PACKING OF BRUCINE CO-CRYSTALS
Robert O. Gould,1 Fokke J.J. Dijksma,1 Paul Taylor2 and Malcolm D. Walkinshaw,2
1Department of Chemistry,
The University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ,
Scotland
2Department of Structural Biochemistry,
The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JR,
Scotland.
The alkaloid brucine has long been used as a resolving agent,
mainly for chiral molecules capable of existing as anions,
because of its strong stereoselectivity in crystalline salt
formation. We will present a survey of some 30 structures
containing brucine in terms of their packing, showing the
environements which brucine normally provides. By far the most
important packing modes involve sheets separated by layers of
cations and water, these sheets being composed of parallel
ribbons of brucine molecules arranged "head-to-tail" as
in the figure.1 The unit mesh of these structures has
symmetry pg and a mesh of 12.3(3) A (along the ribbons) by
7.7(2) A (normal to the ribbons), these often being recognisable
in the cell dimensions of a crystal. The third cell dimension
depends on the size of the counterion as well as the layer
stacking. The most frequent type has antiparallel layers with a
cell repeat every two layers (ideal symmetry P212121).
A less common variant has parallel layers, ideal symmetry P21,
and a repeat every layer.
Another important mode has the sheets centered with respect to
one another, ideal space group C2221. Less
frequently encountered are structures based on
"head-to-head" ribbons and columnar structures.
1Dijksma, Gould, Parsons, Taylor and Walkinshaw, J. Chem. Soc. Chem. Commun., 1998, 745-746.