Offset-free Measurement of Dipolar Couplings in a Single Crystal and Determination of Molecular Orientation

Abstract

Dipolar couplings are an important source of structure as they provide site specific dipolar splittings for aligned samples and hence are extensively used for the study of membrane proteins in lipid bilayers, liquid crystals and single crystals. Of the many Separated Local Field (SLF) techniques existing to avail this information for static oriented systems, PISEMA (Polarization Inversion Spin Exchange at Magic Angle) has found to have wide application due to its many favorable characteristics. However the pulse sequence suffers from its inherent sensitivity to proton resonance frequency offset. We have recently proposed a sequence named DAPT (Dipolar Assisted Polarization Transfer: S. Jayanthi et al. Chem. Phys. Lett. 439, 407, 2007.) for dipolar coupling measurement which is found to be insensitive to proton offsets over wide range. In this presentation, we report the first implementation of the sequence on rigid systems. Experiments were done on a single crystal of N-Acetyl DL-Valine (NAV). Dipolar couplings measured from the 2D DAPT spectrum is used for identifying the orientations of the magnetically in-equivalent molecules in the unit cell. SLF spectrum of NAV is complicated due to the presence of two magnetically in-equivalent molecules in the unit cell and with pairs of splitting for each C(alpha) - 1H and C(beta) - 1H. The molecular orientation has been identified by incorporating the crystal coordinates and constructing a three dimensional rotation matrix about an arbitrary axis using Rodrigues' rotation formula with the aid of a MATLAB program. Dipolar couplings measured from the spectrum is used for all the calculations. The algorithm proposed here is simple and hence can be easily accessible for various other studies aimed for structural elucidation.