The cells are related. You double 2 axes, giving 4x the P2 volume (not double). You gain a C (translational centering in the ab plane) so you have 2x the asu content in C2. Pointless tells you about the space group based on the indexed reflections, so you might want to look at the images to decide whether the P2 indexing simply missed a (commensurate) superstructure with t=~(1/2, 1/2, 0). You can probably distinguish a modulation in the zonal reflections h+k=2n.
Stuff like this happens frequently, with minimal translocations switching from one crystal form to the other. Minor changes in crystallization conditions (additives?) might stabilize one or the other. Also note that superstructure detectability is a matter of resolution. Deviations from a perfect translational symmetry (crystallographic) to NCS (loss of the element) are more significant at high resolution, where the indexing may pick up the extra spots but not at low resolution, where the symmetry still looks (almost) perfect (or, coming from the other side, not detect merging problems and assign higher symmetry). The transition from crystallographic symmetry to related NCS and vice versa is a continuous one and often hard to detect. Best, BR -----Original Message----- From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Markus Heckmann Sent: Thursday, November 9, 2017 2:28 AM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] unit cell is double between 2 forms Dear all, we crystallized a small protein, that gives crystals P2 with cell Cell 53.16 65.73 72.89 90 110.94 90 (has 3 molecules in the asymmetric unit). Tested with pointless. Does not give any other possibility. The other crystal form of the same protein, similar conditions: C2 Cell 109.14 124.37 73.42 90 111.75 90. This has 6 molecules in the a.s.u. Tested with pointless. Does not give any other possibility. The cell length a, b of C2 is twice that of P2. Is it usual to get such crystals from similar conditions or am I missing something? Many thanks, Mark
