Cross-linking of amino acids by formaldehyde. Preparation and 13C NMR spectra of model compounds.

Model cross-linked systems have been prepared by reacting amino acids or alkylamines with formaldehyde and various amino acid model compounds such as 2,4-dimethylphenol (tyrosine), 3-methylindole (tryptophan) and alkylamides (glutamine, asparagine. 13C NMR spectra of the products show the resonances of the formaldehyde-derived methylene carbons in the region 45-60 ppm. Interferences occur from resonances of the alpha-amino acid methine carbons. From the data for these products and other model compounds it has been possible to predict the shifts of the residual methylene carbons in a variety of cross-linked systems. This NMR technique shows promise as a rapid non-degradative method for identification of cross-linking sites.