Cavitation, Tensile Strength, and the Surface Films of Gas Nuclei 



vapor through hydrocarbon surface films, it has been found that a compressed 

 film is somewhat more resistant to the diffusion of water vapor than un uncom- 

 pressed film (14). When the film is tightly packed by compression, the cross - 

 sectional area occupied by each molecule is about 21 A^.* 



The composition of a protein film is summarized by Adam (12). It is dia- 

 grammatically pictured in Fig. 12 as a plan view, looking down upon the sur- 

 face of the water. The linkages forming the backbone of the structure are 

 called peptide links. They bind in sequence the repeating CH, NH, CO, . . . 

 groups. Attached to this backbone are hydrocarbon chains from one to six 

 repeating units long, or chains with a COOH or NH2 group at the end. Each 

 chain is represented by the term R. The endless variety of proteins encoun- 

 tered in nature is due to the various different chains (R), and the different 

 sequences and patterns in which these chains are attached to the polypeptide 

 backbone (23). In an uncompressed form, the side chains R are stretched out 

 on the surface of the water. The R chains are identified by examining the 

 breakdown products of proteins. Several dozen of these amino acids are 

 known (24). 



K considerable compression of the protein were to occur, then the peptide 

 links would tend to pack closely, as shown, with the R chains forced perpendic- 

 ularly out of the surface. The significance of this is that the reoccurring groups 

 in the backbone to which the R chains are attached would then be on as little as 

 3.5 X 4.5 A spacings. This gives an area allotted to each R chain of about 17 A^, 

 somewhat less than the spacing of the chains in a compressed hydrocarbon film. 

 However, the length of the R chains is less than the chain length for a hydro- 

 carbon. 



CO NH CH 



Fig. 12 - Plan view of the structure 

 of a protein film 



*One angstrom unit (A) equals 10"^ centimeters. 



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