The irreversible combination of formaldehyde with proteins

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Abstract
In acid conditions at 100[degree] proteins combine with formaldehyde, some of which cannot be removed by distillation in acid. In the proteins examined, probably all the aspartic acid is in the form of asparagine residues, and the glutamic acid is in the form of glutamine and glutamic acid residues. Formaldehyde forms cross linkages between the glutamic amide groups and other groups, possibly the guanido groups of argenine. These cross linkages can be broken down by distillation in dilute phosphoric acid. Formaldehyde probably combines with the asparagine amide groups to form 6-hydroxytetrahydropyrimidine-4-carboxylic acid, which is stable to boiling dilute phosphoric acid.