Crystallization of Organic Compounds in Reversed Micelles. I. Solubilization of Amino Acids in Water−Isooctane−AOT Microemulsions

Abstract

As a basis for crystallization studies, the solubilization of amino acids (glycine, l-histidine, and l-phenylalanine) in water-in-isooctane microemulsions stabilized by AOT (sodium di-2-ethylhexyl sulfosuccinate) was investigated. The maximum amount of amino acid that could be solubilized was determined by the solid-liquid extraction method, and the effect of the guest molecules (amino acids) on the size and shape of the microemulsion droplets and their thermal properties were determined using SAXS and DSC measurements, respectively. The solubilization of glycine molecules, which primarily dissolve in the water pool, was slightly lower than their solubility in pure water, decreasing with increasing concentration of AOT and increasing with increasing water content in the microemulsion. In contrast, the solubilization of phenylalanine, which is primarily located at the water/oil interface, exceeded several times the solubility in water, the solubilized amount increasing with increasing AOT and/or water concentrations. Histidine had characteristics intermediate between these two extremes. Solubilization of those molecules effected an increase in droplet size. The thermal analysis showed that loading of the microemulsion droplets with glycine has a much stronger effect on the thermal behavior of the emulsified water than has loading with phenylalanine. The low solubilization of glycine as compared to its solubility in pure water can be explained by the state of water within the microemulsion droplets, i.e., part of it is present as free water and part as water bound to the AOT headgroups. The loading of phenylalanine changed the shape of the microemulsion droplets from spherical to ellipsoidal, and with increasing droplet sizes, the [phenylalanine]/[AOT] molar ratio at the interface increased.

Protein folding in a reverse micelle environment: The role of confinement and dehydration

“…These representative structures demonstrate the strong deviations from spherical geometry observed in our simulations of the RM with water loading at w 0 = 6. The distorted Gaussian shape of the unrestrained RM is similar to that observed by Yano et al 39 for reverse micelles with small w 0 values.…”

“…24 The structure of RMs has been studied with experimental techniques including fluorescence, [25][26][27] NMR, [28][29][30] IR, [31][32][33] and small-angle neutron scattering (SANS)/small-angle x-ray scattering (SAXS). [34][35][36][37][38][39] Scattering studies provide limited insight into the size distribution of the RMs as well as the degree of shape fluctuations. In considering the use of RMs as a confining environment for the study of peptide folding and aggregation, it is important to consider the possible role of RM shape fluctuations in any interpretation of the thermodynamics or kinetics of folding and aggregation.…”

Crystallization in Microemulsions: A Generic Route to Thermodynamic Control and the Estimation of Critical Nucleus Size

“…The transient dimer mechanism also enables crystallization to proceed in microemulsions whenever a transient dimer forms between a droplet containing a crystal nucleus and a nucleus-free droplet which contains supersaturated solution, since the crystal nucleus can then gain access to this supersaturated solution and thereby grow (see Figure 6). Crystallization of organic compounds from microemulsions was first studied by Füredi-Milhofer et al in 1999 for the aspartame crystal system, with studies on glycine crystallization (Allen et al, 2002;Yano et al, 2000;Chen et al, 2011) and carbamazepine (Kogan et al, 2007) following. The use of microemulsions in producing both inorganic nanoparticles and macroscopic organic crystals shows that the size of the particulates grown can vary from a few nm to mm, depending upon the nucleation rate, the ability to form stable nuclei, and the extent of surfactant adsorption on the resulting particles.…”

A simple classical model for predicting onset crystallization temperatures on curved substrates and its implications for phase transitions in confined volumes

“…Please note, though, that when comparing experimental and predicted onset crystallization temperatures for crystallization from solution, the assumption of an infinitely thin interface may result in substantial discrepancies if the solute material is also absorbed within the interfacial region. However, this possibility can be accounted for by determining the solubility of the solute material in the microemulsions in the temperature range of interest, as shown by Yano et al 55 Figure 8 shows the variation in T c with R predicted for octadecane in dodecane solutions with different values assuming that the crystallization occurs via nucleation of a metastable rotator phase [56][57][58][59] with values 56 an increase in T c is possible for crystallization from sufficiently concentrated solutions at the smallest ͉R͉ values, because the critical nucleus size is then sufficiently large compared to the substrate size, that its growth is associated with a decrease in the interfacial area, and hence the interfacial energy of the system. This superheating behavior may be accessible for systems that show surface freezing at planar interfaces, although this phenomenon is expected to be frustrated in these highly curved systems.…”