Chiral Selectivity in Inter-reactant Recognition and Electron Transfer of the Oxidation of Horse Heart Cytochrome c by Trioxalatocobaltate(III).

This study delves into the kinetic chirality aspect of outer-sphere electron transfer (ET) between optical active transition-metal complexes and other similar entities, focusing specifically on horse heart cytochrome c and the Λ-/Δ-[Co(Ox)3](3-) pair. The research discovers both structural and operational chiral selectivity for these selected reactants and tries to uncover the reason behind the chirality in ET parameters like work terms, electronic transmission coefficient, and nuclear reorganization free energy. The paper also uses quantum fields like density functional theory and statistical mechanics to substantiate their findings.

Research Context and Main Findings

• The study revolves around the outer-sphere electron transfer (ET) reaction, specifically focusing on optically active transition-metal complexes and metalloproteins. By choosing the pair of Λ-/Δ-[Co(Ox)3](3-) and horse heart cytochrome c (cyt c), the researchers aim to understand the underpinnings of kinetic chirality and distinguish between inter-reactant interaction (or chiral recognition) and ET (or stereoselectivity).
• Through their experiments, the researchers found that Λ-[Co(Ox)3](3-) was the more reactive and more strongly bound enantiomer. They represented this reactivity through the ion-pair formation constant KX and the ET rate constant kET(X), with both KΛ/KΔ and kET(Λ)/kET(Δ) found to be about 1.1-1.2. This indicates the Λ- form is somewhat more active when compared to the Δ- form.

Techniques Used

• The researchers employed a combination of quantum mechanical ET theory, density functional theory, and statistical mechanical computations to analyze chirality and better comprehend the reactants’ behaviour.
• In order to understand how solution ionic strength affects the reacting pair, they also modelled the ion pair K(+)·[Co(Ox)3](3-).

Further Findings and Conclusions

• When studying the complex structure of cyt c, the researchers focused solely on the heme group and its chiral axial ligands, L-His and L-Met. The research also considered different structures for the heme group, including singlet, triplet hemes, and one with partially deprotonated propionic acid side groups.
• Through computational results, they noted that the most favourable inter-reactant configuration was at a very close contact distance, thereby substantiating the concept of a reaction complex and the equivalence of the binding constant to a bimolecular reaction volume.
• The reaction was found to be significantly diabatic with electronic transmission coefficients range between 10(-3) and 10(-2).
• The calculations demonstrated chirality in both the ion-pair formation constant and the electronic transmission coefficient, but no chirality in reorganization and reaction free energy.
• However, due to certain characteristics within the former two factors, the “operational” chirality derived from these parameters is larger than unity, aligning with the experimental data.