the Optical Rotation of the Proteins of Blood Sera. 189 



This regular ratio for the several parts of the absorption curve would 

 ordinarily signify a corresponding difference in the concentration of the 

 substance in solution, but, inasmuch as the percentage concentration of the 

 albumin was the same for all the solutions, both in the experiment and in 

 the ultimate calculations, some other explanation must be found. The work 

 of Kober* and others has shown that aromatic amino acids exhibit selective 

 absorption, while the absorption spectra of several of the aliphatic amino 

 acids and the simpler polypeptides exhibit no selective absorption, and that 

 even 40 mm. layers of - 05 per cent, solutions of some of the latter show no 

 general absorption beyond a wave-length of 2500. In view of the way in 

 which the proteids are built up of amino-acids and similar groups, some 

 exerting selective absorption and some not, and of the difficulty in accounting 

 for some of the properties of the proteins when viewed as chemical entities, 

 it is not unreasonable to regard them as products resulting from an essentially 

 physical association of substances comparable with, but not so chemical as, the 

 association of a salt with its water of crystallisation. It may then be assumed 

 that the aggregate composing human albumin may result from such a union 

 of a substance comparable with that composing horse albumin with a substance 

 or substances possessing no selective absorption. Such an hypothesis of 

 physical association gains some support from the fact that five out of six of 

 the amplitude values, - 72, 072, - 53, - 66, 0"36, 0'20, are approximately 

 simple multiples of 018, and from the view held by Prof. F. Gowland 

 Hopkinsf that even the apparently well-defined crystallised egg albumin is 

 composed of several proteids, and that much the same may be said of serum 

 albumin. It is conceivable that each of the proteins now studied is an 

 aggregate resulting from the physical association of a proteid substance 

 exhibiting selective absorption with various but definite proportions of such 

 simpler bodies as those described above as exerting only general absorption. 

 Had the results now recorded been anticipated, it would have been instructive 

 to determine in each specimen the yield of phenylalanine, tryptophane and 

 similar products exhibiting selective absorption, to see whether the proportion 

 was correlated with the extinction coefficient or not. It would have given 

 some indication of how far the association of the groups is physical or 

 chemical. 



Moreover, if all the curves be re-drawn so as to have the same amplitude, 

 say 0-72 or TO, very little difference will be observed in their form. How- 

 ever, the two albumins would be distinguished from the globulins by the 

 general absorption being slightly greater, extending to about 2450 at the 



* ' J. Biol. Chera.,' vol. 22, pp. 433-441 (1915). 

 t 1 J. Physiol.,' vol. 25, pp. 306-330 (1900). 



