Molecular Structure Determination in Powders by Combined Molecular Modeling and

							                                 Molecular Structure Determination in Powders by
                              Combined Molecular Modeling and Proton Solid-State NMR

                 Bénédicte Elena, Guido Pintacuda, Nicolas Mifsud, Chris J. Pickard* and Lyndon Emsley.

                   Laboratoire de Chimie, UMR 5182 CNRS/ENS Lyon, Ecole Normale Supérieure de
                                      Lyon, 46 allée d’Italie, 69364 Lyon, France.

         * TCM group, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, UK CB3 0HE

         Proton spectroscopy of powdered organic molecules represents a potentially rich source of structural and
dynamic information. With the recent improvement in homonuclear dipolar decoupling sequences used under Combined
                                                       1
Rotation And Multiple Pulse (CRAMPS) conditions, H resolution has been significantly increased to access dipolar
couplings and thus structural information. Proton line-width less than 0.2 ppm (after correction for the decoupling scaling
factor) can be obtained at 700 MHz for crystalline powdered solids, using standard equipment and moderate magic-angle
spinning frequencies.

         We present the refinement of the three-dimensional structure of an organic compound, in powder form and at
natural isotopic abundance, obtained by an approach that combines molecular modeling with experimental proton spin
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diffusion data (PSD) obtained from high-resolution solid-state NMR of protons. Experimental H- H spin diffusion data can
be quantitatively analyzed using a simple framework, in terms of a rate matrix analysis for coherent magnetization
                                              1  1
exchange between sites. The investigation of H- H spin diffusion build-up curves using this approach for the dipeptide -
L-Aspartyl-L-Alanine shows that high resolution magic-angle spinning NMR of protons provides a method to probe
crystalline arrangements, and conformation and orientation of a molecule inside the crystalline unit cell.

         We show progress in the development of computational modeling tools based on the Xplor-NIH structure
                             1  1
calculation program to use H- H spin diffusion curves for crystal structure determination. We use Xplor-NIH to model the
periodic potential for the symmetry related NMR-equivalent molecules in an infinite crystalline environment. We have
integrated the full back-calculation of the spectrum from a trial structure, and the evaluation of the difference with respect
to experimental data. This is achieved using the Python scripting interface in Xplor-NIH, and the integration of a new
potential term coded in C++ that integrates local inter-proton distance restraints via back calculation of the proton spin
diffusion (PSD) behavior. This approach enables us to refine the molecular structure of -Asp-Ala at natural abundance
and in powder form to obtain a group of structures with an average rmsd of 0.1Å, and which deviates from the known
structure by only ~0.2Å. Finally, we show how the structures can be further improved by using ab initio calculations of the
proton chemical shifts.


                      a
                                                               Figure : (a) A set of 150 structures from the
                                                               ensemble of 3000 random structures generated for
                                                                -L-Aspartyl-L-Alanine as a starting point for
                                                               structural refinement. (b) The 16 structures
                                                               determined with the lowest EPSD values after the
                                                               optimization procedure using NMR-PSD.




                      b