A Molecular Model for the Major Conformational Substates in Heme Proteins

We present a molecular model of the major “conformational substates” observed by infrared spectroscopy in carbonmonoxyhemoglobins, myoglobins, and peroxidases, in terms of an electrical perturbation of the CO fundamental vibrational frequencies due to the possibility of H^ϵ2 ↔ H^δ1 tautomerism, and 180° C^β–C^γ ring flips, of distal histidine residues. The model supports a previous interpretation of vibrational frequencies and nuclear magnetic resonance chemical shifts of CO ligands in heme proteins as originating in a weak electric field (dipole and quadrupole interactions with permanent dipole moments, dipole polarizabilities, and shielding polarizabilities), and opens up the possibility of future detailed molecular interpretations of both NMR chemical shifts (of ¹H, ^l3C, and ¹⁵N), as well as IR data, on proteins and other macromolecules.