184 LECTURE IX. 



corresponds with that of the peptones from silk. Many polypeptides are 

 precipitated from dilute solution by phospho-tungstic acid. It is also 

 interesting to note that difficultly soluble amino acids produce polypeptides 

 which are easily soluble, and that difficultly soluble polypeptides often 

 instantaneously become soluble on the introduction of another amino acid. 

 Tetraglycine is difficultly soluble. Leucyl-tetraglycine is easily soluble. 

 As is well known, the peptones are all easily soluble in water, although 

 it is necessary to remember that we are here dealing with mixtures whose 

 components may be capable of keeping each other in solution. The changes 

 which occur in the taste of these substances is also very interesting: for 

 instance, when sweet-tasting amino acids are linked together the resulting 

 product has often a bitter taste. The peptones also, as a rule, taste 

 distinctly bitter. 



We must admit that many analogies exist between the synthetic polypep- 

 tides and the peptones. We can make no sharp distinction in this direction. 

 We must not lose sight of the fact that we are comparing a sharply denned 

 chemical compound with a mixture. The name " peptone " does not 

 indicate any definite compound; in fact, may not even represent distinctly 

 analogous cleavage-products of protein. It is much better to assume that 

 the peptones represent all stages of decomposition between that of albu- 

 moses and the amino acids. 



Although we do not expect at present to obtain positive results by 

 direct comparison in this way, excellent progress has been made by biological 

 experiments. It was of the greatest significance that certain polypeptides 

 were decomposed by the pancreatic ferment 1 in the identical manner as the 

 peptones themselves. Thus glycyl-Z-tyrosine quickly breaks down into its 

 components, glycocoll and Z-tyrosine. It is also especially interesting to 

 note that the racemic polypeptides are broken down asymmetrically; that 

 is, only half of the racemic substance is attacked. 2 The following example 

 is given to illustrate this clearly. If we prepare the polypeptide, alanyl- 

 leucine, from racemic alanine and leucine, we necessarily expect to obtain, 

 according to the theoretical conceptions previously considered, four com- 

 binations, which contain the four active amino acids, I- and d-alanine and 

 I- and d-leucine. One racemic compound contains d-alanine and d-leucine 

 and Z-alanine and Z-leucine (d-alanine-d-leucine + Z-alanine-Z-leucine) . 

 The second is constructed in the following manner: d-alanine-Z-leucine 

 + Z-alanine-d-leucine. The pancreatic ferment, however, splits only 

 one of these two racemic combinations. Experience has already taught 

 us that optically active amino acids in the albumins are split off; hence, 

 we are justified in concluding, that of the two racemic compounds men- 



1 E. Fischer and P. Bergell: Ber. 36, 2592 (1903); 37, 2103 (1904). E. Fischer and 

 E. Abderhalden: Sitzungsber. Akad. Wiss. Berl. 1906; Z. physiol. Chem. 46, 52 (1905). 



2 For further details, see Lecture on Ferments. 



