62 FIXATION 



reaction may occur, with production of a diol and osmic acid. (It 

 may be remarked in passing that the expression osmic acid is here 

 used correctly. See p. 119.) 



\ /" \ /" 



C— 0\ COH 



I /OsOa + 2H2O - I + H2OSO4 



/C^O-^ COH 



H / ^H 



Diol Osmic acid 



That osmium tetroxide could react in this way with many ring- 

 compounds was shown by the German organic chemist, Criegee, 

 in 1936.^^^ One such substance is indene, which is related to indole, 



NH 



HCCH 



2 



H V ^ H 



Ittdole Tryptophane as part 



of a protein chain 



a constituent of tryptophane. It follows from work on related 

 substances ^'^ that the point of attack of osmium tetroxide would 

 be the two ends of the double bond shown here in the five- 

 membered pyrrol ring of indole and tryptophane. 



Osmium tetroxide is capable, in certain circumstances, of 

 joining molecules together. This was shown by Criegee and his 

 associates. ^^^' ^^^ They found that osmium tetroxide acted most 

 readily in this way when it had first combined with pyridine, but 

 there are reasons for supposing that other organic substances 

 might take the place of pyridine. It is clear that osmium can form a 

 link between two ring-compounds by joining with both of them 



^C^O\ II /O-^C 

 I )Os( I 



H H 



Osmium acting as a bridge between tzvo ring-compounds. (The latter are shown 



only in part.) 



in the way previously explained. It is tempting to suppose that 

 the formation of gels from soluble proteins (or of firm gels from 

 weak ones) may result from the ability of osmium to form bridges 

 of this kind between protein chains. It is remarkable that such gels 



