title: Spin-Forbidden Transitions between Electronic States in the Active Site of Rubredoxin.
abstract: Rubredoxin is a small iron-sulfur protein involved in biological electron transfer, which is accomplished by changing the oxidation state of the iron atom in the active site. We investigate the possibility of spin-forbidden transitions between the lowest energy electronic states with different spin multiplicities in the rubredoxin active site models [Fe(SCH3)4]n(n = 2-, 1-, 0) using nonadiabatic transition state theory (NA-TST). The equilibrium structures, minimum energy crossing point structures and Hessians were obtained with density functional theory. The spin-orbit coupling (SOC) was calculated with the complete active space configuration interaction method using the two-electron spin-orbit Breit-Pauli Hamiltonian. We found several crossings between the lowest energy spin states associated with the changes in Fe coordination. However, only triplet/quintet crossings in [Fe(SCH3)4]2-and [Fe(SCH3)4]0, as well as a quartet/sextet crossing in [Fe(SCH3)4]-are characterized by nonzero first-order SOC responsible for transitions between these spin states. The rates of spin-forbidden transitions in the [Fe(SCH3)4]2-complex are 1 and 2 orders of magnitude higher than the rates in the [Fe(SCH3)4]-and [Fe(SCH3)4]0complexes, respectively. These rate differences are related to a large variation of the SOC between the complexes with different charges, which in turn comes from different molecular orbitals involved in the spin-flip transitions. Finally, we demonstrate that the differences between the NA-TST rates and the rates calculated under the assumption of completely spin-allowed transitions could be as large as 4 orders of magnitude. This means that even in qualitative discussions of the reaction mechanisms involving changes in spin states the partially spin-forbidden nature of the transitions between these states must be taken into account.
authors.affiliation: Department of Chemistry, University of Nevada , Reno, 1664 North Virginia Street, Reno, Nevada 89557-0216, United States.
authors.forename: Gwen E
authors.initials: GE
authors.lastname: dePolo
authors.affiliation: Department of Chemistry, University of Nevada , Reno, 1664 North Virginia Street, Reno, Nevada 89557-0216, United States.
authors.forename: Danil S
authors.initials: DS
authors.lastname: Kaliakin
authors.affiliation: Department of Chemistry, University of Nevada , Reno, 1664 North Virginia Street, Reno, Nevada 89557-0216, United States.
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Varganov
mesh_terms: D020134:Catalytic Domain; D004583:Electrons; D007501:Iron / Q000737:chemistry; D008958:Models, Molecular; D012416:Rubredoxins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D012416:Rubredoxins; D007501:Iron