The Degradation of Collagen. II. The Solubilization Process in the Acid pH Range

The partial dissolution of intact collagen has been followed as a function of pH and the duration of extraction at 60%. At this temperature collagen does not undergo heat shrinkage. A certain portion of the collagen dissolves readily at each pH and the amount of this primary solubilization at a given pH approaches a limiting upper value. After the primary solubilization has become extensive at pH 2.0 the breakdown of the collagen structure proceeds at a significantly greater rate. Dye binding data indicate an opening up of the structure of the insoluble residues but no increase in the number of basic groups on the protein (<1 mmole/100 g.). Carboxyl groups are exposed during the degradations, but only up to the amount expected to be available based upon the analysis for glutamic and aspartic acids. The dissolution process also brings about a change in the effective pK of some of the titratable groups. While there is thus little evidence for extensive peptide bond hydrolysis in the residues, the total nitrogen content of theinsoluble residues decreases with increasing degradation whereas the soluble extracts, lower in nitrogen than the original collagen when the fraction solubilized is small, gradually increase in total nitrogen. These data support the view that intact collagen may be composed of a series of related proteins of not quite identical composition. The initialphases of the collagen-soluble collagen (gelatin) transition appear to take place with a very limited amount of peptide bond hydrolysis.