非血红素铁(III)活化氧分子反应的自旋轨道耦合和零场分裂

采用密度泛函理论对原儿茶酚3,4-双加氧酶(3,4-PCD)活化O₂分子的反应机理进行了探讨. 初始复合物, 六重态⁶1的超快形成主要归因于电子交换诱导系间穿越(EISC), Fe d_z:O₂ π^*(z)是主要的交换通道, 在Fe―O键长为0.2487 nm处, 交换重叠积分S_ij=ád_z α|π^*(z) β>=0.3758. 从六重态⁶1 形成四重态中间体⁴1, 有两种效应共存, 即电子交换耦合作用和自旋轨道耦合(SOC)作用, 且相互竞争. 计算结果表明, 自旋轨道耦合(SOC)作用起主导因素(SOC=353.16 cm^-1). 至于O―O键的解离主要取决于儿茶酚(PCA)最高占据分子轨道(HOMO)的电子转移, 非血红素酶的铁中心仅承担PCA向O₂电子转移的缓冲作用.²²

Spin-Orbit Coupling and Zero-Field Splitting in Dioxygen Activation by Non-Heme Iron(Ⅲ)

The mechanism of the O₂ activation by the protocatechuate 3,4-dioxygenase was investigated using density functional calculations. In the initial complex, the ultrafast formation of the sextet ⁶1 was probably the result of electron-exchange-induced intersystem crossing, and Fe d_z:O₂ π^*(z) was the dominant exchange pathway, with an overlap of d_z: O₂π^*(z) was dominant exchange pathway with the overlap of S_ij=ád_z α|π^*(z) β>=0.3758 at an Fe―O bond length of 0.2487 nm. Two coexisting effects, electron spin exchange coupling and spin-orbit coupling (SOC) in the sextet ⁶1, are responsible for formation of the quartet state ⁴1 from the sextet ⁶1. The exchange interaction competes with the SOC interaction as a driving force for spin conversion. The calculated results show that the latter is the dominant factor, because of the larger SOC constant (353.16 cm^-1). In cleavage of the O― O bond, electron transfer from the protocatechuate (PCA) highest occupied molecular orbital (HOMO) plays a vital role. The Fe center of the non-heme enzyme is a buffer to transfer an electron pair from the PCA HOMO to O₂.²²²