I CYTOPLASM 145 



microscope. If the diameter d is calculated from the mean space of 

 161 A^ needed by an amino acid, somewhat largervalues are obtained. 

 The length of the completely unfolded polypeptide chain of the 

 denatured molecule is found by multiplying the chain period of 3.5 A 

 (trans-) or 2.8 A (cis-configuration,seep. 346) by the number of amino 

 acid residues in the molecule. With the period 3.5 A, the lengths L 

 indicated in Table XV are obtained. Of course, these figures are 

 maximal values which are not realized, since the chains will never 

 expand completely, but will assume a bent or curled shape. 



Surface films of proteins. Although globular proteins are soluble in 

 water or salt solution, not all parts of the molecule show an affinity 

 for water. The polypeptide chains which are coiled up in an unknown 

 manner within globular molecules carry hydrophilic and lipophilic 

 (hydrophobic) side groups. The former strive for contact with water 

 but the latter "avoid" it, hence the proteins can be spread as molecular 

 films on the surface of water (Gorter and co-workers, 1955). One 

 milligram of protein can cover a surface from one to more than two 

 and a half square metres; assuming a density of 1.53, this means films 

 of 7.5 to 3 A thickness (Adam, 1941, p. 87). These values show that 

 the molecular film cannot consist of spherical macromolecules, but 

 that these protein globules flatten and uncoil to form protein chains. 

 This spreading of the macromolecule allows all hydrophiUc groups 

 to make contact with the water surface and all hydrophobic groups 

 to turn away from it towards the air. If the surface film is larger than 

 one square metre, it is liquid, i.e. the flattened molecules retain their 

 mobility and may change their relative positions on the water surface. 

 However, as soon as the film is compressed to an area of one square 

 metre, it becomes solid, rigid and insoluble; the molecules lose their 

 individuality and, because they stick closely together, they can no 

 longer be hydrated. They assume the state of fibrous proteins and as 

 such become insoluble. This change of solubility is known as "de- 

 naturation", mentioned on p. 136. Merely shaking a protein solution 

 often suffices to form a foam of insoluble denatured protein. 



If the molecular weight of the protein is known, the area per mole- 

 cule in a surface film of 7,5 A thickness can be calculated, as has been 

 done in Table XV (p. 141). If this area is considered to be circular, 

 the diameter D of the circle can be compared with the diameter d of 

 the globular molecule. For small protein molecules the ratio D/d is 



