The rotational correlation time of a protein is a measurement of the time the molecule rotates through an angle of one radian, and is dependent on the size, shape, and dynamics of the molecule, as well as the bulk physical characteristics of the solvent. Thus, it is directly related to the volume and molecular weigth of the protein. Correlation times can be estimated from NMR spectroscopy in addition to other techniques ( 89BIO8972 , 91JACS4371 , and 94STR609 ). The determination of the rotational correlation time constitutes a primary requirement in the attempt at motion analysis in all molecular sites of the protein, starting from the experimental relaxation data obtained.

NMR Measurement

• NOE measurements in the region of extreme narrowing or outside from low-temperature experiments.
• The most frequently used method is based on the direct fitting of the R₂/R₁ ratios for a set of nuclear spins ( 89BIO8972 , 95JACS12562 , 95BIO5212 , 96BIO4867 , 99CONC51 , 98JPCA812-102 , 97JPCA3246-101 , 98JPCA10573 , 99JB133 , 99JB101 , 99JMR408-139 ). The ¹⁵N T₁/T₂ ratio is a good measure for the rate at which each NH bond vector reorients due to global tumbling and it can be used to define long-order in a protein with a significant rotational diffusion anisotropy. However, this approach does not distinguish between the effects of motional anisotropy and chemical exchange. In contrast to this, the product R1R2 attenuates the effect of motional anisotropy ( 02JACS1852 )
• Globally linked approach ( 99CONC51 , 89JACS4571 ) in which S² and te are fitted for each site for a series or grid of tm values and identifying the tm which minimizes a global error function.

• Local site treatment in which the correlation time is fitted locally for each site, and therefore additional relaxation parameters are required ( 92BIO5269 , 94JMRB211-105 ). A different approach has also been proposed ( 00JMR229-143 ).

• If molecular tumbling is assumed isotropic, the correlation time can be also determined from CSA-DD interference measurements ( 98JACS7905 ).This approach is independent to chemical exchange processes.

Important considerations

Once obtained, the correlation time can be held fixed, and S² and t_e can be obtained ina straightforward manner. However, because S² and te are strongly dependent on the correlation time, an inaccurate determination of tm can result in erronesouly fitted parameters.

The correlation time can be sensitive to the types of relaxation data included in the analysis. Thus, T1, T2 and NOE are routinely collected, but in some cases only T1 and NOE have been used.

The correlation time is related to diffusion coeficients and also to solvent accesible surface area ( 98JB177 ).

Values

Human Ubiquitin (globular protein of 76 residues): 4.1 ns at 300K (95JACS12562).
Major Cold-Shock Protein (CspA) (70 residues): 4.88 ns (98BIO10881).
rat microsomal cytochrome b₅ (98 residues): 5.0+-0.7 ns (98BIO12323).
Calcium-loaded parvalbumin (109 residues): 7.6 ns (98BIO9964)
Photoactive Yellow protein (121 residues): 6.4+-0.6 ns (98BIO12689)
GMH4CO (147 residues): 10.3 ns (98BIO10906)
CDK inhibitor p19^INK4d (166 residues):13.6+-1.1 ns (98JMB221-283)