Analysis of short contacts in crystals of halogenated amino acids: atom–atom interactions vs. energy frameworks

Abstract

We investigated eight crystal structures of a series of chlorinated and iodinated alanine derivatives with different protective groups on carboxyl and amino functionalities. The crystal packing is determined by the H-bonding type interactions, primarily of the amide group, as well as of the acidic hydrogens of the stereogenic center and –CH₂X (X = Cl or I) groups. These types of hydrogen bonding are similar to what is found in nature as seen in the secondary structures of proteins, i.e., α-helices and β-sheets, which are necessary to stabilize the three-dimensional structures of amino acid-based polymers. Two iodinated derivatives demonstrated either type II I⋯I halogen interactions or I⋯O multipolar interactions, whereas no indication of halogen bonding interactions was seen among the chlorinated derivatives. To a large extent, the packing is stabilized by dispersion forces, a conclusion drawn from the analysis of energy lattice networks performed with the help of Crystal Explorer.