The physical chemistry of the proteins / by T. Brailsford Robertson.

  • Thorburn Robertson
Date:
1918
Catalogue details

Licence: In copyright

Credit: The physical chemistry of the proteins / by T. Brailsford Robertson. Source: Wellcome Collection.

    41/508 page 21
    proportion of the nitrogen in proteins is present within their molecules in the form of — NH2 groups. Thus in the case of edestin, as we have seen, only 1.8 per cent of the total nitrogen is present in the form of free — NH2 groups. Now edestin, as Osborne (47) has shown, is insoluble, when in the free condition, in water. It forms an insoluble hydro- chloride containing 14 X 10~5 equivalents of HC1 per gram and, on further addition of acid, passes into solution. Its com- bining-capacity for acids does not remain constant, however, for at neutrality to tropseolin, which corresponds to a free acidity of from 0.01 to 0.001 H+ (60) it neutralizes 127 X 10-5 equiva- lents of acid per gram. Hence, if the acid is neutralized by — NH2 groups of edestin, the number of these groups in each molecule of edestin must be at least JT247_ = 9.* From the former determination it would appear that the molecular weight of edestin is 7000, and this corresponds with the molecular weight indicated by its tyrosin and glutamic acid content (40) (1 mol. tyrosin + 3 mols. glutamic acid +••*). Nine — NH2 groups in this molecule would correspond to over ten per cent of the total nitrogen, or almost exactly to the whole — NH2 content of the arginin, calculated as the free diamino-acid, which the edestin molecule contains. Since only 1.8 per cent of the total nitrogen of the edestin molecule is present therein in the form of — NH2 groups no less than eighty per cent of the neutralizing power of the edestin for acids must be accounted for in some other fashion than by the assumption of a union of the acid with free — NH2 groups. From the investigations of Erb (14), although the exact in- terpretation of his results is in some respects open to question (Chap. IV), it would appear that the combining-weight of egg albumin for acids may be as low as 152, while its molecular weight is, according to Hofmeister, 5400 or some multiple of this. Hence upon the assumption that terminal — NH2 groups bind these acids, there must be at least 35 of them in egg albumin (45), which would correspond to no less than 69 per cent of the total nitrogen in this protein. * Osborne believes that an insoluble “monochlorhydrate” is also formed, containing 7 X 10-6 equivalents per gram which would raise the number of — NH2 groups to 18, but would, at the same time, double the estimate of the molecular weight.
    digitised image 1
    digitised image 2
    digitised image 3