Hammett rho sigma correlation for reactions of horseradish peroxidase compound II with phenols
The rates of reduction of horseradish peroxidase compound II by p-methoxyphenol (4-hydroxyanisole) have been studied from pH 6.0 to 10.5. The kinetics are influenced by an acid group of pKa 8.7 on compound II. The acidic form of compound II is reactive; the basic form is not. Only the electrically neutral, unionized form of p-methoxyphenol is reactive. Fifteen different phenols were reacted with compound II at either pH 7.6 or pH 7.0 (three of them at both pH's). Rate constants varied from zero for p-nitrophenol to 3.2 X 10(7) M-1 for p-aminophenol. The reactive m- and p-substituted phenols yield a rho value of -4.6 +/- 0.5 when plotted according to the Hammett relation. This compares to the rho value of -6.9 obtained for horseradish peroxidase compound I reactions with phenols. The difference in sensitivity of compounds I and II to electron donating substituents on the phenols can be explained in terms of the relative simplicity of the reactions. Electron donation occurs to the electron-deficient porphyrin pi-cation radical of compound I accompanied by single proton addition to the protein. For compound II the electron is fed to the ferryl group at the center of the porphyrin in a reaction accompanied by two proton additions to the ferryl oxygen atom, one from the protein and the other from the substrate or solvent. This is followed by loss of water from the inner coordination sphere of the ferric ion. The relative reactivities of three o-substituted phenols can be explained in terms of steric hindrance which is minimal for a single o-substituent.
- Research Organization:
- Univ. of Alberta, Edmonton
- OSTI ID:
- 6840376
- Journal Information:
- Arch. Biochem. Biophys.; (United States), Journal Name: Arch. Biochem. Biophys.; (United States) Vol. 2; ISSN ABBIA
- Country of Publication:
- United States
- Language:
- English
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560300* -- Chemicals Metabolism & Toxicology
59 BASIC BIOLOGICAL SCIENCES
63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTANT EFFECTS ON LIVING ORGS. AND BIOL. MAT.
AROMATICS
BIOCHEMICAL REACTION KINETICS
BIOLOGICAL EFFECTS
ELECTRONS
ELEMENTARY PARTICLES
ENZYMES
FERMIONS
HYDROXY COMPOUNDS
KINETICS
LEPTONS
MOLECULAR STRUCTURE
ORGANIC COMPOUNDS
OXIDOREDUCTASES
PEROXIDASES
PH VALUE
PHENOLS
REACTION KINETICS