The Structures and Stabilities
of the Complexes of
Biologically Available Ligands with Fe(II) Porphine: An Ab Initio
Study
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Abstract
Fe(II) porphine complexes were investigated at the “MP2/LB”//B3LYP/SB
level of theory where SB and LB denote the small and large basis sets,
6-31+G(d) and 6-311+G(2df,2p), respectively. Solvation effects due
to water and benzene were approximated by the IEFPCM procedure. Ligands
included H<sub>2</sub>O, Cl<sup>–</sup>, and OH<sup>–</sup> and Im (imidazole), CH<sub>3</sub>NH<sub>2</sub>, (CH<sub>3</sub>)<sub>2</sub>S, CH<sub>3</sub>CO<sub>2</sub><sup>–</sup>,
CH<sub>3</sub>S<sup>–</sup>, and CH<sub>3</sub>PhO<sup>–</sup> as models of the side chains of His, Lys, Met, Asp/Glu, Cys, and
Tyr residues, respectively. Fe(II) porphine, <b>2</b>, and the
complexes <b>2</b>(H<sub>2</sub>O), <b>2</b>(Im), <b>2</b>(CH<sub>3</sub>)<sub>2</sub>S), <b>2</b>(CH<sub>3</sub>NH<sub>2</sub>), and <b>2</b>(H<sub>2</sub>O)<sub>2</sub> have
triplet ground states. All pentacoordinated complexes of <b>2</b> with negatively charged ligands have high spin quintet ground states,
while all hexacoodinated complexes with at least one Im ligand have
low spin singlet ground states. With the exception of <b>2</b>(Im)<sub>2</sub> and <b>2</b>(Im)((CH<sub>3</sub>)<sub>2</sub>S), no hexacoordinated complexes are stable in water or benzene.
Redox properties are sensitive to the nature of the environment and
the ligand(s) attached to the iron center. With the exception of the
parent system, <b>2</b><sup><b>+</b></sup>/<b>2</b>, all complexes are predicted to have a negative reduction potential
relative to the standard hydrogen electrode in water. With neutral
ligand(s), the reduction potential is higher in the nonpolar environment.
The opposite is true with negatively charged ligands. The redox activity
of cytochromes, peroxidases, and catalases is discussed in the context
of the model parent systems
