PRESSURE EFFECTS ON ACID-CATALYZED AUTOXIDATION OF OXYMYOGLOBIN

Two opposing factors have been found to affect the rate of autoxidatio n of the fresh meat pigment oxymyoglobin, MbFe(II)O-2, in air-saturate d aqueous acetate buffer with increasing hydrostatic pressure to yield metmyoglobin, MbFe(III): (1) expansion in the transition state, corre sponding to a volume of activation of Delta V-# = +12.7 +/- 0.6 ml mol (-1) (25.0 degrees C, 0.16 M NaCl), in effect decreasing the rate; and (2) an increased specific acid catalysis resulting from a decrease in solution pH and increasing the rate. In the pressure range investigat ed (up to 250 MPa) and at the pH relevant to traditionally deboned mea t (pH = 5.65 at ambient pressure investigated, together with pH = 5.21 in order to obtain mechanistic information), the pressure/pH effect w as found almost to negate the kinetic effect of expansion in the trans ition state. Red-to-brown discoloration during pressure treatment of m eat is accordingly expected to be less significant for hot-deboned and other high-pH meats.