NMR determined Rotational correlation time

Introduction

The rotational correlation time of a protein in solution is the time for a protein to rotate one radian. In the limit of slow molecular motion (τ_c >> 0.5 ns), the correlation time of a protein is related to the ratio of the longitudinal (T₁) and transverse (T₂) ¹⁵N relaxation times, and nuclear frequency (ν_N) according to Eq. 1, which is derived from Eq. 8 in Kay et al. (Ref. 1) by considering only J(0) and J(ω) spectral densities and neglecting higher frequency terms.         

In practice, global ¹⁵N T₁ and T₂ relaxation times for an unknown protein target can be obtained on Bruker or Varian NMR sytems using 1D ¹⁵N-edited relaxation experiments (Ref. 2), by fitting integrated signal in the backbone amide ¹H region of the spectrum as a function of delay time to an exponential decay. One then computes the correlation time using Eq. 1, and compares it to a standard curve of τ_c vs. protein molecular weight (MW) obtained at the same temperature on a series of known monomeric proteins of varying size.  As a general rule of thumb, the τ_c of a monomeric protein in solution in nanoseconds is approximately 0.6 times its molecular weight in kiloDaltons. This approach is reliable up to MW ≈ 25 kDa, where accurate measurement of the diminishing ¹⁵N T₂ becomes problematic. 

Protocols for Bruker and Varian NMR Instruments

• Bruker protocol
• Varian protocol

References

1.    Kay, L.E., Torchia, D.A. and Bax, A. (1989) Backbone dynamics of proteins as studied by ¹⁵N inverse detected heteronuclear NMR spectroscopy:  Application to staphylococcal nuclease. Biochemistry 28, 8972-8979.
2.    Farrow, N.A., Muhandiram, R., Singer, A.U., Pascal, S.M., Kay, C.M., Gish, G., Shoelson, S.E., Pawson, T., Forman-Kay, J.D. and Kay, L.E. (1994) Backbone dynamics of a free and phosphopeptide-complexed Src homology 2 domain studied by ¹⁵N NMR relaxation. Biochemistry 33, 5984-6003.

-- JimAramini - 10 Nov 2009