Fractionation of Gelatin

1. It is possible to fractionate gelatin by means of reprecipitation at 23 degrees C. of a salt-free solution of pH 4.7 into two fractions, one of which is soluble in water at any temperature, and a second one which does not dissolve in water even when heated to 80 degrees C. 2. The proportion of the soluble fraction in gelatin is much greater than of the insoluble one. 3. The insoluble fraction of gelatin does not swell when mixed with water, but it does swell in the presence of acid and alkali which finally dissolve it. 4. Blocks of concentrated gel made by dissolving various mixtures of the soluble and insoluble fractions of gelatin in dilute NaOH swell differently when placed in large volumes of dilute buffer solution pH 4.7 at 5 degrees C. The gel consisting of the insoluble material shows only a trace of swelling, while those containing a mixture of soluble and insoluble swell considerably. The swelling increases rapidly as the proportion of the soluble fraction increases. 5. A 5 per cent gel made up by dissolving the insoluble fraction of gelatin in dilute NaOH loses about 70 per cent of its weight when placed in dilute buffer pH 4.7 at 5 degrees C. A similar gel made up of ordinary gelatin loses only about 20 per cent of its weight under the same conditions. 6. It was not found possible to resynthesize isoelectric gelatin from its components. 7. An insoluble substance similar in many respects to the one obtained by reprecipitation of gelatin is produce on partial hydrolysis of gelatin in dilute hydrochloric acid at 90 degrees C.