476 PROTOPLASM 



composed of a linear fiber and itself an elongated structure, gives 

 substantial thickness to the molecule as a whole. 



We have to deal not only with the question of the external 

 shape of a protein molecule but also with the difficult one of 

 internal arrangement of parts. Any change in the orientation 

 of the amino acids will mean a different protein. The apparent 

 hopelessness of the situation is graphically given by the English 

 chemist J. B. Leathes. He takes the "very simple case" of a 

 protein with only 50 amino links. If 1 link recurs ten times, 

 4 recur four times, and 10 recur twice, then the number of possible 



[50 

 permutations will be, rjQ X (li)'* X (|^)^°. In such a protein of 



50 links, of which only 19 are different, the number of possible 

 arrangements of its parts will be 10'*^ Light takes 300,000 years 

 to travel the length of the Milky Way. This distance, expressed 

 in angstrom units, of which 100,000,000 equal a centimeter, will 

 be less than 10^'^ It is thus clear how great the variations in 

 disposition of the parts of a protein molecule may be and how 

 far we are from being able to map out such a structure. 



Molecular Weight. — Among the many distinguishing proper- 

 ties of the proteins we shall select but a few, the essentially 

 colloidal ones, in which we, as biologists, are primarily interested, 

 though there are others such as optical properties (proteins are 

 laevorotatory) which are equally significant. 



That proteins are colloidal systems there can be no doubt, but 

 until their structure and behavior are better known it cannot be 

 said with certainty whether they are colloidal because of their 

 huge molecules alone or because of a larger structural unit — 

 the micelle. Structure is significant, but we shall do well to 

 remember that we ask of colloidal systems not what they are 

 but what they do. The fact that proteins diffuse slowly, exert 

 little osmotic pressure, exhibit a Tyndall cone, and form jellies 

 is sufficient evidence that they are colloidal quite aside from the 

 nature of their dispersed particles. Protein molecules are 

 sufficiently large to account for colloidal behavior without the 

 need of postulating molecular aggregates — the micelles — even 

 though these may be present. 



The molecular weight of hemoglobin has been variously given, 

 from a minimum of 16,000 to a maximum of 68,000. The lower 

 estimate was based on the assumption that the molecule 



