Stationary and time-dependent carbon monoxide stretching mode features in carboxy myoglobin: a theoretical-computational reappraisal

The stationary and time-dependent infrared spectrum (IR) of the CO stretching mode (νCO) in carboxymyoglobin (MbCO), a longstanding problem of biophysical chemistry, has been modeled through a theoretical-computational method specifically designed for simulating quantum observables in complex atomic-molecular systems and based on a combined application of long time scale molecular dynamics simulations and quantum-chemical calculations. This study is basically focused on two aspects: (i) the origin of the stationary IR substates (termed as A0, A1, and A3) and (ii) the modeling and the interpretation of the νCO energy relaxation. The results, strengthened by a more than satisfactory agreement with the experimental data, concisely indicate that (i) the conformational His64-FeCO relevant substates, i.e., characterized by the formation-disruption of the H-bond between the above moieties, are the main responsible of the presence of two distinct and well separated (A0 and A1/A3) spectroscopic regions; (ii) the characteristic bimodal shape of the A1/A3 spectral region, according to our model, is the result of the fluctuation of the electric field pattern as provided by the protein-solvent framework perturbing the bound His64-CO-Heme complex; and (iii) the electric field pattern, in conjunction with the relatively high density of MbCO vibrational states, is also the main determinant of the νCO energy relaxation, characterizing its kinetic efficiency.