Chemiluminescence from thermal oxidation of amino acids and proteins

Chemiluminescence (CL) with maximum emission in the range 550–650 nm is observed when proteins and certain amino acids are heated in air, and CL intensity is significantly reduced in nitrogen. Of the 20 common amino acids, lysine (Lys) has the highest thermal CL intensity by a factor of ~30 over arginine, threonine and asparagine. This finding differs from previous studies on amino acids and proteins oxidised using free radical initiators or singlet oxygen, where tryptophan was the dominant factor for CL emission. CL from heating solid Lys in air is accompanied by browning and the generation of fluorescent products which are characteristic of advanced glycosylation end products (AGEs) in thermally treated milk proteins. During thermal oxidation, Lys may react with its own carbonyl oxidation products to form fluorescent compounds similar to AGEs via the formation of Schiff bases. The mechanism of thermal oxidation of proteins may be similar to polyamide polymers, where reaction of free primary amino groups with carbonyls to form Schiff bases plays a key role.