5 20 



NA TURE 



[March 29, 1906 



amino-acids Fischer has given the name of polypeptide, 

 or, according lo the number of single amino-acid groups 

 present, di-, tri-, tetra-, &c, peptide. This view of the 

 constitution of proteid matter, which seemed at the outset 

 of the investigation warranted by the nature of the evidence 

 then forthcoming, received ample justification by the 

 very recent isolation of the first natural dipeptide in the 

 process of hydrolysing silk fibroin. But we are antici- 

 pating matters. The first of the polypeptides was obtained 

 by Curtius in 1882, but as its structure is complex and 

 has only been lately ascertained, we will begin with the 

 simpler members prepared by Fischer. 



In 1901 Fischer and Fourneau found that glycine 

 anhydride, which, according to Curtius, is formed when 

 glycine ester is heated in aqueous solution, is partly 

 hydrolysed with mineral acids into glycylglycine, the first 

 and simplest of the dipeptides, 



,CH 2 .CO x 

 NH< NH + H„0 = NH.,.CH,.CO.NH.CH.vCOOH. 



x CO.CH/ 



A year later Fischer found that a third amino-acid or 

 peptide group could be linked to the carbethoxy-derivative 

 of glycylglycine (prepared by the action of chloroformic 

 ester on the dipeptide) by heating it with leucine ester, 

 whereby carbethoxyglycylglycyl-leucine ester resulted, which 

 on hydrolysis is converted into the free acid, 



C„H 5 COo.NH.CH 2 .CO.XH.CH„.CO.NH CH(CjH 9 )C0 2 H. 



The next year saw the introduction of a new method 

 for adding fresh links to the peptide chain by the use of 

 thionyl chloride. This effects the conversion of the end 

 carboxyl group into the acid chloride, and it thus became 

 possible by the subsequent action of an amino-ester to 

 add a new peptide group to the molecule. Thus 

 carbethoxyglycylglycine was converted successively into 

 the acid chloride, and then by the action of glycine ester 

 into carbethoxydiglycylglycine ester, and by a repetition of 

 the process into carbethowtrig'lvcvlglycine ester. Similar 

 compounds with different amino-acids were obtained by 

 this reaction. In all of them, however, the carbethoxy- 

 group at the amino end of the chain refused to be removed, 

 and a new method had to be found for preparing the free 

 polypeptides. This was soon forthcoming. In 1903 Fischer 

 and Otto introduced the chloracyl chlorides for the pur- 

 pose. Glycylglycine ester was first combined with 

 chloracetyl chloride, hydrolysed, and warmed with 

 ammonia, whereby diglycylglycine was formed, 

 ClCH.,.COCl + NH.,.CH„.CO,NH.CH„COOR = 



ClCH 2 .CO,NH.CH 2 .CO,NH.CH 2 .COOR f HC1. 

 ClCH. 2 .CO,NH.CH„.CO,NH.CH„.COOH + NH 3 -= 



NH 2 CH 2 .CO,NH.CH.;CO,NH.CH„COOH + HCI. 

 This method proved extremely fruitful, and led to the 

 production of a variety of di-, tri-, tetra-, and penta- 

 peptides. 



It will be easily conceived how the methods just de- 

 scribed afford the means of lengthening the peptide chain 

 at either end. In the one case an a-chloro- or bromo- 

 acyl chloride is added to the amino-group at one end, or, 

 at the other, the carboxyl group is converted into the acid 

 chloride, for which purpose thionyl chloride has since been 

 replaced by phosphorus pentachloride dissolved in acetvl 

 chloride. In the first case the action of ammonia, in the 

 second that of an amino-acid or another peptide (the ester 

 is not necessary) in presence of alkali, produces the new 

 peptide. By combining the two processes, hexa- and hepta- 

 peptides giving the biuret reaction have been formed from 

 diglycylglycine, and Prof. Fischer confidently predicts the 

 synthesis of still longer chains. In the present year 

 lias also found that two molecules of the methyl 

 ester of diglycylglycine can by heating be combined into 

 'I' 1 ' ester of pentaglycylglycine, which yields the hexa- 

 peptide on hydrolysis. 



If the proteids themselves and the amino-acids to which 

 they give rise comprised optically inactive members, the 

 experimental difficulties in the way of synthesis might be 

 looked upon as approaching solution ; but few of the 

 natural products are inactive, and the question of pre- 

 paring by artificial means active polypeptides must be 



NO. I QOO, VOL. 73] 



faced. This part of the problem has not been neglected. 

 By resolving the amino-acids into their active constituents 

 before linking them together, or by submitting certain 

 inactive members to the selective fermentation of pancreatic 

 juice (for trypsin acts upon some of the polypeptides as it 

 does on proteids), active polypeptides have been obtained. 



In addition to the action of trypsin, the polypeptides 

 exhibit many characteristics of the simpler proteids ; they 

 are for the most part soluble in water ; especially is this 

 the case where the peptide is composed of different amino- 

 acids ; they are insoluble in alcohol, and many of the 

 higher members give the " biuret " reaction. Like the 

 proteids, also, they are quickly and completely hydrolysed 

 by strong hydrochloric acid into amino-acids ; the action 

 of dilute hydrochloric acid and caustic alkalis is, on the 

 other hand, very slow. 



The concluding sections of the address will appeal more 

 especially to physiologists, for they deal with the products 

 of hydrolysis of the proteids themselves. Space will not 

 permit of more than a passing reference to them ; the 

 reader who is interested in the products obtained by the 

 action of pancreatic juice or the combined action of pepsin, 

 hydrochloric acid, and pancreatin must refer to the original 

 memoir. It may, however, be stated that pancreatin 

 yields, in addition to numerous monoamino-acids, a pro- 

 duct which does not give the biuret reaction, but shows a 

 certain resemblance to the artificial polypeptides, and breaks 

 up on hydrolysis with acids into alanine, leucine, glutamic 

 and aspartic acid, as well as proline and phenylalanine. 

 By the successive pepsin and pancreatin digestion the 

 amount of this polypeptide body is diminished, but in its 

 place proline and phenylalanine appear. 



As many of the commoner forms of proteid matter 

 behave in this way, Fischer concludes that proline is an 

 actual constituent of the proteid molecule. For similar 

 reasons he includes tyrosine, leucine, alanine, tryptophane, 

 &c, which always appear in the pancreatic digestion of 

 albumin, a view which is supported by the action of 

 pancreatic juice on the artificial polypeptides containing 

 thc-.i' groups. But of all the facts which point to the 

 polypeptide nature of the albumins, the most convincing 

 is Fischer and Abderhalden's latest discovery of a dipeptide 

 in silk fibroin, which they have identified as glvcyl-d- 

 alanine. The method of preparation is interesting, because 

 it introduces the new principle of combining acid with 

 pancreatic hydrolysis. The silk fibroin is first digested 

 with 70 per cent, sulphuric acid for several days at 1S , 

 then diluted with water, the acid removed with baryta, 

 and the liquid evaporated and submitted to the action of 

 pancreatic juice for eight days. The tyrosine which had 

 then separated was removed, and the esters of the amino- 

 acids were formed in the usual way and heated under 

 reduced pressure at 65 to remove the alcohol and a little 

 glycine and alanine ester. 



From the syrupy residue dissolved in alcohol and satu- 

 rated with ammonia gas (to convert the dipeptide ester 

 into the diketopiperazine), a crystalline precipitate of glycyl 

 alanine anhydride slowly separated. Fischer sees in this 

 discovery a near prospect of obtaining the most important 

 constituents of the natural peptones, and even of the 

 albumoses, and of reproducing them artificially. " But the 

 problem of reproducing true albumins," savs Fischer, "is 

 of far greater difficulty, for their reconstruction from the 

 first products of hydrolysis (peptones and albumoses) will 

 require entirely novel methods, and when these are found 

 their application will probably be a laborious process. 

 One may therefore ask the question whether the eventual 

 success will compensate for the labour expended. This 

 depends, in my opinion, on the profit which biological re- 

 search can derive from it, and this, again, on the manner 

 in which the synthesis has been accomplished. For such 

 a synthesis may be compared to a tourist who rushes 

 through a country in an express train and sees nothing. 

 It is otherwise if the synthesis is constrained to advance' 

 slowly and to construct the molecule step by step. It is 

 then like a traveller journeying on foot, who notes every 

 feature of the road, and tries each side-path before the- 

 right one is found. He not only learns every inch of the 

 country, hut understands the nature of its inhabitants. He- 

 knows his way and can direct others. I can onlv look 



