Investigation of the Hardening Reaction of Gelatin with 13C Labeled Formaldehyde by Solution and Solid State 13C NMR Spectroscopy

Model reactions of the hardening of gelatin with 13C labeled formaldehyde were monitored by 13C NMR spectroscopy in solution and in the solid state. In solution primary attack of formaldehyde first forms methylols of lysine residues and later of arginine residues, which react to a lysine-arginine crosslink. During the drying process of hardened gelatin, in addition to the lysine-arginine crosslink an arginine-arginine crosslink is formed. The rate of crosslink formation is strongly influenced by humidity.