Rubredoxin is an iron-sulfur protein involved in electron transfer in enzymes, which is accomplished by changing the oxidation state of the iron center in the active site. We investigated the intersystem crossings between the lowest energy electronic states of different spin multiplicities in the active site of rubredoxin by calculating the rates of spin-forbidden transitions in the Fe(SCH3)4n- models. The transition rates were calculated for the active sites with the iron center in the oxidation states (II), (III) and (IV) using nonadiabatic transition state theory (NA-TST). The potential energy surface cross sections, minimum energy crossing points (MECP), gradients and MECP energy barriers were obtained with PBE/def2-TZVP, while spin-orbit coupling (SOC) was calculated using complete active space configuration interaction. We found four different crossings between the lowest energy singlet, triplet and quintet states in the Fe(II) and Fe(IV) active sites, as well as two crossings between the lowest energy doublet, quartet and sextet states in the Fe(III) active site. Only two triplet/quintet and one quartet/sextet crossings are characterized by nonzero SOC. The transition rates in Fe(II) active site are one and two orders of magnitude larger than the rates in Fe(III) and Fe(IV) active sites, respectively, which is due to larger SOC in the Fe(II) active site. We conclude that the intersystem crossings could play an important role in the electron transfer in rubredoxin.

Intersystem crossings in the active site of rubredoxin

251st National Meeting (2016)

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