Complexes of Lysine, Histidine, and Arginine with Sulfonated Azo Dyes:  Model Systems for Understanding the Biomolecular Recognition of Glycosaminoglycans by Proteins

The X-ray crystal structures of six salts composed of amino acids and sulfonated azo dyes have been determined, four of them at low temperature (173 K). The compounds are dl-lysine/4-[(4-hydroxyphenyl)azo]benzenesulfonate (HABS) monohydrate (1), dl-lysine/7-hydroxy-8-(phenylazo)-1,3-naphthalenedisulfonate (Orange G) dihydrate (2), l-lysine/Orange G 1.5-hydrate (3), dl-histidine/Orange G trihydrate (4), l-histidine/Orange G trihydrate (5), and tosylarginine methyl ester (TAME)/4-[(2-hydroxy-6-tert-butyl-1-naphthalenyl)azo]benzenesulfonate (“Little Rock Orange,” LRO) (6). By virtue of their basic side chains, these amino acids are the ones most important in the binding interactions between proteins and sulfated macromolecules such as glycosaminoglycans in living systems. The sulfonate salts described here serve as model systems for these interactions. Close intermolecular approaches between the dye sulfonate groups and neighboring amino acids and water molecules are examined, and the graph-set formalism is used to describe packing patterns and to identify corresponding interactions in different crystal structures. The recurrence of certain interactions between sulfonate groups and amino acid functional groups in these small-molecule crystal structures, including numerous interactions mediated by water molecules, suggests specificity that may also be a feature of the interactions between proteins and sulfated biological macromolecules.