Spin Dependent Ligand Binding Reactions of Iron-Containing Complexes and Proteins

Abdullah Ozkanlar, Purdue University

Abstract

(Bio)chemical reactions in which the total electronic spin of the reactants is different from that of the product(s) are called spin-forbidden. Spin forbidden reactions involve spin transitions that only take place via an allowing mechanism, namely spin-orbit coupling. Such spin transitions are believed to take place, with greater probability, at the minimum energy crossing point (MECP) between two interacting spin-dependent potential energy surfaces. Spin forbidden reactions involving iron containing complexes have been studied to understand the role of reactant and product spin degrees of freedom on the reaction rates. The rate constants have been predicted by means of non-adiabatic transition state theory (NATST). We have implemented an ab-initio based computational methodology to predict rate constants of spin forbidden reactions. The relevant MECPs for the reactions studied here have been reported. In order to understand the role of spin transitions on reaction rates, we have studied the spin forbidden binding of singlet carbon monoxide, CO(S=0), singlet dihydrogen, H2(S=0), and singlet ethylene, C2H 4(S=0), to triplet iron tetracarbonyl, Fe(CO)4(S=1), each of which involves one spin transition from triplet to singlet states. The 300K rate constant predicted for CO binding to Fe(CO)4 is ∼ 10-13 cm3s-1molecule-1 , in agreement with the experimental value to within one order of magnitude. The temperature dependent rate constants computed for H2 binding to Fe(CO)4 are ∼ 10-14 cm3s -1molecule-1 showing a trend of faster rate with increasing temperature, in good agreement with the experimental values of 1.97×10 -14 cm3s-1molecule-1 and 2.47×10-14 cm3s-1molecule -1 for 296.5K and 315K, respectively, suggesting that spin transitions play important roles on the rates of this reaction. Finally, the 300K rate constant computed for C2H4 binding to Fe(CO)4 is ∼ 10-13 cm3s-1molecule -1, in good agreement with the experimental value of ∼ 10 -13 cm3s-1molecule-1 also suggesting that the spin transition plays a crucial role on the rate of this reaction. In addition, the spin forbidden binding of triplet dioxygen, O2(S=1), with iron containing heme proteins such as myoglobin has been studied. Finally, a spin forbidden reaction involved in the decomposition cycle of the peroxynitrite anion, OONO-, which is catalyzed by a water soluble iron porphyrin, FeTMPyP, has been studied in the context of NATST.

Degree

Ph.D.

Advisors

Rodriguez, Purdue University.

Subject Area

Physical chemistry|Physics|Biophysics

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