AN ORIENTATION ON SOLVING SUPRAMOLECULAR COMPOUNDS BY VECTOR SEARCH. Randy Israel, Lourdes Infantes, Gezina Beurskens, Paul T. Beurskens, Crystallography Laboratory, University of Nijmegen, The Netherlands

One of the main principles of the DIRDIF program system is the application of direct methods to difference structure factors. This procedure makes optimal use of the phase information available when part of the structure is known. This known part may either consist of one or more heavy atoms or of a molecular fragment found by Patterson vector search techniques. Regarding this last method a suitable fragment can be obtained from the structural databases, molecular modelling or from own experience. Then the orientation of the fragment can be determined fully automatic by the program ORIENT [2]. Especially for supramolecular compounds, which not always are easily solved by ab-initio direct methods, and which usually lack heavy atoms, the vector search method proves to be very effecient. However, known fragments of these structures, are either relatively small or show to a larger extent flexibility, which at the end may lead to failure in solving the crystal structure. Therefore some essential parameters of ORIENT were reinvestaged and tested, resulting in a better way of scanning Patterson space in search of optimal overlap with the interatomic vectors of the search fragment and sorting Patterson vectors used.

[1] Beurskens, P. T., Admiraal, G., Beurskens, G., Bosman, W. P., Gelder, R. de, Israel, R. & Smits, J. M. M. (1994) The DIRDIF program system, Technical Report of the Crystallography Laboratory, University of Nijmegen, the Netherlands.

[2] Beurskens, P. T., Beurskens, G., Strumpel, M & Norman, C. E. 1987) In Patterson and Pattersons, Eds. J. P. Glusker, B. K. Patterson, and M. Rossi, Clarendon Press, Oxford, 356-367.