Vesicles thermodynamic stability

Above, we argued that a portion of a finite-sized membrane can close upon itself to remove edge effects. In this way, vesicles are formed. The thermodynamic stability of vesicles is still a topic of hot debate in the literature, primarily because there are so many scenarios. The SCF analysis of vesicles leads to information on the mechanical parameters for a particular membrane system. 

New glycolipids have to be synthesized to get further insights into liquid crystal properties (mainly lyotropic liquid crystals), surfactant properties (useful in the extraction of membrane proteins), and factors that govern vesicle formation, stability and tightness. New techniques have to be perfected in order to allow to make precise measurements of thermodynamic and kinetic parameters of binding in 3D-systems and to refine those already avalaible with 2D-arrays. Furthermore, molecular mechanics calculations should also be improved to afford a better modeling of the conformations of carbohydrates at interfaces, in relation with physical measurements such as NMR. 

The diversified porous patterns of diatomaceous silicas are on the nano- to submicrometer scale (< 10-300 nm) and these meso- and macropores cannot be mediated by single macromolecules, not even proteins. To mimic these meso- and macroporous structures, a different approach can be applied based on a phase separation process as in the vesicle-mediated macromorphogenesis processes extensivily reviewed in Pickett-Heaps et al.I 1 In this case oil-in-water (0/W) emulsions are applied as a model system. 0/W emulsions are isotropic and thermodynamically stable liquid media with a continuous water domain and an oil domain, which are thermodynamically stabilized by a surfactant as micrometer-sized liquid entities. 

The two-stage model is conceptually very useful because it separates the thermodynamics of insertion from helix assembly. When examining the stability of membrane proteins in the bilayer, the second stage is presumably the most relevant since the extrusion of the protein back into the aqueous environment should be extremely unfavorable. This idea is supported by the experiments discussed below. The two-stage model may not contain sufficient detail to describe the folding process in all cases, however. For example, the F and G helices of bacteriorhodopsin do not spontaneously form helices in vesicles, indicating that these peptides require assistance from the remainder of the protein to fold properly (Hunt et al., 1997). 

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

Fendler et al. [47] carried out experiments on vesicle-stabilized mixed crystals of Zn Cd. S and on CdS particles coated with ZnS. Besides absorption and fluorescence spectroscopy, x-ray diffractrometry was used for structural characterization of the various pure and mixed sulfide particles. In this article, the authors discuss thoroughly the fine interplay between kinetics and thermodynamics governing the rather complicated reaction scheme, finally yielding either separate particles of CdS and ZnS, mixed crystals Zn Cdj. S, or ZnS-coated particles of CdS. The latter may appear either as ZnS islands on the CdS particles or as a closed shell of ZnS surrounding a CdS core. In their experiments on capped particles, the authors of Ref. 47 do not attempt to decide which of the two morphologies occurs. 

The solubility of the monomers of bilayer-forming molecules is usually very low, say, in the range of 10 -10 ° M. Crystals of such amphiphiles immersed in water tend to swell. In this way lamellar liquid crystals (multilamellar vesicles) made up of bilayers packed in large stacks, separated by water molecules, are usually formed. They reach dimensions of a few thousands of nanometers. These lamellar structures may appear in different forms that readily interchange in response to small variations in temperature or composition. Unilamellar vesicles having a radius of a few tens up to a few hundreds of nanometers are derived from the lamellar liquid crystals by mechanical rupturing as occurs in ultrasonic treatment, for example. The unilamellar vesicles are thermodynamically unstable, and, hence, the properties of a unilamellar vesicle dispersion depend on how it was prepared. The colloidal stability of such a vesicle system is determined by the rate of fusion between two vesicles. This rate, in turn, is governed by the rules of colloidal stability discussed in Chapter 16. Anyway, the colloidal stability of unilamellar vesicles allows their use for in vitro studies of physical and chemical bilayer and membrane properties. 

The spectroscopic and thermodynamic data presented above suggest a correlation between molecular order, interfacial packing and the gel to liquid crystalline phase transition temperature. Aqueous phospholipid vesicles above their transition temperature readily form tightly packed, well-ordered monolayers at a water/CCU interface (e.g. DLPC). Vesicles below their transition temperature possess greater stability, forming monolayers which are considerably expanded and which show greater disorder (i.e. DPPC and DSPC). 

The formation of the encapsulating membranes is discussed by Turk-MacLeod et al. the operative strictures of thermodynamics in these processes and in the functional role of cell membranes are elaborated. The competition between vesicles that encapsulate RNA and those incapable of doing so, considered as model protocells, and its relation to the evolutionary fitness of replicator functions, is considered at length in terms of the driving forces of thermodynamics. It is noted that membrane stabilization is a key objective in this competition but this results also in a reduction of permeability, thus diminishing the ability of the protocell to use nutrients. Further evolution of the membrane and its constituents is necessary to overcome this restriction in function. In this respect it is of interest that model protocell membranes composed of particular mixtures of amphiphiles have superior... 

Suspension, electrochemistry of — SoHd particles, liquid droplets, -> vesicles, and biological cells suspended in solutions can cause faradaic and capacitive signals. Not only the size and the stability of the suspended particles but also surfactants determine the electrochemical response. Since colloidal suspensions are in meta-stable states, electrochemical behavior depends on the time-scale of the stabihty. Study on unstable suspension has been directed to aggregation and coalescence of particles, whereas that on stable suspension has to thermodynamics and kinetics as for huge molecules ... 

---