Electron spin resonance spectrum of Tyr-151 free radical formed in reactions of sperm whale metmyoglobin with ethyl hydroperoxide and potassium irridate.

A five-line ESR spectrum was observed at room temperature in reactions of sperm whale metmyoglobin with ethyl hydroperoxide (EtOOH) at pH 9.5 and with potassium irridate at pH's 7.0 and 9.5. A spectrum with the same g value and hyperfine splitting constant appeared in a reaction of sperm whale apomyoglobin with potassium irridate and was assigned to a tyrosyl radical on the basis of optical spectrum data obtained under the same reaction conditions. It was concluded that this radical arose from Tyr-151 for the following reasons. (i) This ESR spectrum could not be observed in the reaction of horse heart metmyoglobin, which lacks Tyr-151. (ii) Sperm whale metmyoglobin no longer gave this spectrum when treated with tetranitromethane (TNM) under conditions in which approximately one tyrosine is lost in sperm whale metmyoglobin but none is lost in horse heart metmyoglobin. (iii) A complex ESR spectrum observed in the reaction of sperm whale metmyoglobin with EtOOH at neutral pH was found to be a mixture of this five-line spectrum and one arising from an unidentified free radical formed in the reaction of horse heart metmyoglobin with EtOOH. The TNM-treated sperm whale metmyoglobin gave the same ESR spectrum as that observed in the reaction of horse heart metmyoglobin with EtOOH.