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Chemical Shift Anisotropy Restraint

The chemical shift anisotropy of $^{15}$ N, $^{13}$ C and $^{31}$ P can be measured in an orienting media and used to provide a restraint in the calculation of protein and DNA structures.

The predicted chemical shift is calculated as follows,

$\begin{displaymath} \delta \sigma = \sum_{i=\{x,y,z\}} \sum_{j=\{x,y,z\}} A_j \sigma_i \cos^2 \theta_{ij} \end{displaymath}$

(30.1)

where is the principal axis of the molecular alignment tensor, $\sigma_i$ is the principal axis of the traceless CSA tensor, and $\theta_{ij}$ is the angle between the corresponding principal axes.

The energy term is $E_{CSA}=K_{CSA} (\delta\sigma_{calc} - \delta\sigma_{obs})^2$ . See Lipsitz and Tjandra (2001) and Lipsitz and Tjandra (2003), as well as Wu, Tjandra, and Bax (2001).

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Xplor-NIH 2025-03-21