Metal Ion Binding by Dipeptides: Structure of the Product Generated in “Aspartame” Hydrolysis, Sodium cyclo(L‐α‐Aspartyl‐L‐phenylalanine) Tetrahydrate, Na[c(L‐Asp‐L‐Phe)] 4H2O

Abstract

Ester cleavage of aspartame (L‐α‐aspartyl‐L‐phenylalanine methyl ester, 1) by equimolar quantities of NaOH is accompanied by intramolecular cyclisation to sodium 3‐benzyl‐6‐(carboxylatomethyl)‐2,5‐dioxopiperazine (2). The solid‐state structure of the crystalline tetrahydrate has been determined by single‐crystal X‐ray diffraction analysis. The cations are arranged in strings, and each sodium atom is in a distorted octahedral environment of six oxygen atoms, including a β‐carboxylato (O1) and an oxo function (O4) of two different dioxopiperazines and four water molecules. No nitrogen coordination is observed. Through the ligand bridging (O1, O4) of the metals and a set of hydrogen bonds involving the amide and carboxylate groups and all water molecules a three‐dimensional network is established.