(2)

CH3·CH(C2H5)·CH(NH2)·COOH
Isoleucine

+ H2O =

CH3·CH(C2H5)·CH2·OH
d-Amyl alcohol

+ CO2 + NH3

The experiments by which these important changes were demonstrated were of a very simple and convincing character [Ehrlich, [1907, 1]]. Two hundred grams of sugar and 3 to 10 grams of the nitrogenous substance to be examined were dissolved in 2 to 2·5 litres of tap water in a 3 to 4 litre flask, the liquid was sterilised by being boiled for several hours, and after cooling 40 to 60 grams of fresh yeast were added and the flask allowed to stand at room temperature until the whole of the sugar had been decomposed by fermentation. In the earlier experiments the amyl alcohols were isolated and identified by conversion into the corresponding valerianic acids, but as a rule the fusel oil as a whole was quantitatively estimated in the filtrate by the Röse-Herzfeld method [Lunge, [1905], p. 571].

The following are typical results. (1) An experiment carried out as above without any addition of leucine gave 97·32 grams of alcohol containing 0·40 per cent. of fusel oil. (2) When 6 grams of synthetic, optically inactive leucine were added, 97·26 grams of alcohol were obtained, containing 2·11 per cent. of fusel oil, which was also optically inactive; 2·5 grams of leucine were recovered, so that 87 per cent. of the theoretical yield of isoamyl alcohol was obtained from the 3·5 grams of leucine decomposed. (3) In the presence of 2·5 grams of d-isoleucine (prepared from molasses residues), 200 grams of sugar gave 93·99 grams of alcohol, containing 1·44 per cent. of fusel oil, which was lævo-rotatory. This corresponds with 80 per cent. of the theoretical yield of d-amyl alcohol from the isoleucine added.

This change, which Ehrlich has termed the alcoholic fermentation of the amino-acids, although brought about by living yeast, does not appear to occur at all when zymin [Ehrlich, [1906, 4]; Pringsheim, [1906]] or yeast-juice [Buchner and Meisenheimer, [1906]] is substituted for the intact organism, nor is it effected even by living yeast in the absence of a fermentable sugar [Ehrlich, [1907, 1]]. The reaction appears indeed to be intimately connected with the nitrogenous metabolism of the cell, and the whole of the ammonia produced is at once assimilated and does not appear in the fermented liquid. Other amino-acids [p088] undergo a corresponding change, and the reaction appears to be a general one. Thus tyrosine, OH·C6H4·CH2·CH(NH2)·COOH, yields p-hydroxyphenylethyl alcohol, or tyrosol [Ehrlich, [1911, 1]; Ehrlich and Pistschimucka, [1912, 2]], OH·C6H4·CH2·CH2OH, a substance of intensely bitter taste, which was first prepared in this way and is probably one of the most important factors in determining the flavour of beers, etc. Phenylalanine, C6H5·CH2·CH(NH2)·COOH, in a similar way yields phenylethyl alcohol, C6H5·CH2·CH2OH, one of the constituents of oil of roses, whilst tryptophane,

C6H4
╱│
HN│
╲│
HC═══C·CH2·CH(NH2)·COOH

yields tryptophol,