Importance of many body effects in the kernel of hemoglobin for ligand binding

TL;DR

This paper introduces a new model for ligand binding in hemoglobin based on many-body effects and valence fluctuations, supported by first-principles calculations, explaining optical spectra and ligand binding energy differences.

Contribution

It presents a novel dynamical orbital selection model for heme ligand binding, incorporating many-body effects via first-principles and dynamical mean-field theory.

Findings

Hund's coupling affects iron 3d density and valence fluctuations.

The model explains infrared optical transitions observed experimentally.

Hund's coupling significantly reduces binding energy imbalance between CO and O2.

Abstract

We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selection process. This scenario may be described as bonding determined by local valence fluctuations. We support this model using linear-scaling first-principles calculations, in combination with dynamical mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein central to human respiration. We find that variations in Hund's exchange coupling induce a reduction of the iron 3d density, with a concomitant increase of valence fluctuations. We discuss the comparison between our computed optical absorption spectra and experimental data, our picture accounting for the observation of optical transitions in the infrared regime, and how the Hund's coupling reduces, by a factor of five, the strong imbalance in the binding energies of heme with CO and O_2 ligands.