FORMATION OF THIAZOLINES WITHIN THE PEPTIDE-CHAIN DURING HEATING OF PROTEINS

L-2-Methyl-4-carboxy-thiazoline was isolated by ion-exchange chromatog raphy and RP-HPLC from heated solutions of N-acetyl-L-cysteine. The st ructure was established by NMR-spectroscopy. The formation of thiazoli nes during the heating of cysteine-containing peptides (glycyl-glycyl- cysteine, glycyl-cysteinyl-glycine, glutathione) was observed by UV an d NMR-spectroscopy and measured by cation-exchange chromatography. The rate of formation was dependent on the pH, the water activity and the temperature. In comparison with untreated samples, a characteristic i ncrease of absorption at 260 nm in heated proteins (beta-lactoglobulin , ovalbumin) was noticed. Using NMR-spectroscopy [two-dimensional: het ero nuclear multiple quantum coherence (HMQC) and total correlated spe ctroscopy (TOCSY)] the formation of thiazolines within heated proteins (beta-lactoglobulin, egg white albumin and bovine serum albumin) coul d be demonstrated unequivocally. The signals of the methine-proton of thiazolines as well as the coupling constants of the thiazoline proton s are very characteristic and not overlaid by signals of protein-bound amino acids.