Uptake of amino-acids by zirconium phosphates. Part 2. Intercalation of L-histidine, L-lysine, and L-arginine by α-zirconium phosphate

Abstract

The uptake of L-histidine (His), L-lysine (Lys), and L-arginine (Arg) by α-zirconium phosphate has been studied at 25 °C. The curves of amino-acid uptake against amino-acid added for His and Arg show only one plateau, while that for Lys exhibits two plateaux indicating that its uptake proceeds in two steps. Each amino-acid forms two intercalated phases at low uptakes in which the interlayer space is probably occupied by a monolayer of the guest molecules placed nearly horizontal to the sheet of the host crystal or in slightly different orientations. The intercalated solids with His and Arg are completely decrystallized at higher loadings becoming gels formed of lath-shaped particles, with the amino-acid uptake capacity being 4.0 mmol g^–1. The Lys intercalate formed initially at the second uptake stage is highly disordered but maintains a certain degree of order in the host lattice. At higher levels of Lys, the second uptake stage results in the formation of a phase Zr(HPO₄)₂(Lys)_1.85·H₂O with an interlayer spacing of 23.1 Å in which the guest molecules are intercalated probably as a bimolecular layer. The striking contrast between the intercalation behaviour of Lys and the other amino-acids is explained in terms of the structural and functional characteristics of the individual acids.