Reduction in crystal symmetry of a solid solution: a neutron diffraction study at 15 K of the host/guest system asparagine/aspartic acid

It has been demonstrated, for the first time by diffraction methods, that a solid solution composed of host and guest molecules can exhibit a crystal symmetry lower than that of the host. The study proves that the symmetry of a solid solution is dependent not only upon the host crystal structure and the guest molecular structure but also upon the surface structure and symmetry of the host crystal. The crystal structures of (S)-asparagine monohydrate (D{sub 2}NCOCH{sub 2}CH(ND{sub 3})CO{sub 2} {times} D{sub 2}O) and of the solid solution (0.848:0.152) (S)-asparagine/(S)-aspartic acid (DO{sub 2}CCD{sub 2}CD(ND{sub 3})CO{sub 2}) monohydrate were refined by using neutron diffraction data obtained at 15 K. The space group of the pure host crystal is P2{sub 1}2{sub 1}2{sub 1} (Z = 4), whereas that of the host/guest crystal is monoclinic P12{sub 1}1 with two molecular sites per asymmetric unit. The ratios of guest/host occupancies of the two independent sites are 0.173:0.827 and 0.132:0.868. The reduction in symmetry is in accordance with the preferred adsorption of guest aspartic acid on the (010) crystal face at half of the orthorhombic, symmetry-related surface sites. Aspartic acid mimics, at the preferred (010) surface sites, molecular asparagine, participating in all hydrogen bonds. At themore » less-favored (010) surface sites a normal N-H{hor ellipsis}O(host) hydrogen bond is replaced by O(hydroxyl){hor ellipsis}O(host) repulsion between lone-pair electrons. 21 refs., 5 figs., 6 tabs.« less