Modeling the hydrogen sulfide binding to heme

The binding of hydrogen sulfide to a model heme compound is investigated by coupled-cluster singles-doubles augmented by a perturbative triple excitations, CCSD(T), and density functional theory, DFT. The minimum energy path for the H₂S addition to an isolated heme center of the heme protein is evaluated by adopting as a model the heme compound FeP(Im) (P = porphyrin; Im = imidazole). The FeP(Im)-H₂S aduct is bound by 13.7 kcal/mol at the CCSD(T) level of theory. Relaxed potential energy curves for the lowest lying spin states of the H₂S to FeP(Im) binding using DFT reveal that the binding process is associated with a "double spin-crossover" reaction with the existence of long-distance van der Waals minima only 5-7 kcal/mol above the FeP(Im)-H₂S ground state. The fact that the energy of the singlet ground state of FeP(Im)-H₂S is so close in energy to the dissociation products FeP(Im) + H₂S points towards the reversibility of the H₂S adsorption/desorption process in biochemical reactions.