In the study of protein backbone dynamics by 15N relaxation measurements, an initial estimation of the isotropic global correlation time, τm, is usually obtained from the average T1/T2 ratio of nuclear spins that do not exhibit slow internal motion and with T2 values not significantly shortened by chemical or conformational exchange processes. Different methods have been used for identification of the rates of internal motion. However, the number of nuclear spins included in the τm estimation is often larger than the number that ultimately can be fitted to a single-order parameter, S2, implying that some nuclear spins involved in the initial τm estimation actually have an effective internal correlation time, τe, not as fast as assumed. As a consequence, τm is underestimated, since internal motion reduces the T1/T2 ratio. This situation becomes more obvious if the molecule has a large τm value because the reduction in T1/T2 ratio arising from internal motion is more significant than for molecules with smaller τm and the same degree of internal motion. This Communication describes a more reliable method for identifying nuclear spins which should be excluded from the τm estimation because of insufficiently rapid internal motion. This results in an improved τm value, giving a much better agreement between the number of nuclear spins fitted successfully to a single-order parameter, S2, and those used in the τm estimation.
|Number of pages||4|
|Journal||Journal of Magnetic Resonance|
|Publication status||Published - 1998|
- N relaxation measurements
- Global correlation time
- Protein dynamics
- Rate of internal motion