J . Am. Chem. SOC.tions. The absorbance was monitored after each injection to assure that phenyl acetate was intact before the enzymic reaction began. The reaction was initiated with the injection of the enzyme in a 50-pL volume to bring the total to 1 mL.The irreversible inhibition of AChE in the presence of the substrate was studied under pseudo-first-order conditions with > 1000-fold excess of inhibitor over enzyme concentration to obtain a signal equal to

No abstract

Two distinct processes characterize the electron-transfer reactions between hemoglobin and arenediazonium tetrafluoroborate salts.In the first, oxidation of hemoglobin by arenediazonium ions that possess polar substituents (p-N02, p-CN, p-CH2COO", and p-PhNH) results in the formation of methemoglobin and arene products obtained by hydrogen abstraction. Their reaction rate constants correlate with those from ferrocyanide oxidation and with Hammett values (p = 3.0). In the second, reactions of hemoglobin with arenediazonium ions whose substituents are more hydrophobic (p-Cl, p-, p-CH3, p-Et, p-Me2CH, p-Me3C, p-CH3(CH2)4CH20, p-CH3Q) form -bonded aryliron(III) complexes. Their rate constants are greater than predicted from the Hammett plot, but there is good correlation with the hydrophobicity parameter . These results are explained by a mechanism in which electron transfer either takes place in the aqueous medium surrounding the hemoprotein, where hydrogen atom abstraction from a hydrogen donor solvent is the preferred process, or at the hydrophobic surface of hemoglobin, after which the neutral aryl radical enters the heme pocket to form the -arylheme adduct.

Two distinct processes characterize the electron‐transfer reactions between hemoglobin and arenediazonium tetrafluoroborate salts.

143ChemInform Abstract The relative second order rate constants for the reduction of (I) by ferrocyanide or decamethylferrocene are correlated with the substituent σ-constants leading to Hammett ρ constants of +4.7 or +3.3, respectively. Applying the Marcus equation for outer-sphere electron transfer, the self-exchange rate constants for the arenediazonium ions are estimated. They agree well with the half-wavepotentials of (I) in sulfolane. It is concluded that the large substituent dependence may be related to the high energy barrier for electron transfer.