Novel Free Radicals Formed by the Amino-Carbonyl Reactions of Sugars with Amino Acids, Amines, and Proteins

Abstract

Formation of novel free radicals in an early stage of the amino-carbonyl reactions of sugars with either amino acids,amines and some peptides were demonstrated by use of ESR spectroscopy. These radicals showed characteristic hyperfine ESR spectra and apparently differed from those observed in melanoidin. Formation of radicals of this type was faster at alkaline pH, but their signals were most stably observed in weakly acidic pH region and disappeared rapidly in other regions. The radicals developed even the mixtures were heated in open test tube but abolished rapidly by aeration. The results of reactions of various sugars or related carbonyl compounds with various amino compounds indicated that presence of sugar-like enedioi structure in the carbonyl compound and a primary amine group in the amino compound are necessary for the formation of the radical products, and that the spectral patterns were mainly influenced by the structure of amino compound, e.g. α-vs β-alanine, but not by the structure of sugar, except for three carbon aldehydes. Analyses of the hyperfine structures of ESR spectra lead to the assumption of the identity of the radical products as N,N’-disubstituted pyrazine radical derivatives. Formation of such free radicals was also investigated on some peptides and proteins.