Spin-orbit coupling and zero-field splitting of the high-spin ferric enzyme-substrate complex: Protocatechuate 3,4-dioxygenase complexed with 3,4-dihydroxyphenylacetate

We used density functional calculations to investigate the electronic origins of the magnetic properties of the high-spin ferric enzyme-substrate complex protocatechuate 3,4-dioxygenase (3,4-PCD). The calculated g-tensors show that ligand-to-metal charge transfer transitions are from the protocatechuate (PCA) and Tyr408 orbitals to the Fe d pi orbitals, which lead to x- and y-polarized transitions. These polarized transitions require a spin-orbit coupling (SOC) matrix element in the z-direction, L-z (z = z'), resulting in a g(z)' value of 2.0158, significantly deviating from 2.0023. A large zero-field splitting parameter value of + 1.147 cm(-1) is due to Delta S = -1 spin-orbit mixing with the quartet states for the sextet ground state, accounting for around 73% of the SOC contribution. The SOC matrix elements indicate that the high-spin d(5) system Fe(III), 3,4-PCD-PCA is a weak spin-crossover compound with an SOC of 31.56 cm(-1).