The interaction of oxyhemerythrin (HrO2) with cyanate and azide ions (X-) is biphasic. The rapid loss of HrO2 is monitored at 500 nm using stopped flow. The dependence of the pseudo-first order rate constant on [HrO2], [X-], and [O2] can be interpreted by the scheme HrO2 in equilibrium Hr + O2 K1 Hr + X-in equilibrium HrX- k2, k-2, k2 At 25 degrees, pH = 6.3 and I = 0.15 M; for X- = CNO-, k2 = 59 M-1 S-1 and k-2 = 0.019 S-1; and for X- = N3-, k2 = 1.6 X 10(2) M-1 S-1 and k-2 = 0.10 S-1. The

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... es of K1K2 from kinetics are in good agreement with those obtained from spectral and [O2] determinations of equilibrated mixtures of Hr, CNO-, and O2. The second step has been analyzed separately by flow observations of the small spectral changes accompanying addition of X- to deoxyhemerythrin. The slower, stoichiometric formation of methemerythrin cyanate or azide has also been studied and second order rate constants for reaction of HrO2 with X- determined (N3-, 0.34 M-1 S-1 and CNO-, 0.08 M-1 S-1 at 25 degrees and pH = 6.3). These are compared with those for accelerated formation of the methemerythrin ligand adduct in the same conditions in presence of NO2- (0.31 M-1 S-1), HCO2- (8.3 X 10(-3) M-1 S-1), F- (0.12 M-1 S-1), Cl- (1.4 X 10(-4) M-1 S-1), and imidazole (4 X 10(-3) M-1 S-1 at pH 8.0). No rapid pre-equilibria are observed in the case of the latter ligands. The effect of pH is examined.