The Effect Of Glycerol On Membrane Solubilization By Nonionic Surfactants

Abstract

Some protocols for solubilizing membrane components by surfactants use glycerol as a co-solute for stabilizing the native state of proteins. We have studied the effect of glycerol on the surfactant - lipid system in terms of surfactant self-association (critical micelle concentration, CMC), membrane partitioning (partition coefficient K and αHmic), and the onset of membrane solubilization (i.e., bilayer-to-micelle transition, characterized by a specific surfactant-to-lipid mole ratio in the membrane denoted Resat) by isothermal titration calorimetry (ITC). One effect expected for glycerol is its tendency to ‘salt out’ hydrophobic molecules. This promotes all aggregation phenomena (K increases, CMC decreases) but has little effect on the balance between the aggregates (Resat ∼ const.). Furthermore, glycerol gradually dehydrates the polar head group, which renders the effective molecular shape more favorable for a bilayer (K increases) whereas micellization is much less affected (CMC ∼ const) so that bilayers are stabilized compared to micelles (Resat increases). Our results indicate that the behavior of the sugar based surfactant octyl glucoside seems governed by the salting out effect, whereas for ethylene oxide surfactant C12EO8, headgroup dehydration seems to be the key effect explaining the effects of glycerol.