th  - 1987 - 51st ENC Conference

WF22
ORIENTATION OF THE 15N CHEMICAL SHIFT TENSOR
IN A POLYCRYSTALLINE DIPEPTIDE
C.J. HartF.ell., T.K. Pratum and G.P. Drobny
Department o/Chemistr!t, Universitlj o/ Washington, Seattle, WA. 98195
ABSTRACT
In an effort to circumvent the ambiguities inherent in a determination of the
orientation of a chemical shift tensor in the molecular frame of a polycD'stalline solid:
we have pursued a study of the mutual orientation of three tensor interactions. In
this study of polycrystalline L-[1-13Ci - alanyl-lilSN ~-alanine we have oriented the lSN
chemical shift tensor in the molecul~ frame by reference to two dipole interactions.
The 13C-ISN and lSN-H dipole interactions axe probed using a modification of the
experiment of Stoll, Vega and Vaughan. The I H dipole-modulated 13C dipole-coupled
lSlN spectrum is obtained as a transform of the data in t2.
From simulations of the experimental spectra, two sets of polar angles are de-
termined relating the 13C-lSN and lSN-H dipoles to the IsN chemical shift tensor.
We have determined refined values for the polar angles Bc~ and c~c~ describing the
orientation of the 13c-lsN bond to the tensor. The angle ~3cx describing the angle
between 033 and the 13C- lSN bond is 106 = and the angle ac.~ describing rotation
about 033 is 5 c. The angle ~3~H is 19 =. This experiment verifies the results of the
13C dipole-coupled aSN powder spectra and simulations showing that or3 lies in the
peptide plane and o~2 is nearly perpendicular to the plane.