Protein interfacial dynamics

Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide what appears to be a very unique and well-suited medium for solubilizing proteins, amino acids, and other biological molecules in a nonpolar medium. The medium consists of small aqueous-polar nanodroplets dispersed in an apolar bulk phase by surfactants (Fig. 1). Moreover, the droplet size is on the same order of magnitude as the encapsulated enzyme molecules. Typically, the medium is quite dynamic, with droplets spontaneously coalescing, exchanging materials, and reforming on the order of microseconds. Such small droplets yield a large amount of interfacial area. For many surfactants, the size of the dispersed aqueous nanodroplets is directly proportional to the water-surfactant mole ratio, also known as w. Several reviews have been written which provide more detailed discussion of the physical properties of microemulsions [1-3]. 

Murray, B.S., Dickinson, E. (1996). Interfacial rheology and the dynamic properties of adsorbed films of food proteins and surfactants. Food Science and Technology International (Japan), 2, 131-145. 

A situation that commonly occurs with food foams and emulsions is that there is a mixture of protein and low-molecular-weight surfactant available for adsorption at the interface. The composition and structure of the developing adsorbed layer are therefore strongly influenced by dynamic aspects of the competitive adsorption between protein and surfactant. This competitive adsorption in turn is influenced by the nature of the interfacial protein-protein and protein-surfactant interactions. At the most basic level, what drives this competition is that the surfactant-surface interaction is stronger than the interaction of the surface with the protein (or protein-surfactant complex) (Dickinson, 1998 Goff, 1997 Rodriguez Patino et al., 2007 Miller et al., 2008 Kotsmar et al., 2009). 

The new generations of experiments are aimed at linking dynamical studies of these and other processes to the function. We have already begun research in this direction. In a recent publication [9] we reported studies of the femtosecond dynamics of an RNA-protein complex and then compared the results with those obtained for in vivo (E. Coli) transcription anti-termination activities. In two other studies we measured the activity of the protein Subtilisin Carlsberg, discussed above, to a substrate, and the role of hydration in interfacial binding and function of bovine pancreatic phospholipase at a substrate site. The goal in all these studies is to relate structures to the dynamics and hopefully to key features of the (complex ) function. 

An excellent comprehensive review of all theoretical and practical aspects of dynamic and static interfacial tension measurements written by the most prolific authors in the field of protein adsorption. Contains a wealth of additional references that the interested reader may consult to gain additional understanding of the field of research. 

Figure 3 shows the dynamic IFT of soybean oil/water interfaces under expansion with constant flow rate as a function of the relative change of the interfacial area, with various surfactants in the oil and aqueous phases, respectively. The IFT is lowest if both phases contain surface active additives, and it hardly changes due to the presence of the fast adsorbing, low molecular emulsifier SPAN 80 in the oil phase. The increase of the dynamic IFT with the interface expansion is most pronounced with 0.01 % BSA in the aqueous phase due to the slow adsorption of the protein. 

Edited by T. Kajiyama and M. Aizawa Vol. 5 Foam and Foam Films. By D. Exerowa and P.M. Kruglyakov Vol. 6 Drops and Bubbles in Interfacial Research. Edited by D. Mobius and R. Miller Vol. 7 Proteins at Liquid Interfaces. Edited by D. Mobius and R. Miller Vol. 8 Dynamic Surface Tensiometry in Medicine. By V.M. Kazakov, O.V. Sinyachenko,... 

As discussed above in Chapter 3, ellipsometry and quartz crystal microbalance (QCM) approaches provide a useful insight into the adsorption of both the supporting interfacial assembly and the proteins themselves. Beyond monitoring the adsorption dynamics and the structural integrity of the biomolecule, the orientation of the active site is of prime importance. For example, if the active site itself binds to the self-assembled monolayer, transport of the substrate or co-enzyme may be blocked. 

The lipidic cubic phase has recently been demonstrated as a new system in which to crystallize membrane proteins [143, 144], and several examples [143, 145, 146] have been reported. The molecular mechanism for such crystallization is not yet clear, but the interfacial water and transport are believed to play an important role in nucleation and crystal growth [146, 147], Using a related model system of reverse micelles, drastic differences in water behavior were observed both experimentally [112, 127, 128, 133-135] and theoretically [117, 148, 149]. In contrast to the ultrafast motions of bulk water that occurs in less than several picoseconds, significantly slower water dynamics were observed in hundreds of picoseconds, which indicates a well-ordered water structure in these confinements. 

Protein function at solid-liquid interfaces holds a structural and a dynamic perspective [31]. The structural perspective addresses macroscopic adsorption, molecular interactions between the protein and the surface, collective interactions between the individual adsorbed protein molecules, and changes in the conformational and hydration states of the protein molecules induced by these physical interactions. Interactions caused by protein adsorption are mostly non-covalent but strong enough to cause drastic functional transformations. All these features are, moreover, affected by the double layer and the electrode potential at electrochemical interfaces. Factors that determine protein adsorption patterns have been discussed in detail recently, both in the broad context of solute proteins at solid surfaces [31], and in specific contexts of interfacial metalloprotein electrochemistry [34]. Some important elements that can also be modelled in suitable detail would be ... 

It is reasonable and widely accepted that the back-transfer process of proteins and other solutes is governed by an interfacial process and by a coalescence of the reverse micelles at the oil-water interface. According to the previous report, alcohol promotes the fusion/fission of the reverse micelles [11]. Such a modification in the dynamic property of reverse micellar droplets also affects the coalescence of the droplets and the bulk aqueous solution, and in this study results in an assistance in the release of proteins from the droplets. However, besides the alcohol addition, the appropriate pH and salt concentration in the recovery aqueous phase is required for protein release from the droplets into the recovery phase. The salt concentration leads to an osmotic effect and results in a swelling of the droplets in the presence of alcohol. The swelling droplets would... 

It appears that the short time dynamics of water molecules at or near the hydrophilic model surface and at a soluble protein surface is much slower than that of the bulk water. It is important to note that the more significant slow dynamics of interfacial water is reflected in the long residence time for jump diffusion. This suggests that there may be a common underlying mechanism for the slowing down of the single-particle dynamics of interfacial water. 

Note. The table contains twice. Although we use the same symbol for the interfacial dialtional viscosity (commonly measured at large strain) and the dynamic Interfaclal dilatlonal viscosity (small strain, oscillatory measurements) the values obtciined may differ (as found for instance for some protein monolayers). 

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