Toward Understanding High-Z ' Organic Molecular Crystals Through The Superspace Method: The Example Of Glycyl-L-Valine

The high-Z' (Z' = 7) structure of glycyl-L-valine has been redetermined at 298 K, using synchrotron radiation and exploiting the superspace approach. The analysis of the diffraction data reveals that the structure can be described as a commensurately modulated crystal structure with superspace group P2(1)2(1)2(1)(0 sigma(2)0)000 and modulation wave vector q = (0, 2/7, 0). A new Z' = 1 phase has been discovered for this compound at 323 K that is related to the known Z' = 7 phase. The origin of the modulated phase has been explored by analyzing intermolecular interactions and molecular conformations in terms of t-plots and through a comparison to (1) the newly discovered high-temperature phase, (2) a hypothetical Z' = 1 structure at 298 K, and (3) a similar Z' = 1 structure of the related compound glycyl-L-leucine. The results show that the conformational flexibility of the glycyl-L-valine molecule ensures optimization of the hydrogen-bond network which causes the modulation and thus a high Z' value of the supercell. This study highlights the elegance and convenience of the superspace approach to explain the occurrence of at least part of the rarely occurring high-Z' structures of molecular compounds.