Spread Monolayers of Protein

Fig. 3. A spread monolayer of protein decreases fe for the absorption of CO (from the gas phase at 1 atm. partial pressure) into water (28). The surface is cleaned thoroughly before the experiment, and the contra-rotating stirrers in the liquid are running at 437 r.p.m. The surface is nonrotating in aU the experiments described here, and when fct is reduced to 1.1 X lO"" cm. sec.", all random surface movements are also eliminated.

Fig. 9. Reduction in kt by spread monolayer of protein, due to prevention of transfer of momentum from ethylacetate, and to reduction in turbulence in the aqueous phase near the interface. Experimental data used here are taken from reference (60).

Spread monolayers of proteins have been previously reviewed by Gorter (2) and by Neurath and Bull (3) and by Bateman (4). 

If a small amount of protein solution is suitably spread at the surface of an aqueous substrate, most of the protein will be surface-denatured, giving an insoluble monomolecular film before it has a chance to dissolve. The techniques already described for studying spread monolayers of insoluble material can, therefore, be used in... 

A monolayer of protein spreads at the air-water interface. The amount of protein spread is equivalent to 0.80 mg m" and gives a surface tension lowering of 0.035 mN mWhat is the molecular weight of the protein ... 

In order to spread on a water surface and form a stable film a substance has to have hydrophilic and hydrophobic groups in it. The bottom side of a spread film of protein must be predominantly hydrophilic, while the side directed towards the air must be predominantly hydrophobic, otherwise a protein monolayer would not be stable on the water surface. That the air side of a protein monolayer is predominantly hydrophobic is shown by the experiments of Bull (70) who deposited single monolayers of egg albumin at various pressures on glass slides and measured the contact angle between water and the deposited film. The film deposited at 15 dynes pressure showed an adhesion tension of about 100 ergs per sq. cm. The adhesion tension of a pure hydro-... 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

As described here, the monolayer of a lipid can be formed by different spreading methods. The thermodynamics of the Ilcol analysis is given in the literature (Birdi, 1989). The monolayer collapse has been shown to provide much information also in the case of protein monolayers. 

Arnold and co-workers attempted to prepare imprinted metal-coordinating polymers for proteins [25]. For this purpose, efforts were made to prepare metalcoordinating molecularly patterned surfaces in mixed monolayers spread at the air-water interface or liposomes. This approach was termed as molecular printing and is illustrated in Fig. 6.6. In this process, a protein template is introduced into the aqueous phase, which imposes a pattern of functional amphiphiles in the surfactant monolayer via strong interactions with metal-chelating surfactant head groups. The pattern is then fixed by polymerising the surfactant tails. The technique has also been employed for two dimensional crystallisation of proteins [26]. 

Another spreading method, often applied when insoluble monolayers of soluble components (polymers) are to be made. Involves a rod that is placed on the bottom of the container emd that has its roughened tapered top Just penetrating the upper phase, as shown in fig. 3.3b. The rod should consist of material that is preferentially wetted by the lower phase, so that it remains covered with a film of the lower liquid. The tip of the syringe is placed on the top of the rod and the solution is spread in the direction of the cUTOws, shown in fig. 3b. In this way Trumlt, who invented this method, successfully spread protein molecules from an aqueous solution on em air-water interface. ... 

Crosslinking of protein monolayers by mercuric ion (MacRitchie, 1970) and silicic acid (Minones et al., 1973) has been reported. These studies are relevant to poisoning by heavy-metal ions and to silicosis, effects that seem likely to result from attack on the cell membrane proteins. Crosslinking by mercuric ion was detected by a spectacular increase in surface viscosity and a decrease in compressibility when a number of proteins (BSA, insulin, ovalbumin, and hemoglobin) were spread on 0.001 M mercuric chloride solution. Poly-DL-alanine was unaffected whereas poly-L-lysine and poly-L-glutamic acid were affected in a similar manner to the proteins, indicating that mercuric ion interacts with the ionizable carboxyl and amino groups on the protein side—chains. Silicic acid similarly caused protein monolayers... 

When protein solutions are shaken, insoluble protein is often seen to separate out (Bull and Neurath, 1937). The coagulation occurs at the interface and may be observed when protein is allowed to adsorb from solution at a quiescent interface (Cumper and Alexander, 1950) or when spread protein monolayers are compressed (Kaplan and Frazer, 1953). This is an interesting type of phase separation in which a three-dimensional coagulum is formed from the two-dimensional monolayer, once a certain critical value of the interfacial pressure is exceeded. The concentration of protein in the monolayer when the critical pressure is reached may be thought of as the solubility in the interface under those conditions. When this concentration is exceeded, precipitation occurs. A simple model may help to illustrate how free energy considerations govern the coagulation. 

---