63.4 kG. For horse myoglobin, the two magnetic field strengths yield essentially identical τ_R values. In the case of human hemoglobin, the difference between the two τ_R values is slightly outside the estimated experimental error. For the relatively nonspherical protein molecules hen egg-white lysozyme and bovine serum albumin, the τ_R values obtained at 14.2 kG are about twice as long as those obtained at 63.4 kG. The use of the axially symmetric ellipsoid model for the analysis of α-carbon relaxation data is examined. This model does not provide a significantly better agreement between the correlation times obtained at 14.2 and 63.4 kG (for lysozyme and albumin) than the isotropic model.