Metal-ligand and hydrogen bonding in the active site of Fe(III)-, Mn(III)- and Co(III)-myoglobins.

We investigated in this work the strength of metal-ligand bonding in complexes formed between Fe(III)-, Mn(III)- and Co(III)-myoglobin and methanol, water, nitrite, and azide, serving as neutral and ionic prototype ligands, for the ε and δ protonation forms of the myoglobin distal histidine. In total, 24 complexes and 12 associated gas phase models were investigated combining a QM/MM protocol with our local vibrational mode analysis at the PBE0/6-31G(d,p)/AMBER level of theory. According to our results, complexes with methanol and water ligands form weaker metal-ligand bonds than those with nitrite and azide ligands. Furthermore, the strength of the metal-ligand bonds depends on the protonation form of the distal histidine. Among the three metals investigated in this study, Fe, the metal found in native myoglobin, turned out to be the most versatile candidate, providing the broadest range of metal-ligand bond strengths. We also analyzed potential hydrogen bonds formed between the ligand and the distal histidine of the heme pocket. The ε tautomer of histidine forms weaker O⋯H type hydrogen bonds whereas the δ tautomer forms stronger N⋯H type hydrogen bonds. Overall, our findings identify the strength of both metal-ligand and hydrogen bonds (fully captured by our local vibrational mode analysis) as a key parameter determining the catalytic activity and function of myoglobins. This is particularly relevant when considering neutral versus ionic ligands and other metals such as Mn or Co as alternatives to Fe. The insights gained through our investigation offer valuable guidance for strategically fine-tuning existing artificial myoglobins and designing new, versatile variants. We hope that our QM/MM - local mode analysis protocol will become a valuable addition to the research community's toolkit.