Pseudo Chemical Shift Anisotropy Restraint
Upon alignment of oligonucleotides in a magnetic field, the downfield
TROSY component of the 
C-
H doublet changes its resonance
frequency as a result of residual C-H dipolar coupling
(RDC) and residual 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 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 C
doublet component, it is applicable to systems with larger rotational
correlation times than RDC measurement.
The target value for the restraint is the 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).
Subsections Xplor-NIH 2024-09-13