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

Lingling Lü,Yuancheng Zhu,Xiaofang Wang,Surui Zhao,Yongcheng Wang,Guofang Zuo,Feng Guo

doi:10.1007/s11434-012-5316-7

Lingling Lü, Yuancheng Zhu + Show 5 more

Open Access

https://doi.org/10.1007/s11434-012-5316-7

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Abstract

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 dπ 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 ΔS = −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 d5 system Fe(III), 3,4-PCD-PCA is a weak spin-crossover compound with an SOC of 31.56 cm−1.

Highlights

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 sextet state geometric model of 3,4-PCD-PCA was optimized with B3LYP using 6-31+G(d) as the basis set
This calculation identifies the orientation of the molecular axes, in which the x- and y-axis are in the plane of the Tyr408 ring and z is out of the Tyr408 plane axis

Summary

Computational details

The activate-site geometric structure of 3,4-PCD-PCA was obtained from the averaged crystallographic coordinate of P. putida 3,4-PCD complex with PCA (PDB code 3PCA) [12]. Spin-unrestricted DFT calculations were performed using Gaussian 03 [13] to geometrically optimize the active model described above. The double-zeta 6-31+G(d) basis set was used in geometric optimization with two commonly used functionals: Becke’s 1988 exchange functional with the correlation function of Perdew (BP86), and Becke’s three-parameter hybrid functional with the correlation function of Lee, Yang and Parr (B3LYP). Frequencies and thermodynamic parameters were calculated, and all frequencies were found to be real. The graphical output of the molecular orbitals was generated with the program Molekel [14]

Calculations of ZFS tensor and g-tensor

SOC calculations

Results and discussion

Method

Conclusions