Sulfur K-Edge XAS and DFT Calculations on P450 Model Complexes: Effects of Hydrogen Bonding on Electronic Structure and Redox Potentials.

Hydrogen bonding (H-bonding) is generally thought to play an important role in tuning the electronic structure and reactivity of metal–sulfur sites in proteins. To develop a quantitative understanding of this effect, S K-edge X-ray absorption spectroscopy (XAS) has been employed to directly probe ligand–metal bond covalency, where it has been found that protein active sites are significantly less covalent than their related model complexes. Sulfur K-edge XAS data are reported here on a series of P450 model complexes with increasing H-bonding to the ligated thiolate from its substituent. The XAS spectroscopic results show a dramatic decrease in preedge intensity. DFT calculations reproduce these effects and show that the observed changes are in fact solely due to H-bonding and not from the inductive effect of the substituent on the thiolate. These calculations also indicate that the H-bonding interaction in these systems is mainly dipolar in nature. The -2.5 kcal/mol energy of the H-bonding interaction was small relative to the large change in ligand–metal bond covalency (30%) observed in the data. A bond decomposition analysis of the total energy is developed to correlate the preedge intensity change to the change in Fe–S bonding interaction on H-bonding. This effect is greater for the reduced than the oxidized state, leading to a 260 my increase in the redox potential. A simple model shows that E° should vary approximately linearly with the covalency of the Fe–S bond in the oxidized state, which can be determined directly from S K-edge XAS. [ABSTRACT FROM AUTHOR]