j^Z FINE-STRUCTURE OF PROTOPLASM II 



Table XV gives examples of this multiple series (cf. K. H. Meyer, 

 1940a, p. 409). It shows how the Svedberg units combine in 2's, 4's, 

 8's, i6's etc. There are, however, not only multiples of 2, but also of 5 

 (e.g. 24), a fact which recalls the Bergmann-Niemann rule. Up to 

 384 units may be combined in one molecule. The aggregation or 

 dissociation of these large particles depends on p^ conditions. 



Since the nitrogen content of proteins is 16%, the average mole- 

 cular weight of the amino acids in proteins is 6.25 x N = 87.5, if no 

 allowance is made for basic amino acids with more than one N-atom. 

 With this figure, the approximate number of amino acids in globular 

 protein molecules can be calculated. The Svedberg unit contains 

 about 200 (which is near to the figures of 2^ x 3 = 192 or 2^ x 3^ 

 = 216) and the largest particles mentioned in Table XV contain more 

 than 75,000. 



Globular protein molecules can be photographed in the electron 

 microscope (Fig. 84a, b, p. 126). The average space needed by an 

 amino acid (Fig. 181, p. 365) is 3.5 X 4-6 X 10 A^ = 161 A^. In the 

 electron microscope a sphere of diameter 50 A can be readily recog- 

 nized. Its volume is 50=^ x n/G A^ = 65,500 A^. This corresponds to 

 about 400 amino acids. Protein molecules with two Svedberg units 

 must therefore be easily visible in the electron microscope, while the 

 Svedberg unit itself is just at the limit of the resolving power. 



A similar result is obtained if we remember (Fig. 31b, p. 34) that 

 in an aliphatic chain the carbon atoms are lined up at intervals of 

 1.25 A, the distance between neighbouring chains being 5 A. Thus 

 40 X 10 X 10 = 4000 carbon atoms can be placed in a cube of 50^ A=^. 

 This would yield a molecular weight of 48,000, which, again, corre- 

 sponds roughly to 2 Svedberg units. 



A third determination is possible based on the average density of 

 proteins, which is 1.33. Knowing the absolute weight of a Svedberg 

 unit (17,600 divided by the Loschmitt number 6.06 X lo^^), the 

 volume of the molecule can be calculated. Considered as a sphere, its 

 diameter is 34.5 A. In Table XV the size of the macromolecules in 

 the multiple series of globular proteins has been calculated in this way 

 (Frey-Wyssling, 1949a), the dimensions found being as shown in 

 Fig. 84a, b (p. 126). As a result we may note that globular macro- 

 molecules of protein with at least 400 amino acids or a molecular 

 weight of about 40,000, are within the resolving power of the electron 



