Coordination of Deprotonated Ferrous Heme with CO and O2 in the Gas Phase: Influence of Spin-Orbit Splitting and Charge.

The ligation of 4-coordinated ferrous heme has been investigated in the gas phase using singly deprotonated heme anions to act as a model for neutral heme. The equilibrium of dioxygen and carbon monoxide ligated to deprotonated heme was observed in the 250-150 K temperature range in a Paul ion trap. The van't Hoff equation was used for the determination of ligand binding energies with CO or O2. Density functional theory calculations have been undertaken to analyze the mechanism of bonding that evolves through spin-orbit crossing between the triplet heme reactant and the singlet carbon monoxide-ligated product. The deprotonated heme anion is a good model for neutral heme, since the negative charge on the carboxylate end of the propionate group of heme is shown to have little influence. Deprotonated heme anions yield a ligation enthalpy of 4-coordinated heme with O2 or CO, which is surprisingly low (respectively, ΔH = 20.6 ± 1.7 and 37.9 ± 1.6 kJ mol1).