PHYSIOLOGICAL CHEMISTRY. 



red precipitate forms after a time if much tyrosine is taken. With only 

 a minute amount a red color only may result. It will be remembered that 

 this reaction is not confined to tyrosine alone, but is given by many ben- 

 zene derivatives containing a hydroxyl group attached to the nucleus. 

 Hence phenol gives the test distinctly. 



By heating a little of the crystalline residue with 2 or 3 cc. of strong 

 sulphuric acid solution follows. On adding a drop of formaldehyde solu- 

 tion a red color is produced which becomes green on heating further with 

 addition of some glacial acetic acid. 



The solution left after filtering off the tyrosine is concentrated still 

 more, which finally causes a separation of leucine. The concentration is 

 continued until a volume of about 5 cc. is reached. Crystals which have 

 separated may be examined under the microscope. To the concentrated 

 liquid about 20 cc. of alcohol is added, the mixture heated on the water- 

 bath to the boiling point and then allowed to stand until cold. It is then 

 filtered. The filtrate contains most of the leucine present. In the pre- 

 cipitate there is peptone. Evaporate the alcoholic liquid slowly to dry- 

 ness, take up the residue with water, add some lead hydroxide (lead oxide 

 with a little alkali), boil and filter. From the filtrate remove the excess 

 of lead by means of hydrogen sulphide; filter again and concentrate the 

 liquid to a small bulk for the crystallization of the leucine. 



Examine some of the leucine crystals under the microscope. They ap- 

 pear as spherical bunches of very fine needles. Often the needle structure 

 is not visible. Hydrochloric acid and weak alkali solutions dissolve the 

 needles on the slide. 



Leucine gives some marked chemical tests. Dissolve some of the crys- 

 tals in water, add sufficient sodium hydroxide to give a good alkaline 

 reaction and then a few drops of copper sulphate solution. The precipi- 

 tate of copper hydroxide which forms at first redissolves, giving place to 

 a blue solution containing a compound of leucine and copper. 



Leucine may be oxidized to yield valeric acid. On this behavior a test 

 is based. To some of the crystalline residue containing leucine add 3 

 drops of water and 2 or 3 grams of solid potassium hydroxide. Heat in 

 a test-tube until the alkali melts. The leucine decomposes, giving off am- 

 monia. Allow the mass to cool, add enough water to dissolve the residue 

 and then enough dilute sulphuric acid to give a sharp reaction. On apply- 

 ing heat the odor of valeric acid becomes evident Through the alkaline 

 oxidation carbon dioxide is split off. 



Albumoses and Peptones. These bodies are contained in 

 the residue insoluble in alcohol left above after separation of 

 the leucine. This residue is washed more completely with 

 strong alcohol and dissolved finally in water. The solution 

 may be tested for albumose and peptone by methods already 

 given. 



