PDBe – bringing structure to biology

S. Velankar, G. J. Kleywegt
2012 Acta Crystallographica Section A Foundations of Crystallography  
It is well known that, in the gas phase, biphenyl (BP) adopts a twisted conformation about the ring-ring C-C bond [1] and cyclobutane (CB) adopts a puckered conformation about its ring diagonal [2] . In both cases, though, both BP and CB can adopt higher-energy planar conformations in some crystal structures. These observations, particularly for BP, have led some scientists to infer that conformations observed in crystal structures are biased due to 'crystal packing effects' and that such
more » ... mations are not transferrable to other scientific applications. In the present contribution we compare molecular conformations of BP and CB derivatives as observed in crystal structures taken from the Cambridge Structural Database (CSD) [3] with their optimised geometries in the gas phase. Only ~16% of both BP and CB fragments are exactly planar in relevant crystal structures while >60% have conformations close to the lowest energy conformers in gas-phase. The planar conformers only occur when crystallographic and molecular symmetry elements coincide. Extensive lattice energy scans and crystal structure prediction calculations for parent BP and a CB derivative clearly show how, in very particular cases, higher conformational energies of BP and CB are almost exactly compensated by reductions in the overall lattice energies. Such situations, however, are very rare -due to the symmetry requirements-and only compensate for conformational energy differences of up to 10 kJ/mol. Crystal structure conformations are, therefore, good guides to conformational preferences in other phases [4].
doi:10.1107/s0108767312098285 fatcat:r6cmjcrq3vgxfoboyzfigctdsy