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

Abstract

Hydrogen 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 pre-edge 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 Hbonding interaction in these systems is mainly dipolar in nature. The −2.5 kcal/mole 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 pre-edge 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 330 mV 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.

S K-Edge XAS and DFT Calculations on Square-Planar NiII−Thiolate Complexes: Effects of Active and Passive H-Bonding

“…The data obtained here show a very different behavior than the experimental results we observed in Ref. 22 for the hemethiolate system; there is no decrease in Ni-S bond covalency but there is a shift in pre-edge energy due to H-bond. DFT calculations reproduce this behavior and indicate that it is the result of the differences in M-S bonding interactions between a thiolate and a high-spin d 5 Fe III which involves π and σ bonds and a low-spin d 8 Ni II which involves only σ bonds.…”

“…22. While the pre-edge intensity was reduced upon H-bonding in the Fe III case there is no resolvable change in intensity in the Ni II case.…”

“…Details of the experimental configuration for low energy studies have been described previously. 22 The energy calibration, data reduction and error analysis follow the methods described in Ref. 16.…”

X-ray Absorption Spectroscopy and Density Functional Theory Studies of [(H3buea)FeIII-X]n- (X = S2-, O2-, OH-): Comparison of Bonding and Hydrogen Bonding in Oxo and Sulfido Complexes

“…53 This indicates that the nature of the H-bonding interaction is mainly dipolar, in agreement with previous results, where the dipole stabilizes the charge on X 2-which reduces the covalency and weakens the Fe-X bond. 54 This is also evident from the increase of NPA charges on the X 2-ions on H-bonding (Table 5). These calculations also indicate that with both waters and point dipoles, the covalency is reduced in the π but not σ orbital (Table S2, Supporting Information).…”

The Influence of the Proximal Amide Hydrogen Bonds and the Proximal Helix Dipole on the Catalytic Activity of Chloroperoxidase

“…The sulfur of Cys29 has two hydrogen bonding interactions with the amides of Ala31 and Leu32. The roles of the NH•••S hydrogen bonds in P450, as well as in NOS and CPO, have been postulated to be passive (protection of the axial sulfur; prevention of the formation of the deactivated species P420, as well as the prevention of the interaction with diatomics), [7][8][9] as well as active (modulation of the "push effect" and the fine tuning of Fe-S bond covalency) 8,[10][11][12] and is the primary subject of this dissertation.…”