pH-dependent permeation of amino acids through isolated ivy cuticles is affected by cuticular water sorption and hydration shell size of the solute

Resumo

As permeabilidades de aminoácidos para membranas cuticulares isoladas de hera (Hedera helix L.) foram medidas em diferentes pH. As permeâncias cuticulares foram mais baixas para a forma iônica em pH 6, seguidas pela forma catiônica em pH 1. As permeâncias mais altas foram obtidas para a forma aniônica em pH 11.. Este achado sugere que a permeação ocorre nas vias cuticulares polares. O efeito do pH nas propriedades de transporte cuticular foi analisado de acordo com o modelo de membrana porosa considerando o caráter polieletrolítico da cutícula em termos de porosidade, tortuosidade e seletividade de tamanho da via cuticular aquosa que é alterada pelo pH. Um aumento do teor de água e da permeabilidade da membrana cuticular foi causado pela dissociação de grupos ácidos fracos com o aumento do pH, levando a um aumento da cutícula induzido por cargas negativas fixas.

Abstract

The permeabilities of amino acids for isolated cuticular membranes of ivy (Hedera helix L.) were measured at different pH. Cuticular permeances were lowest for the zwitterionic form at pH 6, followed by the cationic form at pH 1. Highest permeances were obtained for the anionic form at pH 11. Permeances were not correlated with octanol/water partition coefficients and decreased at a given pH with increasing molar volume of the solute. This finding suggests that permeation takes place in the polar cuticular pathways. The effect of pH on the cuticular transport properties was analysed according to the porous membrane model considering the polyelectrolytic character of the cuticle in terms of porosity, tortuosity, and size selectivity of the aqueous cuticular pathway which is altered by pH. An increase of water content and permeability of the cuticular membrane was caused by the dissociation of weak acidic groups with increasing pH leading to a swelling of the cuticle induced by fixed negative charges. In addition, the pH-dependent size of the hydration shell of the amino acids was identified as a secondary factor explaining the variability of cuticular permeances.