Pseudo Chemical Shift Anisotropy Restraint

Upon alignment of oligonucleotides in a magnetic field, the downfield TROSY component of the $^{13}$C-$^1$H doublet changes its resonance frequency as a result of residual $^{13}$C-$^1$H dipolar coupling (RDC) and residual $^{13}$C chemical shift anisotropy (RCSA), and the sum of these two second rank tensors is referred to as the pseudo-CSA. The experimentally measured difference in the resonance frequency of the $^{13}$C TROSY component in the aligned and isotropic samples is referred to as residual pseudo-CSA (RPCSA), and it can be used directly as a restraint during structure calculation. Because measurement of the RPCSA involves detection of the narrow TROSY $^{13}$C doublet component, it is applicable to systems with larger rotational correlation times than RDC measurement.

The target value for the restraint is the $^{13}$C chemical shift difference of the TROSY component between the aligned and isotropic samples. In principle, the code is applicable to any chemical shift tensor where one of the components (d33) is orthogonal to the plane specified by the 3 atoms that define the local molecular frame. In practice, the supported chemical shift tensors relate only to the H-linked base carbons within A-RNA and B-DNA for the following nucleotide types: A, C, U, G, dA, dC, dT, dG. The potential term is closely modelled on the chemical shift anisotropy term (Chapter 30), as the shift and the energy terms take the same form. This term calculates internally field-dependent pseudo chemical shift tensors and uses them in the data fit.

For more information see Grishaev, Ying, and Bax (2006).

Xplor-NIH 2023-11-10