Hydrophobicity of oligopeptides: a voltammetric study of the transfer of dipeptides facilitated by dibenzo-18-crown-6 at the nitrobenzene/water interface

Abstract

For 13 dipeptides with un-ionizable amino acid side chains, their transfer at the nitrobenzene/water interface facilitated by dibenzo-18-crown-6 (DB18C6) has been studied by ion transfer voltammetry. On the condition that the pH of the water phase is adequately low compared with the pK₁ (=3.0–3.3) of the dipeptide, its protonated form transfers across the interface as a univalent cation. The reversible half-wave potential determined at pH 2.3 by cyclic voltammetry has been found to show good correlations with the hydrophobicities of the dipeptides estimated by some previously proposed hydrophobicity scales of amino acid residues. Especially, an excellent correlation with R=0.991 has been obtained for the “effective’' hydrophobicity scale proposed by Akamatsu and Fujita (J. Pharm. Sci., 1992, 81, 164), in which the steric effect of amino acid side chains seems to be correctly incorporated.