Diffusion NMR Studies of Diol-boronates: Implications for Membrane Transport Carrier Design

Diffusion coefficients for various neutral trigonal and anionic tetrahedral cyclic diol-boronate esters in chloroform and acetonitrile were measured using PFG-NMR. As expected, bulk diffusion of neutral trigonal boronate decreases with molecular weight. However, diffusion of tetrahedral boronate anion is affected by the structure of the tetralkylammonium counter-cation, Q⁺, and the polarity of the solvent. In weakly polar chloroform, the slow rates of Q⁺/boronate diffusion indicate tight ion-pairing and ion-pair aggregation. In the more polar solvent acetonitrile, the diffusion rates depend on the structure of Q⁺. When Q ⁺ is tetrabutylammonium, the boronate diffusion coefficient is low suggesting that there is strong ion-pairing, but when Q⁺ is tetraoctylammonium the boronate diffusion is remarkably high suggesting that there is very little ion-pairing. The results have relevance to the boronic acid mediated transport of sugars through liquid organic membranes, a process that is known to be controlled by diffusion of the transient sugar-boronate esters. The diffusion data show how the design of boronate transport carriers can be fine-tuned to produce higher membrane transport fluxes.