Hypercondensation of an Amino Acid: Synthesis and Characterization of a Black Glycine Polymer

A granular material was obtained by thermal polymerization of glycine at 200 degrees C. It has been named thermomelanoid because of its strikingly deep-black color. The polymerization process is mainly a dehydration condensation leading to conventional amide bonds, and also CC double bonds that are formed from CO and CH2 groups (hypercondensation). Spectroscopic data, in particular from C-13 and N-15 solid-state cross-polarization magic angle spinning (CP-MAS) NMR spectra, suggest that the black color is due to (cross-)conjugated CC, CO, and NH groups. Small glycine peptides, especially triglycine, appear to be key intermediates in the formation of the thermomelanoid. This has been concluded by comparing the thermal behavior of gly(n) homopeptides (n=2-6) and glycine. The glycine polymerization was accompanied by the formation of small amounts of byproducts. Notably, a few percent of alanine and aspartic acid could be detected in the polymer. By using C-13-labeled glycine, it was shown that these two amino acids formed through a common pathway, namely CC bond formation between glycine molecules. The thermomelanoid is hydrolyzed by strong acids and bases at room temperature, forming brown solutions.