Experimental determination of microsecond reorientation correlation times in protein solutions

Reorientation correlation times in protein solutions are key determinants for feasibility and quality of NMR experiments. Yet, their accurate estimate is not easy, especially in the case of very large proteins. We show that nuclear magnetic relaxation dispersion (NMRD) can accurately determine reorientation times up to the microsecond range. A theoretical description for the analysis of the NMRD profiles is provided, and the protein reorientation time is shown to be provided by the longest correlation time among those needed to reproduce the experimental profile. Measurements are performed using samples of the archaeal proteasome double ring α7α7 and of αB-Crystallin in glycerol solutions.