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 (\3.0È3.3) of the dipeptide, its pK 1 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 "" e †ective ÏÏ hydrophobicity scale proposed by Akamatsu and Fujita (J. Pharm. Sci., 1992, 81, 164), in which the steric e †ect of amino acid side chains seems to be correctly incorporated.

Biphasic Electrospray Ionization for the Study of Interfacial Complexes

“…3) + by the oxygen of the ether crown. 7 This spectrum shows a yield of complexation of roughly 15%. Of course, this yield depends on the MS optics parameters used (the optimization was performed on m/z = 313.1 Th).…”

“…[46][47][48] Ion transfer processes across the interface to the organic phase of the ionic species present in the aqueous phase (e.g., Pb 2+ , FF + ) have also to be taken into account: the latter not only because a significant voltage drop that has to be applied in order to obtain a stable electrospray, but also because the presence of an ionophore in the organic phase usually facilitates the ion transfer processes across the interface via complex formation, as has been shown previously in numerous studies. [5][6][7] Finally, after the desolvation processes inherent to the electrospray ionization process, that are still not very clear under these conditions, the products are posteriorly analyzed by MS. As expected, this type of pathway would enhance the reaction of compounds with a high affinity for the interface, such as phospholipids or porphyrins.…”

“…7,37 Here, DB18C6 was used in DCE in order to extract the dipeptide FF from an aqueous solution. The mass spectrum (Fig.…”

“…8,9 Electrochemistry has been developed over the last decades as a very efficient tool to study polarized liquid-liquid interfaces, because it is important to accurately control the potential drop across the interface between two immiscible electrolyte solutions (ITIES). 10,11 This control of the polarization can be used to drive or monitor adsorption or desorption processes 8,9,12 as well as to conduct and investigate ion 7,13 and/or electron transfer reactions across the interface. 14 Assisted ion transfer reactions are an important class of charge transfer reactions where ligands or ionophores in the organic phase can interfacially complex aqueous ions to extract them to the organic phase.…”

“…The Taylor cone is the locus of very large surface tension gradients resulting in a recirculating Marangoni flow generating a very active mixing in the solution. 3,4 Biphasic liquid systems have found many applications ranging from the classical solvent extraction [5][6][7] to the study of heterogeneous reactions. Biphasic liquid systems have also been proposed to mimic biological interfaces.…”