Studies on the influence of pH and pancreatin on 13C-formaldehyde-induced gelatin cross-links using nuclear magnetic resonance.

Carbon-13 nuclear magnetic resonance (13C-NMR) and 13C-enriched formaldehyde (13CH2O) were utilized to observe cross-linking in gelatin. Thus, when a 6% solution of gelatin in water was treated with 2000 ppm 13CH2O at 20 degrees C, the 15 hr 13C-NMR spectrum of the crosslinked gel showed peaks representing carbinolamines (methylols) of arginine and lysine, as well as a peak ascribed to a methylene cross-link between arginine and lysine. Similar results were obtained when these cross-linking reactions were conducted using only 100 ppm 13CH2O. When pancreatin (1% w/v) was added to the solution of 6% gelatin cross-linked with 2000 ppm 13CH2O, the gel began to revert to a clear fluid solution. After incubation for 24 hr at 37 degrees C, the 13C-NMR spectrum of this solution confirmed the presence of the methylols of lysine and arginine, and the lysine-arginine cross-link. When 13CH2O (2000 ppm) was added to a 6% solution of gelatin at pH 13.0, the arginine methylol and the lysine-arginine crosslinks were produced. The 13CH2O-induced crosslinking of gelatin at pH 2.0, however, yielded the lysine methylol as the sole product.