| Abstract
| - Mechanistic studies have been carried out on a copper monooxygenase model system, reaction 1 ⇌ 2 → 3, in acetone solvent, using low-temperature stopped-flow kinetics techniques, both rapid-scanning diode-array and variable high-pressure (to 100 MPa) methods. Kinetic and thermodynamic parameters have been ascertained, including activation volumes for individual steps: ΔV⧧(k1) = −15 ± 2.5 cm3/mol, ΔV⧧(k-1) = + 4.4 ± 0.5, and ΔV⧧(k2) = −4.1 ± 0.7 cm3/mol.
- We provide a mechanistic study of a monooxygenase model system and detail low-temperature stopped-flowkinetics studies in acetone as solvent, employing both the use of rapid-scanning diode-array observation andvariable high-pressure (20−100 MPa) techniques. The dicopper(I) complex employed is [Cu2(H-XYL-H)]2+ (1),with the H-XYL-H ligand wherein a m-xylyl group links two bis[2-(2-pyridyl)ethyl]amine units. This reacts withO2 reversibly (k1/k-1) giving a peroxo−dicopper(II) intermediate [Cu2(H-XYL-H)(O2)]2+ (2), which thereuponirreversibly (k2) reacts by oxygen atom insertion (i.e., hydroxylation) of the xylyl group, producing [Cu2(H-XYL-O-)(OH)]2+ (3). Activation parameters are as follows: k1, ΔH⧧ = 2.1 ± 0.7 kJ/mol, ΔS⧧ = −174 ± 3 J/(K mol);k-1, ΔH⧧ = 80.3 ± 0.8 kJ/mol, ΔS⧧ = 77 ± 3 J/(K mol); k2, ΔH⧧ = 58.2 ± 0.2 kJ/mol, ΔS⧧ = −5.8 ± 0.9 J/(Kmol). These values are similar to values obtained in a previous study in dichloromethane. At low temperaturesand higher concentrations, the situation in acetone is complicated by a pre-equilibrium of 1 to an isomer form.The present study provides the first determination of activation volumes for individual steps in coppermonooxygenase reactions. The data and analysis provide that ΔV⧧(k1) = −15 ± 2.5 cm3/mol and ΔV⧧(k-1) =+4.4 ± 0.5 cm3/mol for formation and dissociation of 2, respectively, while ΔV⧧(k2) = −4.1 ± 0.7 cm3/mol; avolume profile for the overall reaction has been constructed. The significance of the findings in the present studyis described, and the results are compared to those for other systems.
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