Surface films of proteins

Surface films of proteins. Although most proteins are soluble in water they are usually sufficiently adsorbed to leave the interior of a solution almost completely for the surfaoe, provided this is large enough. Also, if a protein is placed in a suitable manner at the surface of water, it will often spread out to a thin film, which is of the order one amino-acid in thickness not only is the protein arranged in a monomolecular layer on the surface, but the complex protein molecules are themselves unfolded so that every amino-acid has its own place on the surface. Hence proteins can be studied by methods appropriate for insoluble films. 

Rideal and his collaborators found that solid particles of protein spread very rapidly on a clean surface their method of spreading is to coat a quartz fibre, first covered with paraffin wax, with a little protein this is dipped into the water surface, and the amount of protein passing into the surface ascertained by weighing the fibre on a simple micro-balance.1 By this method somewhat larger areas can sometimes be obtained, and films formed from proteins which cannot be made to form homogeneous films when spread by Gorter s technique and in certain cases the average thickness of the fully spread film is only about 3 A. 

Protein films are coherent their molecules are much too large for gaseous films to be detectable. They are also very compressible in all cases measured the area diminishes considerably between 2 and 12 dynes, 

The composition of a protein is, in general, a series of polypeptide linkages1 

Evidently very little force is required to remove the side chains nearly completely from the surface, since the compressibility above 1 sq. m. per mg. is so large. Gliadin contains a large proportion of the dibasic acid, glutamic, as well as smaller amounts of diamino acids, so that it is a 

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These surface films of proteins are not infrequently formed on solutions in which the proteins are quite soluble, so that it may be argued that the protein has been altered or denatured by its unfoldment in the surface film, in such a way as to render it less soluble or completely insoluble. Whether the unfoldment and spreading of the molecule always results in the loss of solubility is not proven Gorter has been able to remove pepsin from a surface on which it has been spread by means of a fine net pulled up through the surface, and subsequently dissolved the material in water, finding that it retained its normal properties, including proteolytic activity. 

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