BORONIC ACIDS MEDIATE GLYCOSIDE TRANSPORT THROUGH A LIQUID ORGANIC MEMBRANE VIA REVERSIBLE FORMATION OF TRIGONAL BORONATE ESTERS

The ability of phenylboronic acid and 3-(1-adamantylcarboxamido)phenylboronic acid to mediate the transport of p-nitrophenyl beta-D-glucopyranoside (glucoside), p-nitrophenyl beta-D-galactopyranoside (galactoside), p-nitrophenyl beta-D-mannopyranoside (mannoside), and p-nitrophenyl beta-D-xylopyranoside (xyloside) through a liquid dichloroethane membrane in the presence and absence of trioctylmethylammonium chloride was determined. In the absence of the lipophilic ammonium cation, the boronic acids transported significant amounts of galactoside, suggesting that transport was mediated by reversible formation of a trigonal boronate ester. Extraction and transport experiments as a function of pH provided confirming evidence for this transport mechanism. The apparent order of diol selectivity for the trigonal boronate transport pathway was observed to be cis-alpha,gamma-diol>cis-alpha,beta-diol approximate to trans-alpha,gamma-diol much greater than trans-alpha,beta-diol. Uphill transport of galactoside was achieved by including boric acid in the receiving phase and lipophilic boronic acid in the organic layer, representing a functionally biomimetic example of active transport driven by group translocation.