Interfacial denaturation

The complexity and diversity of structures in the native proteins eluded any attempt to produce some simple conformation that accounted for their interfacial properties. The study of synthetic polypeptides with non-polar side chains has provided good evidence to support the view that the a-helix can be stable at the air-water interface (5), and it is therefore possible that the interfacial denaturation of proteins is mainly a loss of the tertiary structure (6, 7, 8). Since for a typical protein an a-helix takes up about the same area per residue as the p conformation, it can be accommodated as easily. Moreover, like the p conformation but unlike a more randomly coiled structure, it is linear and therefore compatible with a plane surface without loss of configurational entropy (5). In this respect a plane surface may favor an ordered over a more random structure. The loss of solubility of the spread protein can then be attributed to intermolecular association between hydrophobic side chains exposed as a result of the action of the interface on the polar exterior of the molecules. 

Achieving specific and covalent inhibition of hpolytic enzymes is a difficult task, because of mutuaUy non-exclusive processes such as interfacial denaturation, changes in interfacial quahty [45], and surface dilution phenomena [46]. Further-... 

In spite of a tremendous amount of work, several very important aspects of protein denaturation in solution remain ill-known or controversial, because of theoretical and experimental difficulties. These difficulties are still much more acute in the case of the much less studied interfacial denaturation of proteins interfacial denaturation... 

Protein denaturation can be caused by a large number of physical and chemical factors (for an overview, see for example Refs. 1-3). We shall focus here on thermal denaturation (in solution), since it is obviously of prime importance in food science and technology, and also for fundamental reasons of its direct links with the thermodynamics of protein unfolding. Interfacial denaturation will be treated more succinctly, for less information is available on it. 

The process of the unwinding of DNA at the negatively polarized surface also takes place at the graphite electrode so that this interfacial process seems to be independent of the chemical nature of the adsorbent [114]. The interfacial denaturation of DNA has been observed at the positively charged surfaces of the graphite and silver electrodes [108, 114]. This is why a simple polarization effect of the electric field at... 

It is interesting that the surface denaturation of dh DNA is inhibited by the formation of covalent interstrand cross-links formed in DNA by bifunctional platinum coordination complexes - cis-diamminedichloro-platinum (II) and its trans-isomer [115]. The half-time of the interfacial denaturation of dh DNA in alkaline medium of ionic strength of 0.5 is c. 20 s [102] and is lowered with decreasing ionic strength [112]. The experiments carried out on dh DNAs of various base content indicate that the segments in dh DNA rich in adenine.thymine pairs are more susceptible to this kind of denaturation than those rich in guanine-cytokine pairs [115]. The surface denaturation at the mercury electrode has also been observed in dh RNA and dh complexes of synthetic polydeoxy- and polyribonucleotides [112, 116]. 

Surface films much more viscous than the bulk of the material occur also at a constant temperature. Thus, proteins denature at interfaces and produce interfacial films. Evaporation of the solvent into the atmosphere leaves a layer of higher concentration on the surface of the solution. As, usually, higher concentration means higher viscosity, a viscous surface film results. Again, the loss of solvent may be so great that the films solidify and a solid foam of an indefinite persistence is obtained. As a first approximation, the foam on milk consists of evaporated milk. 

Interfacial interaction between silicone and protein/starch microparticle, 3 and the use of polysiloxanes having hydrophilic groups for the stabilization of proteins against denaturation, 4 were studied. 

Kinsella (13, 14) summarized present thinking on foam formation of protein solutions. When an aqueous suspension of protein ingredient (for example, flour, concentrate, or isolate) is agitated by whipping or aeration processes, it will encapsulate air into droplets or bubbles that are surrounded by a liquid film. The film consists of denatured protein that lowers the interfacial tension between air and water, facilitating deformation of the liquid and expansion against its surface tension. 

Emulsification is a stabilizing effect of proteins a lowering of the interfacial tension between immiscible components that allow the formation of a protective layer around oil droplets. The inherent properties of proteins or their molecular conformation, denaturation, aggregation, pH solubility, and susceptibility to divalent cations affect their performance in model and commercial emulsion systems. Emulsion capacity profiles of proteins closely resemble protein solubility curves and thus the factors that influence solubility properties (protein composition and structure, methods and conditions of extraction, processing, and storage) or treatments used to modify protein character also influence emulsifying properties. 

Immobilization has other advantages it can slow enzyme deactivation by inhibiting protease attack and minimizing shear, interfacial, temperature, or solvent denaturation. As for the scarcity of some potentially very useful enzymes, it may be only a temporary problem. The development of cloning techniques, and probably the very increase in demand will result in lower prices. One spectacular instance is sialyl aldolase (see Table I). Industrial production of this enzyme by the gene-cloned strain of Escherichia coli has been reported.1,2 Sialylaldolase is now available from Toyobo at a moderate price. 

Because direct electrochemistry is observed only after the problems of interfacial specificity, compatibility, and denaturation have been overcome, it should provide us with a most powerful tool for investigating protein adsorption at surfaces. The studies of genetically engineered cytochrome c variants described in Section II serve as an illustration of this application. 

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