Compounds of several classes have been prepared as models for groupings that could possibly be formed during the treatment of proteins with formaldehyde.
The formation of cyclic methyleneimines from primary aliphatie amines and formaldehyde (Cambier and Brochet, Bull.Soc.Chim, 1895, (3), 13, 392) has been confirmed and a compound formilated by Kempf (Annalen, 1890, 256, 219) as a methylene bis amine has been shown to be the cyclic methyleneimine. Both the above compounds are methylene bis secondary amines are unstable to mineral acids.
Further examples of N-hydroxymethyl amides (Sinhorn, Annalen, 1905, 343, 207) have been prepared and approximate dissociation constants in weak alkali measured. They have also been shown to be unstable to acids.
Several methylene bis amides have been prepared and shown to be highly stable to acid and alkaline hydrolysis.
Derivatives of N-hydroxymethyl amides in which the hydroxyl group is replaced by: (a) alkoxy, (b) thic-alkyl, (c) acyloxy, (d) primary and secondary amino groups, have been prepared. These are all unstable in various degrees to acids and alkalis and such groups are not likely to confer increased acid and alkali resistance to proteins, bearing in mind, however, that the stability of a group in a macromolecule is not necessarily of the same order as that in a compound of small molecular weight.
N-benzoyl tyrosine has been found to be less reactive to formaldehyde than tyrosine. A condensation product has been isolated from an alkaline medium but no reaction has been observed in acid, neither could condensation be brought about between hydroxymethyl amides and N-benzoyl tyrosine.
Several model compounds containing a peptide link have been synthesised by way of phthalyl-amino intermediates. Reaction with formaldehyde has been attempted under a variety of conditions, in each case with a negative result. It is, therefore, thought unlikely that the peptide group plays any part in the reaction of proteins with formaldehyde.