Abstract
Some imidazolium-based ionic liquids were used to determine zwitterionic or non-zwitterionic structures of glycine and p-amino benzoic acid, as model amphoteric compounds, in their corresponding isoelectric point. To do this, the partitioning behaviors of both compounds between the ionic liquid and aqueous phase at different pH values were investigated. The results revealed that due to having different pH-dependent chemical structures, each compound showed different partitioning behavior. This observation was considered as a basis for introducing a green technique for understanding the real chemical structures (species), i.e. zwitterionic or non-zwitterionic structures of amphoteric compounds such as amino acids in the aqueous solutions. This study revealed the existence of a non-zwitterionic (neutral) structure for p-amino benzoic acid and a zwitterionic structure for glycine in their corresponding isoelectric points.
Highlights
Charge characterization and speciation of amino acids is of interest in biological sciences for better understanding of the metabolic processes in tissue membranes and in analytical chemistry for improving and designing new separation and measurement procedures
Few studies about the charged structures of amino acids in aqueous phase are available for checking the studies reported by many authors
In 1916, Adams showed that an amino acid might possess a zwitterionic structure, i.e. +NH3–R–COO−, rather than a non-zwitterionic classical structure, i.e. NH2–R–COOH, in aqueous solutions.[9]
Summary
Charge characterization and speciation of amino acids is of interest in biological sciences for better understanding of the metabolic processes in tissue membranes and in analytical chemistry for improving and designing new separation and measurement procedures. +NH3–R–COO−, rather than a non-zwitterionic classical structure, i.e. NH2–R–COOH, in aqueous solutions.[9] opposite ideas were reported.[10] In this regard, in 1930, Harris was able to provide direct evidence through doing many titrations and experiments to show that an amino acid molecule does possess, in preponderant proportion, a zwitterionic rather than a non-zwitterionic classical structure.[11] In a recent study, amino acids were considered as neutral species,[12] while previous studies showed that they can exist in zwitterionic form. It is necessary to investigate the real amino acid species in aqueous solutions
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