The vibronic theory of activation and quantum chemical intermediate neglect of differential overlap (INDO) calculations are used to study the activation of carbon monoxide (change of the C-O bond index and force field constant) by the imidazole complex with heme in dependence on the distortion of the porphyrin ring, geometry of the CO coordination, iron-carbon and iron-imidazole distances, iron displacement out of the porphyrin plane, and presence of the charged groups in the heme environment. It is shown that the main contribution to the CO activation stems from the change in the sigma donation from the 5 sigma CO orbital to iron, and back-bonding from the iron to the 2 pi orbital of CO. It follows from the results that none of the studied distortions can explain, by itself, the wide variation of the C-O vibrational frequency in the experimentally studied model compounds and heme proteins. To study the dependence of the properties of the FeCO unit on the presence of charged groups in the heme environment, the latter are simulated by the homogeneous electric field and point charges of different magnitude and location. The results show that charged groups can strongly affect the strength of the C-O bond and its vibrational frequency. It is found that the charges located on the distal side of the heme plane can affect the Fe-C and C-O bond indexes (and, consequently, the Fe-C and C-O vibrational frequencies), both in the same and in opposite directions, depending on their position. The theoretical results allow us to understand the peculiarities of the effect of charged groups on the properties of the FeCO unit both in heme proteins and in their model compounds.

The fluorescence spectra of the wild-type green fluorescence protein (wt-GFP) and the anionic form of p-hydroxybenzylidenedimethylimidazolone (p-HBDI), which models the protein chromophore, were obtained in the 80-300 K temperature range in glycerol/water solvent. The protein spectra have pronounced and well-resolved vibronic structure, at least at lower temperatures. In contrast, the chromophore spectra are very broad and structureless even at the lowest temperatures. Analysis of the spectra shows that the experimentally observed red-shift of the protein spectrum upon heating is apparently caused by quadratic vibronic coupling of the torsional deformation (TD) of the phenyl single bond of the chromophore to the electronic transition. The broad spectra of the chromophore manifest the contribution of different conformations in the glycerol/water solvent. In particular, the lowest-temperature spectrum reflects the distribution over the same TD coordinate in the excited electronic state, which essentially contributes to the asymmetry of the spectrum. Upon heating, motion along this coordinate leads to a configuration from which the radiationless transition takes place. This narrows the distribution along the TD coordinate, causing a more symmetric fluorescence spectrum. We were able to reconstruct the broad, structureless fluorescence spectra of p-HBDI in glycerol/water solutions at various temperatures by convoluting the original wt-GFP spectra with the function describing the distribution of the transition energies of the p-HBDI chromophore. Thus, both the fluorescence broadening and increase in radiationless transition upon removal of the protein chromophore to bulk solvent are consistent with decay by a barrierless TD of the phenyl single bond.

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The quantum chemical calculations, vibronic theory of activation, and London-Pople approach are used to study the dependence of the C-O vibrational frequency, 17O isotropic chemical shift, and nuclear quadrupole coupling constant on the distortion of the porphyrin ring and geometry of the CO coordination, changes in the iron-carbon and iron-imidazole distances, magnitude of the iron displacement out of the porphyrin plane, and presence of the charged groups in the heme environment. It is shown that only the electrostatic interactions can cause the variation of all these parameters experimentally observed in different heme proteins, and the heme distortions could modulate this variation. The correlations between the theoretically calculated parameters are shown to be close to the experimentally observed ones. The study of the effect of the electric field of the distal histidine shows that the presence of the four C-O vibrational bands in the infrared absorption spectra of the carbon monoxide complexes of different myoglobins and hemoglobins can be caused by the different orientations of the different tautomeric forms of the distal histidine. The dependence of the 17O isotropic chemical shift and nuclear quadrupole coupling constant on pH and the distal histidine substitution can be also explained from the same point of view.

The causes of the strong coupling of the iron-histidine vibration to the Soret resonance in the resonance Raman spectra of deoxyhemoglobin, myoglobin, and peroxidase are explored, using the vibronic theory. It is shown that the extent of the iron displacement out of the plane of the porphyrin nitrogens is the main structural parameter controlling the Fe-NHis band features, such as the dependence of its frequency and intensity on the protein conformation and number of the axial ligands, time evolution after the photolysis of the diatomic complexes of the proteins under consideration, and inverse relationship between the changes Fe-NHis and v4 porphyrin breathing mode frequencies.