Conclusion

1
Atomic Refinement with Correlated Solid-State NMR
Restraints
Richard Bertram, Tom Asbury, Felcy Fabiola, J. R.
Quine, Timothy A. Cross and Michael S. Chapman.
Kasha Institute of Biophysics, Florida State
University, Tallahassee, FL 32306
The Dipolar/Chemical Shift Plot The
relationship between the dipolar coupling and
chemical shift can be seen when both are
projected to the frequency plane (Fig. 2).
Dipolar signs in regions A and B are resolvable,
limiting the area of degeneracy to region C.
Solid-State Refinement Solid-state refinement
includes the use of additional energy terms for
chemical shift and dipolar coupling. The
correlation technique provides a more precise
dipolar energy function, leading to better
refinements.
Introduction The dipolar coupling tensor in
solid-state NMR has a sign degeneracy which
hinders its use as a restraint for structure
refinement. It is possible to remove a large
fraction of these degeneracies by correlating the
anisotropic dipolar coupling data with
anisotropic chemical shift measurements. Using
synthetic data generated from x-ray crystal
structures of membrane proteins, we demonstrate
that incorporating this correlation can improve
the refinement of atomic structures.
Energy Landscape of Dipolar Coupling Fig.
5 compares the dipolar coupling energy landscapes
with and without chemical shift correlation.
Fig. 2 Frequency Plot of ? and ?
A random sampling of peptide plane orientations
with respect to the magnetic field shows that
most of the sign degeneracies can be resolved in
this manner (Fig. 3).
Fig. 5 Dipolar Coupling Energy Landscape
Refinement Results Atomic refinement was
performed with software developed and freely
distributed by FSU. It is located at
http//www.sb.fsu.edu/rsref/Distribution/software
_distribution.htm Results using the correlated
and non-correlated energy functions are shown in
Fig. 6.

Fig. 1 B Field in the Chemical Shift PAS
Fig. 3 Random Sampling of Peptide Plane
Orientations
Chemical Shift The chemical shift ? is an
asymmetric tensor which makes an angle ? with the
dipolar axis u. A chemical shift tensor is
defined for the peptide plane of each residue.
The magnetic field B can be expressed in this
reference frame (Fig. 1).
Data from Bacteriorhodopsin Fig 4 shows
synthetic data generated from the
Bacteriorhodopsin atomic structure. The number of
sign degeneracies is drastically reduced when
dipolar coupling data is correlated with chemical
shifts.
Dipolar Coupling The dipolar coupling tensor ?
reflects the angle between a covalently bounded
atom pair and the B field. However, dipolar
coupling measurements only give the magnitude of
?, not the sign.
Fig. 6 Refinement Data for Bacteriorhodopsin
Conclusion Correlating dipolar coupling and
chemical shift data helps resolve dipolar sign
degeneracies and improves atomic refinement.
Funding NIH grant PO1-GM064676 and FSU Center
of Excellence grant S-1303-0633
Fig. 4 Synthetic Bacteriorhodopsin Data