PRODUCTS OF HYDROLYSIS 65 



phur. The assumption of the presence of two atoms of sulphur in the 

 protein molecule may be considered to be established if the cystine 

 which results on hydrolysis is an actual constituent of the molecule. 

 Proteins which contain 0*4 per cent, of sulphur must therefore have a 

 molecular weight of at least 15,000 if the preparations in which this 

 quantity of sulphur is found do not consist of mixtures of sulphur-con- 

 taining and sulphur-free proteins. 



With the exception of one or two unsatisfactory observations, no 

 proteins have been described which contain no sulphur whatever. 

 Only one carefully studied vegetable protein has yet been obtained 

 which contains so small a proportion of sulphur as to make the ex- 

 istence of sulphur-free protein in any way probable. This protein is 

 vicilin, obtained from several leguminous seeds, some preparations of 

 which have been found to contain as little as 0*1 per cent, of sulphur. 

 The sulphur content of various preparations of vicilin, which were 

 obtained by Osborne and Campbell (330, 331, 332, 334) by fractional 

 precipitation, fell between O'2 and O'l per cent. The accuracy of the 

 determinations in these different fractions was established by repeated 

 closely agreeing determinations, and there can be no doubt that dif- 

 ferences in the sulphur content of these fractions actually existed. In 

 view of this variable content in sulphur and of its very small total 

 amount, it is possible that these preparations of vicilin were mixtures 

 of different proportions of sulphur-free and sulphur-containing protein. 

 In no other case has any evidence of the possible existence of sulphur- 

 free protein yet been obtained. In all other carefully studied and pro- 

 perly analysed proteins the sulphur content has been found to be very 

 constant and in many of them it has been established with a high 

 degree of accuracy. 



Numerous attempts have been made to establish a definite ratio 

 between the sulphur which can be thus split off as sulphide by boiling 

 with alkalies, and the total sulphur of the protein. In every case the 

 amount of sulphide sulphur thus obtained was less than the total 

 sulphur, and it was for a long time assumed that this fact showed the 

 presence of two different forms of sulphur in the protein molecule. 

 Experiments with cystine show that not more than two-thirds of the 

 sulphur which this substance contains can be converted into sulphide 

 by boiling with alkali, and that, unless special care is taken, the pro- 

 portion obtained is but little more than one-half. The fact that only 

 a part of the protein sulphur can be converted into sulphide gives, in 

 most cases, no evidence of the existence of two forms of sulphur in the 

 protein molecule, but the fact that the proteins yield cystine on hydro- 



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