The ability of phenylboronic acid, [3-( 1-adamantylcarboxamido)phenyl]boronic acid, and diphenylborinic acid to extract and transport p-nitrophenyl /?-D-glucopyranoside (glucoside), p-nitrophenyl 6-D-galactopyranoside (galactoside), and p-nitrophenyl 8-D-mannopyranoside (mannoside) through a liquid organic membrane, in the presence of trioctylmethylammonium or tetrabutylammonium chloride, was determined. Under the conditions examined, glycoside transport was facilitated by the reversible formation of covalent tetrahedral, anionic glycoside-boronate complexes, which partitioned into the organic membrane as lipophilic ion pairs. The results of various experiments indicated the rate-limiting step in the transport process was diffusion of the solutes through the narrow unstirred boundary layers adjacent the organic/aqueous interfaces. A plot of glycoside transport rate versus glycoside extraction constant, K,,, formed an approximate bell-shaped relationship. Maximal transport occurred when the carrier admixture had an extraction constant of log Kc,(,,,) -2.2. Under low extraction conditions (Kcx < Kcx(max))r movement of the glycoside from the receiving phase into the organic membrane was the rate-determining step, and under high extraction conditions (Kcx > Kcx(max)), exit from the membrane into the receiving phase was rate-determining. Because transport was dependent on K,,, an analysis of the structural and environmental factors that controlled transport could be reduced to an analysis of the factors that changed K,, relative to Kex(max). The factors examined included the following; pH, boron acid acidity, diol structure, polarity of the organic layer, boron acid lipophilicity, glycoside lipophilicity, quaternary ammonium lipophilicity, and the presence of competing lipophilic anions. The importance of Kcx(mx) as the parameter determining transport stereoselectivity is discussed.
