CLEAVAGE OF THE IMIDAZOLE RING IN HISTIDYL RESIDUE ANALOGUES REACTED WITH PEROXIDIZING LIPIDS

Abstract
Degradative reactions initiated by peroxidizing lipids on the histidyl imidazole side‐chain were studied by reacting the histidyl residue analogues N‐benzoylhistidine and hippurylhistidyl‐leucine with methyl linoleate in a low‐moisture model system resembling the structure of freeze‐dried foods. We found that, after incubation at 51‐57°C and 75% relative humidity for 3 wk, the imidazole side‐chain was cleaved by peroxidizing methyl linoleate to yield asparagyl and aspartyl side‐chains along with other compounds at lower concentrations. The degradation of the histidyl imidazole ring (pKa= 6.0–6.5) increased significantly at pH ≥ 7, indicating that the free base form was the labile species.