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abstracts
Two polymorphs are known to exist under ambient conditions for a number of amino acids (three for glycine). While investigations at high pressure have revealed a number of additional polymorphs, temperature-induced changes are rare. Low-temperature structures with modified side-chain conformations were identified for L- and DL-cysteine. Furthermore, racemates with linear side chains, such as DL-methionine and the non-standard DL-aminobutyric acid (DL-Abu), DL-aminopentanoic acid (DL-norvaline, DL-Nva) and DL-aminohexanoic acid (DL-norleucine, DL-Nle), undergo major crystalline rearrangements on transitions between P21/c and C2/c space groups [1], some of them entropy driven (disordering). As for the corresponding enantio-pure amino acids, we recently described related P21 and I2 structures at 105 K for L-Abu, both with Z' = 4 [2]. A short side-chain C-C bond (1.426 Å) in the only available CSD entry for L-Nle (at 298 K) [3] lead us to suspect that disorder could have been overlooked in the original refinement. L-Nva has not been described previously. We now present single-crystal X-ray determinations between 105 and 405 K for L-Abu, L-Nva and L-Nle, showing phase behavior of unprecedented complexity. For L-Abu and L-Nva we find three different forms in this temperature interval, while four different phases were found for L-Nle. Its known C2 structure with Z' = 1 prevails between 200 and 390 K, and the side chain is indeed disordered 2:1 over two positions. Above 390 K disorder is extensive; the space group remains C2 but cell parameters change. Upon cooling new low-temperature forms are observed at 200 and 170 K. Both are modulated, but to a different extent: data collected at 100 K reveal an almost commensurate phase, while the 180 K phase is fully incommensurate. This is, to our knowledge, the first observation of modulated structures for an amino acid, and also the first observations of major crystalline rearrangements akin to those seen for the corresponding racemates.
abstracts
The first diffraction patterns measured for crystals of 2-chloro-benzo-1,3,2-dithiarsole (C6H4S2AsCl) showed an exceptionally large triclinic cell. After routine data collection the structure could be solved without difficulty; 17 independent molecules were found. A successful conventional refinement of the 170 independent non-H atoms was possible if restraints were applied (similar bond lengths and angles for all molecules; rigid-bond restraints; three sets of 8 anisotropic displacement parameters for the S and C atoms). At convergence R, wR2 were 0.042, 0.107 for 859 variables, nearly 4K restraints and more than 24K unique reflections of which 8774 have I>2σ(I). All displacement ellipsoids were positive definite. The display program Mercury revealed that the molecules form ribbons with a core of closely spaced As and Cl atoms. Because the ribbon is obviously modulated, and because Z' = 17 is both very large and prime, the possibility of an incommensurate structure had to be considered. A new integration of the original frames using EVAL14 gave a modulation vector with components 5.012(2)/17, -3.187(2)/17, 8.016(3)/17; the modulation is clearly incommensurate in the b* direction. Refinement with JANA2006 (811 variables, 11,119 unique reflections, no restraints) gave R, wR2 values 0.045, 0.116. The overall packing is determined by the stacking of the aromatic rings and probably by the segregation of interacting As and Cl atoms. A conventional refinement of a disordered, average (Z' = 1) structure revealed two basic orientations of the C6S2 plane that must be correlated in several directions if impossibly short intermolecular contacts are to be avoided. Along the modulation vector q the orientation of the C6S2 plane varies smoothly, but q is not a direction in which the molecules are in contact. The reasons for the unusual modulation will be discussed, as will the signs that a modulated, high-Z' molecular crystal structure is actually incommensurate.
