, Volume 36, Issue 2, pp 79-102

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

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Abstract

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.