Dear all:
I tried to back-calculate chemical shift anisotropy (CSA) from a model that's
generated by XPLOR-NIH but my results seem to be off.
In XPLOR-NIH model calculation the model is already aligned along the z-axis as
in AssignFit by:
vz=unitVec(oTensor.zAtom().pos() - oTensor.oAtom().pos())
vy=unitVec(oTensor.yAtom().pos() - oTensor.oAtom().pos())
vx=unitVec(oTensor.xAtom().pos() - oTensor.oAtom().pos())
AtomSel("all").apply( Rotate(transpose(Mat3(vx[0],vy[0],vz[0],
vx[1],vy[1],vz[1],
vx[2],vy[2],vz[2]))) )
sigma11 = 57.3, sigma22 = 81.2, and sigma33 = 227.8, sigma_iso = 122.1, so for
the input to "setsigma" we used, sigma_xx = 64.8, sigma_zz = -105.7 and
sigma_yy = 40.9. Beta = -17.
In XPLOR-NIH the CSA is calculated as
calcedShift = sum_{i,j} A_i sigma_j * cos(theta_{i,j})^2 / DaScale
I calculated it following "Application of solid-state NMR restraint potentials
in membrane protein modeling", and it's
sigma = sigma11 * e1z * e1z + sigma22 * e2z * e2z + sigma33 * e3z * e3z
I figured those two ways of calculating CSA should be equivalent, but what I
got from the latter way is different from what's given in the XPLOR-NIH model
viols file.
Is there something that's missing?
Thank you.
Jian
