Manipulation of Stationary-Phase Acid-Base Properties by a Surface-Buffering Effect

Abstract

The presence of residual amine groups in sur face bound, silica-based phenylboronic acid phases lowers the apparent pK a of the acid groups. This "surface buffering" effect permits boronate-saccharide complexation chemistry to occur at much lower pH values than is typically the case. The broader implications of the de liberate use of such effects are discussed.Lochmuller, Wilder and Marshall ( 1 ) observed an apparent lowering of the pKa for surface-bound quinazolines using photothermal spectrometric titration approach and further evidence for the mechanism of this change in acid base behavior has recently been reported by Lochmuller and Hill (2). It appears that sitesite interactions (charge repulsion) and "surface buffering" by residual amine groups combine to produce the apparent change. Mat lin and Davidson (3 ) also observed interactions between the neutral species picramidopropyl and propylamine in a "mixed", bonded phase by photοthermal spectrometry.One of the problems with silica-based chromatographic materials is their relatively high solubility in mobile phases of pH higher than 7.5 units. It is interesting to speculate that at least for some important ionic equilibria, the acid-base chemistry of bound molecules might be manipulated by deliberately intro ducing a "surface buffering" effect.We chose to explore this possibility by studying the complexation of saccharides by boronic acid anion because of the importance of saccharides and the nor mally high pH conditions required (pH 8-10) Gilham had postulated that the local positive or negative charges on cellulose in fluenced the pKa of phenylboronic acid at low mobile phase ionic strength (4 ).In addition, Lochmuller and Amoss first demon strated the advantage of "mixed" bonded phases in systems where the complexation constants are large and contribute to poor trans fer kinetics (5 ). Later Karger used this approach to improve the performance of bound metal complex phases (6).Partial derivati zation of amine silicas might be expected to improve the efficien cies obtained with boronate phases by the same reasoning.

Chiral discrimination of monosaccharides by monolayers of a steroidal boronic acid

“…The interaction between phenylboronic acids and diols2-4 as well as sugars and related compounds has been studied over the past few years. '-" These studies established not only the structure of the complexes but quantified their stability '-' and showed possible applications for membrane sensors," matrix-supported receptors 11,12 or boron neutron capture therapy 1 7 q 1 * as well. A number of phenylboronic acids 1914*1 and diphenyldiboronic acids ',16 possess unique properties such as high sensitivity towards sugars together with pronounced selectivities for mono-and disaccharides due to facile complexation between boronic acids and 1,2or 1,3-diols not only in organic and aqueous alkaline, but also in neutral solution^.…”