179 
properties of the acid were developed and could be titrated as any organic 
acid. 
S. P. L. Sorensen worked out the details and made it possible to estimate 
amino acids quantitatively by means of formaldehyde. It is well known that 
amino acids, such as are formed by the hydrolysis of proteins, especially 
milk proteids, are neutral to phenolpthalein, have both an acidic group, 
earboxyl and a basic (amino) group. These exist in the same molecule and 
being the alpha amino acids neutralize each other, or in other words we have 
an amphoteric molecule, but as soon as formaldehyde is added, it reacts 
with the alkaline or basic group forming a methylene compound and leaving 
the acid group free to act. 
For example: 
/NH, i CH, 
CH, CH + iC0.2 = CH, — CH, EO 
COOH COOH 
(Alanine) (Formaldchyde) 
/N = CHe pit = Gil. 
CH; — CH + KOH = Cu; — Cr + 29 
COOH CoCr. 
From Emil Fisher’s researches on protein and polypeptids there is no 
doubt that the protein molecule is conposed of amino acid units. The 
carboxyl group (—COOH) of one amino acid is combined with the amino 
group (—NH,) of another amino acid, forming peptids, di, tri, ete., to poly- 
peptids. For example, glycyl-glycine composed of two units of gyleine. 
CH: — CONQI CH, — COOH CH: CO — CH: — COOH 
| | = | | + HO 
HUN HY— N —H H.N NH 
(Glycine) (Glycine) (Glycil-Glycinc) 
Likewise different units may combine, as example, alanyl-glycyl-tyrosine 
From which we see that each peptid has one carboxyl group (—COOH) 
. ,@ e 
acidic and one amino group (—NH)) basic. Now if the protein molecule 
is built up from amino acids, we can expect it to split up into simpler mole- 
