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 
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.
![\begin{displaymath}
E_{CSHIFT}=K_{CSHIFT}[(CA_{calc}-CA_{obs})^2+(CB_{calc}-CB_{obs})^2].
\end{displaymath}](img739.png) |
(23.1) |
Subsections
Xplor-NIH 2025-03-21