What is the best crystal size for collection of X-ray data? Refinement of the structure of glycyl-L-serine based on data from a very large crystal.

The dipeptide Gly-L-Ser was crystallized as part of a study on hydrogen-bonding patterns in the structures of dipeptides. Hydrogen-bond donors and acceptors have been assigned ranks (1 is best, 2 is next best etc.), and the observed hydrogen-bond connectivity is compared with the hypothetical pattern in which the rank n donor associates with the rank n acceptor (n = 1, 2,.), and with the pattern observed in the retroanalogue L-Ser-Gly, which contains the same functional groups. Crystallization of the title compound produced very bulky crystals. Rather than reducing the size of one of these before data collection, three data sets with different exposure times were collected with a Siemens SMART CCD diffractometer on a very large specimen (2.2 x 2.0 x 0.8 mm). The crystal was subsequently shaped into a 0.30 mm-diameter sphere for collection of two additional data sets. The discussion of the refinement results focus on the effect of absorption correction for the various data sets, and a comparison of geometrical and thermal parameters. One advantage of using a large crystal, the great speed with which data can be obtained, has been exemplified by collection of a complete data set of good quality in less than 25 min.