Kinetics of the photoinduced dissociative electron transfer reduction of the antimalarial endoperoxide, Artemisinin

Abstract

The rate constants (k) for reactions between a series of excited singlet state donors and the antimalarial agent, Artemisinin (ART), were measured in acetonitrile using fluorescence quenching techniques. A plot of log(k) correlates with the excited state oxidation potential of the donor, ED•+/D∗ , while a similar plot of log(k) versus the singlet energy of the donor, Es, which if linear would indicate an energy transfer reaction process, shows a poor correlation. The results suggest that the determined rate constants are for dissociative electron transfer (ET) from the excited state donor to the O–O bond in ART. Using our recently determined standard dissociative reduction potential for ART, E0 diss, the rate constants are related to the free energy of ET, 1G0 ET. Analysis of the kinetic data as a function of 1G0 ET correlates well with theories of ET modified for the non-adiabatic nature of the ET to peroxides. A number of thermochemical parameters are estimated from the analysis, in particular the intrinsic barrier (1G6= 0 ) that is comprised of the solvent reorganization energy (λ) and the bond dissociation enthalpy of the O–O bond.