THEORETICAL-STUDY OF THE DISTAL-SIDE STERIC AND ELECTROSTATIC EFFECTSON THE VIBRATIONAL CHARACTERISTICS OF THE FECO UNIT OF THE CARBONYLHEME PROTEINS AND THEIR MODELS

The vibronic theory of activation and quantum chemical intermediate ne glect of differential overlap (INDO) calculations are used to study th e activation of carbon monoxide (change of the C-O bond index and forc e 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 th e porphyrin plane, and presence of the charged groups in the heme envi ronment. It is shown that the main contribution to the CO activation s tems 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. I t follows from the results that none of the studied distortions can ex plain, by itself, the wide variation of the C-O vibrational frequency in the experimentally studied model compounds and heme proteins, To st udy 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 magnitu de and location. The results show that charged groups can strongly aff ect 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 ca n affect the Fe-C and C-O bond indexes (and, consequently, the Fe-C an d C-O vibrational frequencies), both in the same and in opposite direc tions, depending on their position. The theoretical results allow us t o understand the peculiarities of the effect of charged groups on the properties of the FeCO unit both in heme proteins and in their model c ompounds.