Compounds of Amino Acids as Anions

Abstract

This chapter deals with salts formed from metal cations and amino acid anions. All amino acids can act as monovalent anion; the acidic members glutamic acid and aspartic acid as well as cysteine and tyrosine can form both monovalent and divalent anions. Due to the large number of available cations, a multitude of combinations is possible and in fact found (for the standard twenty amino acids, over 150 crystal structures are published, and many more species have been characterized by other methods). Different hydration states (as reported for the pristine amino acid crystals) occur as well, and the existence of polymorphs is also documented for some cases. The formation of metal–amino acid complexes has been studied in several works (both for a given amino acid and a given cation), and the flexibility of amino acid molecules as ligands allows coordination of cations with different chemical properties (such as charge or ionic radius). Several coordination modes are found for amino acids – the molecules can act as monodentate, bidentate, tridentate, and bridging ligands. The connectivity of the coordination polyhedra of the metal cations is considered – isolated units are frequent, but chains and layers occur as well. Moreover, these units can connect via amino acid molecules to form higher-dimensional structures. Hydrogen bonds (also involving water molecules, both in coordination or in the interstices as crystal water) further stabilize the units, forming relatively stable phases. The frequency of salts varies among the different amino acids, and most examples are reported for glycinate salts, whereas crystals of cations and basic and weakly soluble amino acids are difficult to obtain (e.g., arginate and lysinate salts have not been reported in crystalline state). Finally, the aspect of symmetry is considered (the chirality of most amino acids playing a crucial role), and, consequently, the properties of these salts have impact on applications in the fields of physics, biology, and medicine.