Kinetics of binding of cyanide to sperm whale metmyoglobin.

The kinetics of binding of cyanide to sperm whale metmyoglobin has been studied in the pH range from 4.7 to 10.6 by simple mixing and stopped flow methods with classical and relaxation techniques and the aid of a digital computer. The apparent second order rate constant is strongly dependent on the nature and concentration of the buffer. Extrapolation to zero buffer concentration at constant total ionic strength and pH yields apparent rate constants independent of which buffer is used. The extrapolated rate constants are greatest in the region between pH 7 and pH 8.5, decrease with lower pH values, and decrease more rapidly with higher pH values. The data are interpreted in terms of a mechanism providing for a single ionizable group on the protein with pK = 6.0, the acid dissociation of Fe. Hz0 in the heme with pK = 8.9, and the HCN dissociation with pK = 9.2.