22O 



PRINCIPLES OF GENERAL PHYSIOLOGY 



Whether R is to be looked upon as an ion with both a negative and a positive charge is 

 doubtful. If so, it is formed by giving off both H' and OH' ions and would be represented 

 thus : 



CHoNH 



COO' 



in the case of glycine and is sometimes known as a "hermaphrodite" ion. In Bredig's scheme, 

 however, it is represented as devoid of charges and is probably, in fact, an internal anhydride: 



CR.NH., 



COO. 



As such, we must suppose that the two groups combined have opposite charges, so that it is 

 not impossible that they might exist as such on a single ion. An interesting suggestion 

 is made by Bredig (1894, p. 323), as to the length of the chains which can exist without 

 self-neutralisation. If a sufficiently long chain could be formed, having opposite charges 

 at the ends, it should be possible by optical means to detect an orientation to an electrical 

 current passed through the solution. Bredig, himself, was unable to detect any sign of 

 this in the case of betaine. 



It is unnecessary to remark that an ion with two opposite charges moves to neither 

 electrode, being equally attracted to both, so that it can take no part in the conduction 

 of a current. In this aspect, it is not, in any case, entitled to the name of an ion, 

 in Faraday's sense. 



As we have seen above (page 105), there is no evidence that an amino-acid can combine with 

 the positive and negative ions of a neutral salt simultaneously. A "hermaphrodite" ion 

 should be able to do this. 



The various ions enumerated above as present in solutions of the amino-acids 

 exist in very small concentrations, so that their electrical conductivity is very low, 

 especially in the case of the mono-amino-monocarboxylic series. The acidic and 

 basic groups are mutually antagonistic, so that both dissociation constants are 

 very small. The mono-amino-monocarboxylic acids are very weak indeed, both as 

 acids and as bases. The carboxyl group is a little stronger as acid than the NH., 

 group is as base, so that the acid properties very slightly preponderate. 



When we have another COOH or another NH group added on, as in aspartic 

 acid or lysine respectively, the acidic function is considerably increased in the first 

 case and the basic function in the second. 



From Winkelblech's investigations (1901) it is interesting to note that, when the strength 

 of the acid group considerably exceeds that of the basic one, as in taurine (amino-ethyl- 

 sulphonic acid), salts are formed only with bases, not even with acids as strong as hydrochloric 

 acid. Conversely, if the basic group is considerably stronger than the acid one, as in betaine 

 (tri-methyl-glycine), then salts are formed only with acids. It is also somewhat unexpected to 

 find that, comparing glycine, alanine, leucine, sarcosine and betaine, the stronger acid is at 

 the same time the stronger base, but the fact appears to hold only for the mono-amino-mono- 

 carboxylic series. 



As to the methods of determining the two dissociation constants, one of these is 

 that of conductivity measurements of their salts with hydrochloric acid and with 

 sodium hydroxide, and another is that of hydrolytic dissociation. The papers by 

 Lunden (1908) and by Winkelblech (1901) may be consulted. 



I insert here the values of the dissociation constants of a few amphoteric 

 electrolytes, at 25, taken from Lunden's work (1908, p. 81). 



