184. PROTEINS AND CARBOHYDRATES 
2. Secondary Protein Derivatives. Products of the further hydrolytic 
cleavage of the protein molecule. 
(a) Proteoses. Soluble in water, uncoagulable by heat and pre- 
cipitated by saturating their solutions with ammonium sulphate or zinc 
sulphate. 
(b) Peptones. Soluble in water, uncoagulable by heat, but not 
precipitated by saturating their solutions with ammonium sulphate. 
(c) Peptids. Definitely characterized combinations of two or more 
amino acids, the carboxyl group of one being united with the amino 
group of the other, with the elimination of a molecule of water. 
Structure of Proteins. Proteins are combinations of amino acids 
the unions being between the carboxyl group of one acid and the amino 
group of the other. The following will serve to illustrate the linkage. 
i i 
H iH H H—C—H H H—C—H 
| Ff | f® | | 
H—C—C= N—C—CC —=H-—C—- C—N—C—C—0 
| Hw | (NB OT LL LNG 
H EH NH20 H H 
Glycine Alanin Glycyl-alanin 
The free amino and carboxyl groups on the glycyl-alanin molecule 
are free to react in the same way. This yields infinite possibilities with 
regard to the size of the protein molecule. Fischer has done much 
toward synthesizing proteins from these smaller units the amino acids. 
The amino acids which have been isolated from proteins may be 
classified as follows: 
I. Monoamino Monobasic Acids. 
Glycocoll, a-amino acetic acid. 
H 
/ 40 
H—-C—oZ0H 
NH» 
Alanin, a-amino propionic acid. 
