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Mechanism and Driving Force of NO Transfer from S-Nitrosothiol to Cobalt(II) Porphyrin:  A Detailed Thermodynamic and Kinetic Study

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journal contribution
posted on 2007-01-22, 00:00 authored by Xiao-Qing Zhu, Jian-Yu Zhang, Jin-Pei Cheng
The thermodynamics and kinetics of NO transfer from S-nitrosotriphenylmethanethiol (Ph3CSNO) to a series of α,β,γ,δ-tetraphenylporphinatocobalt(II) derivatives [T(G)PPCoII], generating the nitrosyl cobalt atom center adducts [T(G)PPCoIINO], in benzonitrile were investigated using titration calorimetry and stopped-flow UV-vis spectrophotometry, respectively. The estimation of the energy change for each elementary step in the possible NO transfer pathways suggests that the most likely route is a concerted process of the homolytic S−NO bond dissociation and the formation of the Co−NO bond. The kinetic investigation on the NO transfer shows that the second-order rate constants at room temperature cover the range from 0.76 × 104 to 4.58 × 104 M-1 s-1, and the reaction rate was mainly governed by activation enthalpy. Hammett-type linear free-energy analysis indicates that the NO moiety in Ph3CSNO is a Lewis acid and the T(G)PPCoII is a Lewis base; the main driving force for the NO transfer is electrostatic charge attraction rather than the spin−spin coupling interaction. The effective charge distribution on the cobalt atom in the cobalt porphyrin at the various stages, the reactant [T(G)PPCoII], the transition-state, and the product [T(G)PPCoIINO], was estimated to show that the cobalt atom carries relative effective positive charges of 2.000 in the reactant [T(G)PPCoII], 2.350 in the transition state, and 2.503 in the product [T(G)PPCoIINO], which indicates that the concerted NO transfer from Ph3CSNO to T(G)PPCoII with the release of the Ph3CS radical was actually performed by the initial negative charge (−0.350) transfer from T(G)PPCoII to Ph3CSNO to form the transition state and was followed by homolytic S−NO bond dissociation of Ph3CSNO with a further negative charge (−0.153) transfer from T(G)PPCoII to the NO group to form the final product T(G)PPCoIINO. It is evident that these important thermodynamic and kinetic results would be helpful in understanding the nature of the interaction between RSNO and metal porphyrins in both chemical and biochemical systems.