Detergents are widely used in modern in vitro biochemistry
and
biophysics, in particular to aid the characterization of integral
membrane proteins. An important characteristic of these chemicals
in aqueous solutions is the concentration above which their molecular
monomers self-associate to form micelles, termed the critical micellar
concentration (CMC). Micelles are supramolecular assemblies arranged
with the hydrophobic portions oriented inward and the hydrophilic
head groups positioned outward to interact with the aqueous solvent.
Knowledge of the CMC is not only of practical relevance but also of
theoretical interest because it provides thermodynamic insights. Isothermal
titration calorimetry (ITC) is a powerful method to determine CMCs,
as it furnishes additional information on the enthalpy and entropy
of micellization. Here we describe our extension of previous methods
to determine CMCs and other thermodynamic parameters from ITC demicellization
curves. The new algorithm, incorporated into the stand-alone software
package D/STAIN, analyzes ITC demicellization curves by taking advantage
of state-of-the-art thermogram-integration techniques and automatically
providing rigorous confidence intervals on the refined parameters.
As a demonstration of the software’s capabilities, we undertook
ITC experiments to determine the respective CMCs of n-octyl β-d-glucopyranoside (OG), n-dodecyl β-d-maltopyranoside (DDM), and lauryldimethylamine N-oxide (LDAO). Motivated by the fact that in vitro membrane
protein studies often require additives such as precipitants (e.g.,
polyethylene glycol (PEG)), we also carried out ITC demicellization
studies in the presence of PEG3350, finding in all cases that PEG
had significant effects on the thermodynamics of detergent micellization.
Aqueous mixtures of two or more surfactants are often employed for research or industrial purposes because such mixtures offer advantages over single-surfactant systems. This is particularly true for mixtures of fluorocarbon (FC) and hydrocarbon (HC) surfactants, which display a broad range of mutual miscibilities in mixed micelles. Unfortunately, the prediction and even the experimental elucidation of the micellar mixing behavior of surfactant mixtures remain challenging, as evidenced by conflicting results and conclusions derived from diverse, and often complex, mixing models. One of the most intriguing questions is whether certain combinations of FC and HC surfactants form only one type of mixed micelle or rather demix into two micelle populations, namely, FC-rich and HC-rich ones. Here, we demonstrate a novel approach to the model-free analysis of critical micellar concentrations (CMCs) of surfactant mixtures that is based on a fit of the experimental data with cubic splines using a stringent thermodynamic criterion for mixing. As a proof of principle, we analyze CMC values determined by isothermal titration calorimetry and confirm the conclusions with the aid of combined H- andF-NMR spectroscopy. Specifically, we show that aqueous mixtures of an FC maltoside and an HC maltoside conform with the assumption of only one type of micelle regardless of the mixing ratio, whereas combining the same FC surfactant with an HC surfactant carrying a zwitterionic phosphocholine headgroup gives rise to two coexisting micelle populations at high mole fractions of the FC maltoside.
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