An NMR Investigation of Fast Proton Transfer along the Surface of Cationic Micelles

Abstract

The surfaces of proteins and membranes are dotted with a multitude of proton binding sites, some of which are only a few tenths of a nanometer apart. This proximity between proton binding sites could be at the origin of the fast proton transfers occurring at the biological scale.[1] In order to understand this phenomenon more precisely, we investigated the mechanism of proton transfer at the surface of cationic micelles composed of amphiphilic ammonium chloride salts. Proton transfer rates were obtained from the overall analysis of the DNMR (dynamic nuclear magnetic resonance) signals in experiments varying the pH and the salt concentrations of aqueous solutions at 25 8C.[2-4] Exchange rates of the acidic proton of amphiphilic ammonium salts BH+ with R = n-C 8 H 17 (DMOA) or n-C 10 H 21 (DMDA) increase dramatically upon going from dilute non-micellar to concentrated micellar solutions, thus shifting the NMR kinetic window towards acidic pHs by ca. 2 units. Experiments performed at constant pH and at variable salt concentrations C reveal a sharp rate increase as C is brought beyond the critical micellar concentration (CMC); such operations can constitute a very sensitive new procedure for CMC determination. Herein, the influence of the pH and of the structure of the ammonium salts is examined in order to elucidate the mechanisms contributing to proton exchange in micelles.As a first step, proton transfers on DMOA were studied at a range of concentrations between 0.01 and 0.25 m, well below the CMC, and at pH 3 to 5 in order to bring exchange rates within the NMR kinetic window (1-30 s À1). 1 H NMR spectra were recorded at 400 MHz with suppression of the water resonance. Exchange rates were obtained from total line shape analysis of the N-methylic pattern (see Figure 1). The deprotonation of BH + into its conjugate base B is the slow and measurable step, followed by an immeasurably fast reprotonation of B, which ends the proton exchange. In the process, the nuclear spin state of the exchanging proton randomly flips between a and b; this is equivalent to observing an NMR site exchange between the two components of the coupled N-methylic doublet with a frequency k NMR (s À1

Complexation of Cu(II) by Original Tartaric Acid−Based Ligands in Nonionic Micellar Media: Thermodynamic Study and Applications

“…1 It has been established that micelles can cause an acceleration or inhibition of a given reaction relative to the equivalent reaction in aqueous media. These effects have been described in the literature for a large variety of reactions, [2][3][4][5][6] but few thermodynamic data are available concerning complexation reactions in micellar conditions, 7 and controversy persists regarding the structure of the complexes solubilized in the micelles. [8][9][10][11] In the present work, the effect of nonionic micelles composed of Brij 58 (C 16 EO 20 ) on the complexation of Cu(II) by original tartaric acid-based ligands, 12 named C x T (Figure 1), is investigated.…”

“…It has been established that micelles can cause an acceleration or inhibition of a given reaction relative to the equivalent reaction in aqueous media. These effects have been described in the literature for a large variety of reactions, − but few thermodynamic data are available concerning complexation reactions in micellar conditions, and controversy persists regarding the structure of the complexes solubilized in the micelles. − …”