Aggregation and other intermolecular interactions of biological buffers observed by capillary electrophoresis and UV photometry

Abstract

Electrophoretic and photometric experiments strongly indicate that monovalent anions, which arise by deprotonation of the nitrogen atom in zwitterionic Good’s buffers 3-(cyclohexylamino)-2-hydroxy-1-propanesulfonic acid (CAPSO) and 3-morpholinopropanesulfonic acid (MOPS), spontaneously aggregate. Cationic migration of sanguinarine (SA) and chelerythrine (CHE) in highly alkaline 1,3-bis[tris(hydroxymethyl)methylamino]propane (Bis–Tris–propane), in which the concentration of cations of both alkaloids is negligible, may be explained by the existence of an aggregate, which contains uncharged sanguinarine or chelerythrine and one monovalent cation of Bis–Tris–propane at least. Tendency of tris(hydroxymethyl)aminomethane (Tris), bis (2-hydroxyethyl)iminotris(hydroxymethyl)methane (Bis–Tris) and Bis–Tris–propane cations to ion pairing with synthetic cluster borane anions and with fused silica markedly rises up with the size and charge of these cations. The drop in mobility of cluster borane compounds sometimes exceeds 50% of their mobility found at identical pH and ionic strength in buffers with sodium cation. The electroosmosis drop approached 70% if background electrolyte contained Bis–Tris–propane cations instead of sodium cations. Nitrate, taken as a model inorganic ion, and four randomly chosen organic anions interacted markedly less with Tris, Bis–Tris and Bis–Tris–propane cations than cluster borane anions. 2-( N-morpholino)ethanesulfonic (MES) acid anions present in background electrolyte affect the ion pairing of Tris, Bis–Tris and Bis–Tris–propane cations with anionic analytes and, in this way influence also mobilites of these anionic analytes. Limited hydrophilicity at least one of interacting species appears to be the most probable cause of observed intermolecular interactions of biological buffers.