DETECTION OF METHYLATED ASPARAGINE AND GLUTAMINE RESIDUES IN POLYPEPTIDES

A residue of γ-N-methylasparagine (γ-NMA) is found at position β-72 of many phycobiliproteins. δ-N-Methylglutamine is present in some bacterial ribosomal proteins. γ-NMA was synthesized by reacting the ω-methyl ester of aspartate with methylamine and δ-N-methylglutamine by reaction of pyroglutamate with methylamine. These derivatives and the ω-methyl esters of aspartate and glutamate were characterized by melting point, by thin-layer chromatography, by amino acid analysis, by NMR spectroscopy, and after conversion to the phenylthiohydantoin (PTH) derivative. The γ-NMA residues in peptides from allophycocyanin, C-phycocyanin, and B-phycoerythrin were stable under the conditions of automated sequential gas-liquid phase Edman degradation. On HPLC, PTH-γ-NMA coeluted with PTH-serine and was accompanied by a minor component eluting just prior to dimethylphenylthiourea. Similar results were obtained on manual derivatization of synthetic γ-NMA to prepare the PTH derivative. The PTH-δ-N-methylglutamine standard eluted near the position of dimethylphenylthiourea under the usual conditions employed for the identification of PTH-amino acid derivatives in automated protein sequencing. © 1990.