Molecular Structure m Protoplasvi 



51 



of amino acid residues. Thus a protein of molecular weight 36,000 

 with about 300 amino acid residues of average molecular weight, 

 120, will have a chain length of about 1,000 A, a width of about 

 10 A, and a thickness of 4.5 A. 



Although these generalized chain dimensions are well enough 

 established, the configurations which the chains may take and the 

 resulting sizes, shapes, and structural details in the formation of 



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Fig. 4. Model photograph of a portion of a protein chain, made to scale 

 using known atomic radii and bond angles. Black balls indicate carbon atoms; 

 dark grey balls marked X indicate nitrogen atoms; light grey balls marked 

 with a dot indicate oxygens; small white balls, hydrogen atoms. 



submicroscopic particles is less satisfactorily known. Practically all 

 of our knowledge concerning these comes from investigations of 

 proteins in bulk which have been treated in various ways for pur- 

 poses of purification. Material of this sort has been subjected to 

 many experimental procedures in attempts to determine the exist- 

 ence of elementary protein particles, or molecules, and their sizes, 

 shapes, and structural details. 



Investigations as to size or sizes of the elementary particles have 

 been made by means of osmotic pressure and diffusion measure- 

 ments (34, 35), analytical methods (37), the ultracentrifuge (9, 

 38), and determination of the unit cell dimensions in crystalline 

 proteins (36 and 39). From this work it appears that the protein 

 particles may belong to groups of molecular weight 17,500, or some 

 multiple of this value, as 35,000 or greater; or submultiple, as 9,000. 



