Centre of Excellence in Biocrystallography, Department of Chemical and Pharmaceutical Sciences, University of Trieste, 34127 Trieste, Italy.
Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, and INSTM, UdR Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy.
Molecules. 2018 Dec 19;23(12):3368. doi: 10.3390/molecules23123368.
Crystallization of tetraphosphonate cavitand Tiiii[H, CH₃, CH₃] in the presence of positively charged amino acids, namely arginine, lysine, or histidine, afforded host-guest complex structures. The X-ray structure determination revealed that in all three structures, the fully protonated form of the amino acid is ditopically complexed by two tetraphosphonate cavitand molecules. Guanidinium, ammonium, and imidazolium cationic groups of the amino acid side chain are hosted in the cavity of a phosphonate receptor, and are held in place by specific hydrogen bonding interactions with the P=O groups of the cavitand molecule. In all three structures, the positively charged α-ammonium groups form H-bonds with the P=O groups, and with a water molecule hosted in the cavity of a second tetraphosphonate molecule. Furthermore, water-assisted dimerization was observed for the cavitand/histidine ditopic complex. In this 4:2 supramolecular complex, a bridged water molecule is held by two carboxylic acid groups of the dimerized amino acid. The structural information obtained on the geometrical constrains necessary for the possible encapsulation of the amino acids are important for the rational design of devices for analytical and medical applications.
四膦酸 Cavitand Tiiii[H, CH₃, CH₃]在带正电荷的氨基酸（精氨酸、赖氨酸或组氨酸）存在下结晶，形成主客体配合物结构。X 射线结构测定表明，在所有三种结构中，氨基酸的完全质子化形式由两个四膦酸 Cavitand 分子双位络合。氨基酸侧链的胍基、铵基和咪唑基阳离子被膦酸受体的空腔容纳，并通过与 Cavitand 分子的 P=O 基团的特定氢键相互作用固定在适当位置。在所有三种结构中，带正电荷的α-铵基与 P=O 基团形成氢键，并与第二个四膦酸分子空腔中的水分子形成氢键。此外，还观察到 Cavitand/组氨酸双位络合物的水辅助二聚作用。在这个 4:2 的超分子复合物中，桥接水分子被二聚氨基酸的两个羧酸基团固定。获得的关于可能封装氨基酸所需的几何约束的结构信息对于分析和医疗应用的设备的合理设计很重要。
