Lipophilicity of acetylcholine and related ions examined by ion transfer voltammetry at a polarized room-temperature ionic liquid membrane

Ion transfer voltammetry at a polarized room-temperature ionic liquid (IL) membrane was used to evaluate the standard Gibbs energy of ion transfer from water to IL. This quantity was considered to be a measure of the ion lipophilicity, which is one of the factors playing a role in the extraction and transport processes in the two-phase liquid and liquid membrane systems. On this basis, the lipophilicity of several biologically active ions was compared, namely of neurotransmitter acetylcholine (ACH(+)) and several related ions including choline (CH+, precursor for ACH(+)), muscarine (MUS+, agonist of the muscarinic ACH(+) receptors), protonated atropine (ATH(+), antagonist of the muscarinic ACH(+) receptors), protonated scopolamine (SAH(+), antagonist of the muscarinic ACH(+)), and the tetramethylammonium ion (TLM+) representing their charged moiety. Cyclic voltammetric measurements were carried out using a 4-electrode cell with the IL membrane composed of highly hydrophobic tridodecylmethylammonium tetralcis[3,5-bis(tiifluoromethyl)phenyl] borate. Analysis of the voltammetric data provided the values of the standard Gibbs energy of ion transfer (in kJ mol(-1) in parentheses), which follow the order of ions TMA+ (14.6) < ACH(+) (16.2) similar to ATH(+) (16.4) < MUS+ (20.3) similar to SAH+ (20.4) < CH+ (24.1) indicating their rather weak and similar lipophilicity. An analysis of the effect of pH on the voltammetric behavior of ATH+ and SAH+ suggested that the partition coefficient for the neutral bases AT and SA is likely to be fairly small and, hence, that the lipophilicity of these neutral bases is also rather weak. Comparable theoretical values of the electrostatic contribution to the theoretical standard Gibbs energy of ion transfer were obtained by the density functional theory calculations, which accounted for the solvent effect by using the polarizable continuum model (PCM). On the other hand, an estimate of the (neutral) solvophobic contributions to the standard Gibbs energy of ion transfer that is based on the empirical Uhlig formula predicts a significant lipophilic effect increasing with the ion size. This effect is likely to be compensated by the strong dipole-dipole and specific (e.g., H-bond) interactions stabilizing most of the studied ions in the water environment. These conclusions are supported by an analysis based on the voltammetric data reported in literature for the ion transfer across the water/nitrobenzene interface, and the DFT calculations performed in the present study.