Stability, films, protein structure

Proteins and polypeptides comprise an essential class of biopolymers Their structures are diverse and complex, and provide the basis for their activity, stability, and functon. CD spectroscopy is an effective tool for characterizing these structures, whether in solution or in films or micelles. The sample requirements are minimal and CD can be used to assess structural changes in any type of protein. Therefore, CD is becoming an important method in the determinination of the stability of proteins. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

The data indicate that penetration of the monolayer occurs when the polyoxyethylene nonionic surfactants are injected into the substrate. Polar portions of surfactant molecules interact with their counterparts on the protein film through permanent dipole attraction and dipole-induced-dipole (van der Waals) interaction, and electrostatic attraction. The nature of this adsorption disrupts hydrogen bonding which partially stabilizes the tertiary protein structure. This makes the macromolecules more amenable to unfolding, and this has been observed with proteins in the presence of ionic surfactants (22) as well. 

Nicolini C, Erokhin V, Antolini F et al. thermal stability of protein secondary structure in Langmuir-Blodgett films. Biochim Biophys Acta 1993 1158 273-278. 

Hydrophobic interactions which are enforced (entropy driven) by the nature of water are the principle forces behind protein folding (6). They facilitate the establishment of other stabilizing interactions (7,10). Hydrophobic interactions, being of fundamental importance to protein structure, are very relevant to the functional properties of many food proteins, especially caseins. These forces affect solubility, gelation, coagulation, micelle formation, film formation, surfactant properties and flavor binding (7,10). 

LbL assembly improves the stability of immobilized proteins. For example, GOD immobilized in the LbL films keeps its high activity for more than 14 weeks at 4°C [180]. Most enzymes are denaturated and lose their activity at high temperature, but immobilization of GOD in the LbL films drastically enhances thermostability. A significant decrease in activity was not detected even after incubation at 50 °C. Immobilization of protein molecules in films through interaction with a polyelectrolyte matrix effectively prevents denaturation of protein structures. An interesting advantage of the LbL assembly over the LB technique was found in... 

The structure of whipped cream is quite complex. A coating of milk protein surrounds small globules of milk fat containing both solid and liquid fats. These globules stack into chains and nets around air bubbles. The air bubbles are also formed from the milk proteins, which create a thin membrane around the air pockets. The three-dimensional network of joined fat globules and protein films stabilizes the foam, keeping the whipped cream stiff. 

Such differences in the secondary structure behavior with respect to temperature can be explained by suggesting that molecular close packing of proteins in the film is the main parameter responsible for the thermal stability. In fact, as in the case of BR, we have close packing of molecules even in the solution (membrane fragments) there are practically no differences in the CD spectra of BR solution at least tiU 75°C (denaturation takes place only for the sample heated to 90°C). RC in solution begins to be affected even at 50°C and is completely denatured at 75°C, for the solution contains separated molecules. 

The Langmuir-Blodgett (LB) technique was successfully applied for the deposition of thin protein layers (Langmuir and Schaefer 1938, Tiede 1985, Lvov et al. 1991). LB organization of protein molecules in film not only preserved the structure and functionality of the molecules, but also resulted in the appearance of new, useful properties, such as enhanced thermal stability (Nicolini et al. 1993 Erokhin et al. 1995). 

Comparative study of LB films of cytochrome P450 wild type and recombinant revealed similar surface-active properties of the samples. CD spectra have shown that the secondary structure of these proteins is practically identical. Improved thermal stability is also similar for LB films built up from these proteins. Marked differences for LB films of wild type and recombinant protein were observed in surface density and the thickness of the deposited layer. These differences can be explained by improved purity of the recombinant sample. In fact, impurity can disturb layer formation, preventing closest packing and diminishing the surface density and the average monolayer thickness. Decreased purity of... 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

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