Observations on the crystallization of animal proteids / by F. Gowland Hopkins and S.N. Pinkus.

  • Hopkins, Frederick Gowland, Sir, 1861-1947.
Date:
[1898?]
Catalogue details

Licence: Public Domain Mark

Credit: Observations on the crystallization of animal proteids / by F. Gowland Hopkins and S.N. Pinkus. Source: Wellcome Collection.

    5/12 (page 131)
    Upon adding to egg-white an equal bulk of saturated ammonium sulphate solution as directed by Hofmeister, the fixed alkali present in the proteid liberates a considerable quantity of ammonia, frequently in sufficient amount to produce a marked ammoniacal odour, and always enough to affect litmus-paper when this is held above the mixture. In preparing products by Hofmeister’s method we early came to the conclusion that this free alkali greatly interfered with the separa¬ tion of crystals; and we found that after careful neutralization with acetic acid the mixture when exposed as usual in flat dishes yielded a crop of crystals much more rapidly and satisfactorily than when not so neutralised. Further observations showed that, by working with a slight excess of acetic acid, the process becomes much easier still, and has the great advantage that absolutely no evaporation is required to produce crystals, while the yield is larger. We have found that, working with fresh eggs, it is sufficient to add to the filtered mixture of egg-white and ammonium sulphate enough acetic acid to produce a slight permanent precipitate. It should be noticed that the very smallest excess begins to produce precipitation, and the acid should be diluted and added cautiously from a burette. It is well to wait between each addition so that a precipitate may be dis¬ tinguished from the bubbles of gas which are evolved from carbonates present. When a faint permanent precipitate remains the mixture is left to stand in a closed vessel. In 24 hours or less a large proportion of the proteid will have separated in microscopic rosettes of acicular crystals quite unmixed with amorphous material, and this in the entire absence of evaporation, and therefore wholly without fear of contamina¬ tion with ammonium sulphate crystals. Even if sufficient acid be added to produce a fairly bulky precipitate, there is, as a matter of fact, no necessity to filter it off, for it is a remarkable fact that this precipitate, though amorphous at first, becomes rapidly crystalline on standing. As time elapses the amount greatly increases and finally the whole product is found to be uniformly crystalline1. The use of acetic acid in the manner we have described renders the preparation of crystalline egg-proteid quite the simplest of operations. We shall best illustrate the exact details of the procedure which we 1 Gabriel has shown that the globules obtained at the earlier stages of Hofmeister’s process, on standing for some weeks in closed vessels with the mother-liquor may spontaneously change into needles, loc. cit.