82 INTIMATE ASSOCIATIONS OF INORGANIC IONS 



holding power is now only 0.04, so that (0.18 0.04) 0.14^ 

 is the figure representing the acid fixed by the proteins alone. 

 This represents 0.51$ of hydrochloric acid itself. Now this is 

 rather a considerable amount; and its physiological significance 

 is that, within pretty wide chemical limits, no free acid 

 can reach the living tissues, for the circulating proteins 

 can combine with them and so constitute a protective mechanism 

 against acidosis or an excessively acid condition of blood. These 

 native or serum-proteins, therefore, behave in an amphoteric 

 fashion, for they can fix alkalies like acids and acids like 

 alkalies. This explains how serum is acid to phenolphthalein, 

 and alkaline to methyl-orange, while it is physico-chemically 

 neutral. This double power proteins possess is now believed to 

 be due to their polypeptide composition. This means that after 

 any number of amino-acids have united together in chain 

 fashion, there will be left an amidogen group at one end and a 

 carboxyl group at the other, thus conferring a chemical polarity 

 or what is otherwise called "residual affinity." 



Thus the dipeptide glycyl-glycin is formed, 



NH 2 CH 2 COOH + H NH CH 2 COOH, 



which gives us NH 2 CH 2 .CO-NH-CH 2 -COOH + H 2 0, a com- 

 pound is basic on account of NH 2 and acidic on account of the 

 COOH. 



Hence owing to its acidity, glycin can form the copper 

 salt thus 



CH 2 NH 2 CO 0-^ 



rrr f-(r\ r\ J^ Cu J anc * owing to its basicity it can unite 

 IN ri GO O^-^ 



with an acid like benzoic and form hippuric acid thus ; 



C 6 H 5 CO 'OH + H 1 NH CH, COOH C 6 H 5 CO NH CH 2 COOH 



+ H 2 0. 



The union of oxygen with haemoglobin is, however, not 

 merely an adsorption due to residual affinities, for it is strictly 

 mono-molecular, and the reduced form of the pigment is differ- 

 ent from the oxidised in colour and therefore in spectrum. 



But not merely are acids and inorganic substances united 



