The present study successfully implemented the ab initio
quantum
mechanical charge field molecular dynamics (QMCF MD) formalism for
the investigation of structural and dynamical properties of hydrated
cobalt–porphyrin complexes. Considering the significance of
cobalt ions in biological systems (for instance, vitamin B12), which
reportedly incorporate cobalt ions in a d6, low spin, +3 state chelated
in the corrin ring, an analog of porphyrin, the current study is focused
on cobalt in the oxidation states +2 and +3 bound to the parent porphyrin
lead structures embedded in an aqueous solution. These cobalt–porphyrin
complexes were investigated in terms of their structural and dynamical
properties at the quantum chemical level. The structural attributes
of these hydrated complexes revealed the contrasting features of the
water binding to these solutes, including a detailed evaluation of
the associated dynamics. The study also yielded notable findings in
regard to the respective electronic configurations vs coordination,
which suggested that Co(II)-POR possesses a 5-fold square pyramidal
coordination geometry in an aqueous solution containing the metal
ion coordinating to four nitrogen atoms of the porphyrin ring and
one axial water as the fifth ligand. On the other hand, high-spin
Co(III)-POR was hypothesized to be more stable due to the smaller
size-to-charge ratio of the cobalt ion, but the high-spin complex
demonstrated unstable structural and dynamical behavior. However,
the corresponding properties of the hydrated Co(III)LS-POR
revealed a stable structure in an aqueous solution, thus suggesting
the Co(III) ion to be in a low-spin state when bound to the porphyrin
ring. Moreover, the structural and dynamical data were augmented by
computing the free energy of water binding to the cobalt ions and
the solvent-accessible surface area, which provide further information
on thermochemical properties of the metal–water interaction
and the hydrogen bonding potential of the porphyrin ring in these
hydrated systems.