, Volume 36, Issue 2, pp 79-102

Quantifying Lipari–Szabo modelfree parameters from 13CO NMR relaxation experiments

Rent the article at a discount

Rent now

* Final gross prices may vary according to local VAT.

Get Access


It is proposed to obtain effective Lipari–Szabo order parameters and local correlation times for relaxation vectors of protein 13CO nuclei by carrying out a 13CO-R1 auto relaxation experiment, a transverse \(^{13}\hbox{CO CSA}/^{13}\hbox{CO}-{^{13}\hbox{C}}\upalpha\) CSA/dipolar cross correlation and a transverse 13CO CSA/13CO–15N CSA/dipolar cross correlation experiment. Given the global rotational correlation time from 15N relaxation experiments, a new program COMFORD (CO-Modelfree Fitting Of Relaxation Data) is presented to fit the 13CO data to an effective order parameter \(S^{2}_{\rm CO}\) , an effective local correlation time and the orientation of the CSA tensor with respect to the molecular frame. It is shown that the effective \(S^{2}_{\rm CO}\) is least sensitive to rotational fluctuations about an imaginary \(\hbox{C}\upalpha-\hbox{C}\upalpha\) axis and most sensitive to rotational fluctuations about an imaginary axis parallel to the NH bond direction. As such, the \(S^{2}_{\rm CO}\) information is fully complementary to the 15N relaxation order parameter, which is least sensitive to fluctuations about the NH axis and most sensitive to fluctuations about the \(\hbox{C}\upalpha-\hbox{C}\upalpha\) axis. The new paradigm is applied on data of Ca2+ saturated Calmodulin, and on available literature data for Ubiquitin. Our data indicate that the \(S^{2}_{\rm CO}\) order parameters rapport on slower, and sometimes different, motions than the 15N relaxation order parameters. The CO local correlation times correlate well with the calmodulin’s secondary structure.