Monolayer protein

We have observed that such proteins as CaM and bovine serum albumin (BSA) can be developed at the air-water interface to form monolayer protein films. In previous works, the developed BSA monolayer was stabilized by cross-linking with a bifunctional reagent immediately after the preparation of protein monolayer. The BSA thin film thus prepared can be employed as a passive material, e.g., an ultrathin protein film for a matrix of enzyme-linked immunosorvent assays. 

Figure 8-12. Left Monolayer protein voltammetry of P. furiosus ferredoxin on an Au(lll)-electrode modified by a mercaptopropionic acid (MPA) SAM, cf, Fig. 8-lOC 5 mM phosphate buffer, pH 7.9. Scan rate 5 mV s . Middle In situ STM image of P. furiosus ferredoxin molecules in electron transport action on the same MPA-modified Au(lll)-electrode surface Ar-atmosphere. Working electrode potential -0.35 V (SCE), bias voltage -0.35 V. Tunneling current 0.10 nA. Right Schematic view of the P. furiosus ferredoxin molecule on the MPA-modified Au( 111 )-surface. From ref. 137 with permission.

Differences between Fixed Monolayer, Protein Clefts and Artificial Receptors... 

Important consequences of this feature can be readily assessed by assuming that monomeric units of structural protein forming the primary monolayer protein coat (see Section II, A) are regular hexagons of side 58 A whieh eorrespond to 64,000 M.W. units (see Section III, B, 2 and IV, A, 1 and 2). It must be emphasized that neither the shape or size assumed for subunits are of critical importance to the conclusions of the following demonstration. 

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. 

Proteins, like other macromolecules, can be made into monolayers at the air-water interface either by spreading, adsorption, or specific binding. Proteins, while complex polymers, are interesting because of their inherent surface activity and amphiphilicity. There is an increasing body of literature on proteins at liquid interfaces, and here we only briefly discuss a few highlights. 

A monolayer of Streptavidin containing 1.75 mg of protein/m gives a film pressure of 0.070 erg/m at 15°C. Calculate the molecular weight of the protein, assuming ideal-gas behavior. 

Prime K L and Whitesides G M 1991 Self-assembled organio monolayers—model systems for studying adsorption of proteins at surfaoes Science 252 1164-7... 

Harder P, Grunze M, Dahint R, Whitesides G M and Laibinis P E 1998 Molecular conformation in oligo(ethylene glycol)-terminated self-assembled monolayers on gold and silver surfaces determines their ability to resist protein adsorption J. Rhys. Chem. B 102 426-36... 

Feldman K, Hahner G, Spencer N D, Harder P, and Grunze M 1999 Probing resistance to protein adsorption of oligo(ethylene glycol)-terminated self-assembled monolayers by scanning force microscopy J. Am. Chem. Soc. at press... 

DiMilla P A, Folkers J P, Biebuyck H A, Harter R, Lopez G P and Whitesides G M 1994 Wetting and protein adsorption of the self-assembled monolayers of alkanethiolates which are supported on transparent films of gold J. Am. Chem. Soc. 116 2225-6... 

Guemouri L, Ogier J and Ramsden J J 1998 Optical properties of protein monolayers during assembly J. Chem. 

Films or membranes of silkworm silk have been produced by air-drying aqueous solutions prepared from the concentrated salts, followed by dialysis (11,28). The films, which are water soluble, generally contain silk in the silk I conformation with a significant content of random coil. Many different treatments have been used to modify these films to decrease their water solubiUty by converting silk I to silk II in a process found usehil for enzyme entrapment (28). Silk membranes have also been cast from fibroin solutions and characterized for permeation properties. Oxygen and water vapor transmission rates were dependent on the exposure conditions to methanol to faciUtate the conversion to silk II (29). Thin monolayer films have been formed from solubilized silkworm silk using Langmuir techniques to faciUtate stmctural characterization of the protein (30). ResolubiLized silkworm cocoon silk has been spun into fibers (31), as have recombinant silkworm silks (32). 

RAIRS is routinely used for the analysis of chemically modified surfaces - surface systems in electrochemistry [4.277], polymer research [4.266, 4.278], catalysis [4.265, 4.271], selfassembling monolayers [4.267, 4.268], and protein adsorption [4.268, 4.279] have been investigated. 

Proteins that can flip phospholipids from one side of a bilayer to the other have also been identified in several tissues (Figure 9.11). Called flippases, these proteins reduce the half-time for phospholipid movement across a membrane from 10 days or more to a few minutes or less. Some of these systems may operate passively, with no required input of energy, but passive transport alone cannot establish or maintain asymmetric transverse lipid distributions. However, rapid phospholipid movement from one monolayer to the other occurs in an ATP-dependent manner in erythrocytes. Energy-dependent lipid flippase activity may be responsible for the creation and maintenance of transverse lipid asymmetries. 

Highly active catalysts have been produced by adsorption of lipases onto macroporous acrylate beads, polypropylene particles and phenol-formaldehyde weak anion exchange resins. Protein is bound, presumably essentially as a monolayer, within the pores of the particles. The large surface area of the particles (10m2 g 1) means that substantial amounts of protein can be adsorbed, and the pores are of sufficient size to allow easy access of reactants to this adsorbed protein. 

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