Lipoid pH indicators as probes of electrical potential and polarity in micelles

Abstract

Two types of fluorescent pH indicators, a hydroxycoumarin and an aminocoumarin dye, are incorporated by means of long paraffinic chain substituents to neutral, anionic, and cationic micelles. The apparent pK values for the dyes in the micelles are compared to the pK values in aqueous solution. Shifts of the apparent pK are found for charged as well as for uncharged micelles, being different for the two indicators. A comparison of the pK shifts of the indicators in neutral micelles to pK shifts in a series of mixed solvents of different dielectric constant (t) leads us to attribute the shifts in the neutral system to a reduced polarity at the micelle surface for which a value of e 32 is estimated. With respect to charged micelles this polarity effect is responsible for part of the apparent pK shift. It may be revealed by comparing the results for the two indicator types.The value of the effective dielectric constant is found to be similar to that in the neutral micelle. The remaining part of the apparent pK shift may be taken as a measure of the electrical potential at the surface of the charged micelles. Some examples of potential measurements for various counterions are presented. A thermodynamical section attempts to present clear definitions of (and relations between) terms as "apparent pK'\ "interfacial pK", "interfacial pH", and "acidity function".

Effect of pH and Monovalent Cations on the Ionization State of Phosphatidylglycerol in Monolayers

“…According to Eqn (3), these variations mainly stem from changes in $ , , the electrical potential at the interface. As also reported by others [54,, an experimental slope of about 52-53 mV for every tenfold change in ion concentration was found. This slope is somewhat lower than the value of 58 mV which is predicted by the theory, at 20 "C. Surprisingly, such a discrepancy between the theory and experience is observed apparently only with monovalent cations [54,56 -581, not with the bivalent ones for which slopes of 29 mV have been reported [69], in agreement with the theory.…”

“…Surface potentials were carefully measured and the possibility of a systematic experimental error does not seem to be the case with these results. It should be noted that slopes lower than 58 mV have already been reported by different authors who measured changes in surface potentials or in zeta potentials with monovalent ion concentration, for various lipids and on different membrane model system [54,. As there is an apparent contradiction between the theory and experience, instead of performing calculations on the experimental data of Fig.…”

Time‐resolved surface charge change on the cytoplasmic side of bacteriorhodopsin

“…We find that the surface charge at position 101 on the cytoplasmic surface becomes transiently more negative by 0.80 elementary charge per bR. We cannot exclude an alternative explanation that the observed transient pK increase is due to a transient decrease in the dielectric constant (e) of the environment of the dye [12]. In fact, experiments with fluorescein in solvents of different dielectric constants show that its pK increases by 0.7 pH-unit when e decreases from 78 to 50 (data not shown).…”

Development of constant-pH simulation methods in implicit solvent and applications in biomolecular systems

“…In truth, this discussion is similar to proton association/dissociation from/to a amphiphilic molecule of a micelle. The classical works of Mukerjee and Banerjee (1964) and Fernández and Fromherz (1977) suggested a relative simple equation to measure ΔpK based only on the electrostatic interactions and neglecting nonelectrostatic interactions. They also left for debate a fundamental problem: the correct measurement of pK 0 (the reference dissociation constant).…”