Computational modeling of factors that modulate the unique FeNO bonding in {FeNO}(6) heme-thiolate model complexes

Structure and reactivity of chlorite dismutase nitrosyls

Ferric nitrosyl ({FeNO}6) and ferrous nitrosyl ({FeNO}7) complexes of the chlorite dismutases (Cld) from Klebsiella pneumoniae and Dechloromonas aromatica have been characterized using UV-visible absorbance and Soret-excited resonance Raman spectroscopy. Both of these Clds form kinetically stable {FeNO}6 complexes and they occupy a unique region of ν(Fe-NO)/ν(N-O) correlation space for proximal histidine liganded heme proteins, characteristic of weak Fe-NO and N-O bonds. This location is attributed to admixed FeIII-NO character of the {FeNO}6 ground state. Cld {FeNO}6 complexes undergo slow reductive nitrosylation to yield {FeNO}7 complexes. The effects of proximal and distal environment on reductive nitroylsation rates for these dimeric and pentameric Clds are reported. The ν(Fe-NO) and ν(N-O) frequencies for Cld {FeNO}7 complexes reveal both six-coordinate (6c) and five-coordinate (5c) nitrosyl hemes. These 6c and 5c forms are in a pH dependent equilibrium. The 6c and 5c {FeNO}7 Cld frequencies provided positions of both Clds on their respective ν(Fe-NO) vs ν(N-O) correlation lines. The 6c {FeNO}7 complexes fall below (along the ν(Fe-NO) axis) the correlation line that reports hydrogen-bond donation to NNO, which is consistent with a relatively weak Fe-NO bond. Kinetic and spectroscopic evidence is consistent with the 5c {FeNO}7 Clds having NO coordinated on the proximal side of the heme, analogous to 5c {FeNO}7 hemes in proteins known to have NO sensing functions.

Alternative modes of O2 activation in P450 and NOS enzymes are clarified by DFT modeling and resonance Raman spectroscopy

The functions of heme proteins are modulated by hydrogen bonds (H-bonds) directed at the heme-bound ligands by protein residues. When the gaseous ligands CO, NO, or O2 are bound, their activity is strongly influenced by H-bonds to their atoms. These H-bonds produce characteristic changes in the vibrational frequencies of the heme adduct, which can be monitored by resonance Raman spectroscopy and interpreted with density functional theory (DFT) computations. When the protein employs a cysteinate proximal ligand, bound O2 becomes particularly reactive, the course of the reaction being controlled by H-bonding and proton delivery. In this work, DFT modeling is used to examine the effects of H-bonding to either the terminal (Ot) or proximate (Op) atom of methylthiolate-Fe(II)porphine-O2, as well as to the thiolate S atom. H-bonds to Op produce a positive linear correlation between ν(Fe - O) and ν(O - O), because they increase the sp2 character of Op, weakening both the Fe - O and O - O bonds. H-bonds to Ot produce a negative correlation, because they increase Fe backbonding, strengthening the Fe - O but weakening the O - O bond. Available experimental data accommodate well to the computed pattern. In particular, this correspondence supports the interpretation of cytochrome P450 data by Kincaid and Sligar [M. Gregory, P.J. Mak, S.G. Sligar, J.R. Kincaid, Angew. Chem. Int. Ed. 125 (2013) 5450-5453], involving steering between hydroxylation and lyase reaction channels by differential H-bonds. Similar channeling between the first and second steps of the nitric oxide synthase reaction is likely.

Electronic structure and ligand vibrations in FeNO, CoNO, and FeOO porphyrin adducts

The gaseous ligands, CO, NO, and O2 interact with the Fe ion in heme proteins largely via backbonding of Fe electrons to the π* orbitals of the XO (X = C, N, O) ligands. In these FeXO adducts, the Fe-X stretching frequency varies inversely with the X-O stretching frequency, since increased backbonding strengthens the Fe-X bond while weakening the X-O bond. Inverse frequency correlations have been observed for all three ligands, despite differing electronic and geometric structures, and despite variable composition of the "FeX" vibrational mode, in which Fe-X stretching and Fe-X-O coordinates are mixed for bent FeXO adducts. We report experimental data for 5-coordinate Co(II)(NO) porphyrin adducts (isoelectronic with Fe(II)(OO) adducts), and the results of density functional theory (DFT) modeling for 5-coordinate Fe(II)(NO), Co(II)(NO), and Fe(II)(OO) adducts. Inverse ν(MX)/ν(XO) correlations are obtained computationally, using model porphyrins with graded electron-donating and -withdrawing substituents to modulate the backbonding. Computed slopes agree satisfactorily with experiment, provided nonhybrid functionals are used, which avoid overemphasizing high-spin states. The BP86 functional gives correct ground states, a closed-shell singlet for Co(II)(NO) and an open-shell singlet for the isoelectronic Fe(II)(OO), as corroborated by structural data for Co(II)(NO), and the ν(MX)/ν(XO) slope agreement with experiment for both adducts. However, for Fe(II)(OO) adducts, the computed inverse ν(MX)/ν(XO) correlation applies only to porphyrins with electron-donating and withdrawing substituents of moderate strength. For substituents more donating than -CH3, a direct correlation is obtained, the Fe-O and O-O bonds weakening in concert. This effect is ascribed to the dominance of σ bonding via the in-plane dxz(+dz(2))-π* orbital, when electron-donating substituents raise the d orbital energies sufficiently to render backbonding (dyz-π*) unimportant.

CO, NO and O2 as Vibrational Probes of Heme Protein Interactions

The gaseous XO molecules (X = C, N or O) bind to the heme prosthetic group of heme proteins, and thereby activate or inhibit key biological processes. These events depend on interactions of the surrounding protein with the FeXO adduct, interactions that can be monitored via the frequencies of the Fe-X and X-O bond stretching modes, νFeX and νXO. The frequencies can be determined by vibrational spectroscopy, especially resonance Raman spectroscopy. Backbonding, the donation of Fe dπ electrons to the XO π* orbitals, is a major bonding feature in all the FeXO adducts. Variations in backbonding produce negative νFeX/νXO correlations, which can be used to gauge electrostatic and H-bonding effects in the protein binding pocket. Backbonding correlations have been established for all the FeXO adducts, using porphyrins with electron donating and withdrawing substituents. However the adducts differ in their response to variations in the nature of the axial ligand, and to specific distal interactions. These variations provide differing vantages for evaluating the nature of protein-heme interactions. We review experimental studies that explore these variations, and DFT computational studies that illuminate the underlying physical mechanisms.

New light on NO bonding in Fe(III) heme proteins from resonance Raman spectroscopy and DFT modeling

Visible and ultraviolet resonance Raman (RR) spectra are reported for Fe(III)(NO) adducts of myoglobin variants with altered polarity in the distal heme pockets. The stretching frequencies of the Fe(III)-NO and N-O bonds, nu(FeN) and nu(NO), are negatively correlated, consistent with backbonding. However, the correlation shifts to lower nu(NO) for variants lacking a distal histidine. DFT modeling reproduces the shifted correlations and shows the shift to be associated with the loss of a lone-pair donor interaction from the distal histidine that selectively strengthens the N-O bond. However, when the model contains strongly electron-withdrawing substituents at the heme beta-positions, nu(FeN) and nu(NO) become positively correlated. This effect results from Fe(III)-N-O bending, which is induced by lone-pair donation to the N(NO) atom. Other mechanisms for bending are discussed, which likewise lead to a positive nu(FeN)/nu(NO) correlation, including thiolate ligation in heme proteins and electron-donating meso-substituents in heme models. The nu(FeN)/nu(NO) data for the Fe(III) complexes are reporters of heme pocket polarity and the accessibility of lone pair, Lewis base donors. Implications for biologically important processes, including NO binding, reductive nitrosylation, and NO reduction, are discussed.