Carbon Chemical Shift Restraints

Carbon chemical shifts of the alpha and beta carbons of an amino acid have been found to correlate with the phi and psi backbone angles of that amino acid. This has been exploited to provide a refinement term for X-PLOR. Two grids of expected secondary shifts were created from 4 proteins for which $^{13}$C shifts and high resolution X-ray structures were available. These grids are arranged in 2 degree steps of phi and psi.

In the first step, a random coil chemical shift (determined by residue type) is subtracted from the observed chemical shift to get a secondary chemical shift. This secondary shift is compared to the expected secondary shift of the grid point closest to the phi, psi angles of the residue. The energy term is calculated from the difference of the observed secondary chemical shift and the expected one.

\end{displaymath} (23.1)

The reference is Kuszewski, et al. (1995).


Xplor-NIH 2023-11-10