144 PROTEINS AND AMINO- ACIDS [ch. 



The peptones are the only proteins not precipitated by complete 

 saturation with ammonium sulphate. They give the protein colour 

 reactions and are precipitated by tannic acid and lead acetate. 



Expt. 133. Separation and reactions of proteoses. Prepare about 20 gms. of gluten 

 from 50 gms. of flour as in Expt. 135 {d). Put the gluten into a small flask, add 

 100 c.c. of 0*2 % hydrochloric acid and 0*5 gm. of commercial pepsin : add also a 

 little toluol, shake and plug with cotton-wool. Leave in an incubator at 38° C. for 

 two days. (A control experiment should also be made with 100 c.c. of 0*2 % hydro- 

 chloric acid and 0*5 gm. of pepsin. Since pepsin itself gives a biuret reaction, a 

 control is necessary for comparison in the next experiment.) After two days, the 

 incubated mixture is neutralized to litmus with dilute sodium carbonate solution, 

 filtered and saturated while boiling with solid ammonium sulphate. A precipitate of 

 proteoses is formed, which can be gradually collected together as a sticky mass and 

 removed with a glass rod. Dissolve the precipitate i u some hot water, filter and make 

 the following tests : 



{a) Xanthoproteic reaction. A positive result is given. A modification of this 

 reaction is characteristic of most proteoses. Add a few drops of nitric acid : a white 

 precipitate is formed which disappears on heating gently and reappears on cooling. 



(6) MillonJs reaction. A positive result is given. 



(c) Glyoxylic reaction. A positive result is given. 



id) Biuret reaction. A pink or pinkish-violet colour is given. 



(e) Sulphur reaction. A positive result is given. 



(/) Add a little 3 ^Iq tannic acid solution. A precipitate is formed. 



{g) Add a drop of 5 % copper sulphate solution. A precipitate is formed. 



(A) Add a drop of strong acetic acid and then a couple of drops of 5 «/o potassium 

 ferrocyanide. A precipitate is formed which disappears on heating gently and re- 

 appeifcrs on cooling. ^ ^' 



(^) Boil some of the solution. No coagulum is formed. 



Expt. 134. Detection of peptone. The saturated solution, from which the proteoses 

 have been precipitated, is then filtered and to a measured quantity (about 5 c.c.) 

 twice the volume of 40 % sodium hydroxide is added and a drop of 1 % copper 

 sulphate solution. A pink colour appears, due to the presence of peptone. A test 

 should be made with the control solution containing hydrochloric acid and pepsin 

 only. An adequate amount should be saturated with ammonium sulphate, filtered 

 and 5 c.c. tested for peptone. The reaction is less marked than in the actual hydro- 

 lytic product. Concentrate the remainder of the peptone solution on a water-bath 

 and pour off from the excess of ammonium sulphate crystals. Filter and make 

 the following tests : (i) Xanthoproteic, (ii) Millon's, (iii) Glyoxylic, (iv) Tannic acid. 

 A positive result is obtained in each case. 



The Seed Proteins of Certain Plants. 



The proteins present in the seeds of certain genera and species, upon 

 which special investigations have been made, may now be considered. 



It should be borne in mind that there are always several proteins 

 present in the seed. Some are reserve proteins of the cells of the 



