Kinetics and Thermodynamics of the Reaction of Deoxyhemerythrin with Nitric Oxide.

Abstract

Deoxyhemerythrin reacts with NO to form a 1:1 adduct shown spectrophotometrically. The kinetics of the formation have been studied directly by stopped-flow measurements at four different temperatures (0.0-23.6.degree. C). The kinetics of the dissociation have been studied, also by stopped-flow techniques, at five different temperatures (4.0-35.1.degree. C) using three different scavangers [Fe(II)(edta)2-, O2 and sperm whale deoxymyoglobin], which gave similar values. For the formation kf= (4.2 .+-. 0.2) .times. 106 M-1 s-1, .DELTA.Hf.dbldag. = 44.3 .+-. 2.3 kJ mol-1, .DELTA.Sf.dbldag. = 30 .+-. 8 J mol-1 K-1 and for the dissociation kd = 0.84 .+-. 0.02 s-1, .DELTA.Hd.dbldag. = 95.6 .+-. 2.1 kJ mol-1, .DELTA.Sd.fwdarw. = 74 .+-. 7 J mol-1 K-1 (25.degree. C, I = 0.2 M and pH 7-8.1). From the kinetic data the thermodynamic data for the formation of HrNO were calculated: Kf = (5.0 .+-. 0.3) .times. 106 M-1, .DELTA.H- = -51.3 .+-. 3.1 kJ mol-1 and .DELTA.S- = -44 .+-. 11 J mol-1 K-1 (25.degree. C). The kinetic data suggest thta NO occupies the same iron(II) site for deoxyhemerythrin as oxygen does. The equilibrium constant for the formation of Fe(II)(edta)(NO)2- has been redetermined: K1 = (1.45 .+-. 0.07) .times. 107 M-1, .DELTA.H- = -77.5 .+-. 1.5 kJ mol-1 and .DELTA.S- = -123 .+-. 5 J mol-l K-1 (25.degree. C).