Take a deep breath, you just created a quantum entangled state!

Metalloporphyrin systems, organic compounds which contain a metal ion such as heme, the pigment within red blood cells, play a central role in biochemistry.

The ability of such molecules to reversibly bind small ligands is vital for life, particularly in heme, which can bind diatomic molecules such as O$_2$ and CO.  Crucially, heme is the primary transport molecule in human respiration.

Although studied extensively, heme remains a mystery for physicist. Indeed, electronic structure calculations carried out early in the last decades found that whereas O$_2$ binds reversibly, the toxic ligand CO binds irreversibly!

In a recent work appeared in Physical Review Letters, we used state of the art DFT+DMFT quantum calculations (combination of Linear-Scaling Density Functional Theory and Dynamical Mean-Field Theory, using the ONETEP package) to study the binding of oxygen and carbon monoxide to the heme molecule.

We found that quantum effects are important to describe this simple molecule. This is due to the effect of electronic correlations (N-body problem), called "strong correlations", and are induced by the presence of the iron atom, a transition metal ion.

The quantum many-body ground-state of heme is indeed a quantum superposition of different quantum states (it is entangled), and in particular the charge of the iron atom was found to be "fluctuating" (valence fluctuation).

We also found that the Hund's rule, so far neglected to describe this simple molecule, plays a central role in reorganizing the electronic state of this molecule: a 3d orbital-selection process occurs beyond a critical value of the Hund's exchange coupling parameter J, by which out-of-plane orbital hybridization is enhanced and the difference of ligand binding affinities is strongly reduced (Picture below).

This work offers a very detailed picture of the microscopic mechanisms of diatomic ligand binding to heme, and is among the first applications of DFT+DFMT to molecules, including total-energies, spectral properties in very good agreement with experiment, and transient magnetic response calculations.

Journal link: http://prl.aps.org/abstract/PRL/v110/i10/e106402

Importance of Many-Body Effects in the Kernel of Hemoglobin for Ligand Binding

Cédric Weber, David D. O’Regan, Nicholas Hine, Peter B. Littlewood, Gabriel Kotliar, and Mike C. Payne

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