Sulfur K-edge XAS and DFT calculations on nitrile hydratase: geometric and electronic structure of the non-heme iron active site

The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS(-))-, sulfenate (RSO(-))-, and sulfinate (RSO(2)(-))-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO(-) species changes upon protonation as the S-O bond is elongated (by approximately 0.1 A). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe(III) in the active site of NHase as CysS(-), CysSOH, and CysSO(2)(-) both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The pre-edge region of the X-ray absorption spectrum is sensitive to the Z(eff) of the Fe and reveals that the Fe in [FeNO](6) NHase species has a Z(eff) very similar to that of its photolyzed Fe(III) counterpart. DFT calculations reveal that this results from the strong pi back-bonding into the pi antibonding orbital of NO, which shifts significant charge from the formally t(2)(6) low-spin metal to the coordinated NO.     

Electron paramagnetic resonance characteristics of some non-heme low-spin iron(III) complexes

We have recorded the powder EPR-spectra of some near octahedral iron(III) complexes with tridentate ligands donors and analysed their spectra with simple ligand field analysis and for some cases with the angular overlap model (AOM). We have determined the electron praramagnetic resonance (EPR) characteristic of bis 1,4,7-triazacyclonane iron(III)chloride at 4 K and found that it was similar to the characteristics of the so-called 'highly anisotropic low spin' complexes. We have recorded the powder spectra of bis (2,6-bis(benzimidazoly-2-yl)pyridine) iron(III) perchlorate and made an AOM-analyses of the structural similar complex bis-(2,6 (N-carbamoyl)-pyridine) iron(III). With a combination of ligand field analyses and AOM, we could determine the pi-donor properties of these ligands. The same approach have been used to determine the pi-donor properties of the hydroperoxo ligand. Finally we have recorded the powder EPR-spectrum of [Fe(CN)6]3- doped in K3[Co(CN)6] and [Co(NH3)6][Co(CN)6] at 4 and 100 K and in water at 4 K. The spectra are interpreted as the effect of a dynamic Jahn-Teller distortion.     

13C NMR studies of the electronic structure of low-spin iron(III) tetraphenylchlorin complexes

A series of low-spin six-coordinate (tetraphenylchlorinato)iron(III) complexes [Fe(TPC)(L)2]+/- (L = 1-MeIm, CN-, 4-CNPy, and (t)BuNC) have been prepared, and their (13)C NMR spectra have been examined to reveal the electronic structure. These complexes exist as the mixture of the two isomers with the (d(xy))2(d(xz), d(yz))3 and (d(xz), d(yz))4(d(xy))1 ground states. Contribution of the (d(xz), d(yz))4(d(xy))1 isomer has increased as the axial ligand changes from 1-MeIm, to CN(-) (in CD2Cl2 solution), CN- (in CD(3)OD solution), and 4-CNPy, and then to tBuNC as revealed by the meso and pyrroline carbon chemical shifts; the meso carbon signals at 146 and -19 ppm in [Fe(TPC)(1-MeIm)2]+ shifted to 763 and 700 ppm in [Fe(TPC)(tBuNC)2]+. In the case of the CN- complex, the population of the (d(xz), d(yz))4(d(xy))1 isomer has increased to a great extent when the solvent is changed from CD2Cl2 to CD3OD. The result is ascribed to the stabilization of the d(xz) and d(yz) orbitals of iron(III) caused by the hydrogen bonding between methanol and the coordinated cyanide ligand. Comparison of the 13C NMR data of the TPC complexes with those of the TPP, OEP, and OEC complexes has revealed that the populations of the (d(xz), d(yz))4(d(xy))1 isomer in TPC complexes are much larger than those in the corresponding TPP, OEC, and OEP complexes carrying the same axial ligands.     

Dinuclear iron isonitrile complexes: models for the iron hydrogenase active site

Reaction of Fe(2)(mu-S(2)C(3)H(6))(CO)(6) (1) with 2 equiv of t-BuNC affords a disubstitued species Fe(2)(mu-S(2)C(3)H(6))(CN-t-Bu)(2)(CO)(4) (2). The structure of 2 has been determined by X-ray crystallography, which shows that in the solid state both isonitrile ligands are cis to sulfur. In solution, NMR and IR spectroscopy suggest that multiple isomers are present. Protonation of 2 occurs at the Fe-Fe bond to give a cationic complex 3 as four different isomeric species. Complex 3 does not react with deuterium gas (98 psi) in the absence of light. Irradiation of solutions of 3 with visible light under D(2) gas leads to formation of HD.     

Investigating the structural and electronic properties of nitrile hydratase model iron(III) complexes using projected unrestricted Hartree-Fock (PUHF) calculations

Important structural and mechanistic details concerning the non-heme, low-spin Fe(III) center in nitrile hydratase (NHase) remain poorly understood. We now report projection unrestricted Hartree-Fock (PUHF) calculations on the spin preferences of a series of inorganic complexes in which Fe(III) is coordinated by a mixed set of N/S ligands. Given that many of these compounds have been prepared as models of the NHase metal center, this study has allowed us to evaluate this computational approach as a tool for future calculations on the electronic structure of the NHase Fe(III) center itself. When used in combination with the INDO/S semiempirical model, the PUHF method correctly predicts the experimentally observed spin state for 12 of the 13 Fe(III)-containing complexes studied here. The one compound for which there is disagreement between our theoretical calculations and experimental observation exhibits temperature-dependent spin behavior. In this case, the failure of the PUHF-INDO/S approach may be associated with differences between the structure of the Fe(III) complex present under the conditions used to measure the spin preference and that observed by X-ray crystallography. A preliminary analysis of the role of the N/S ligands and coordination geometry in defining the Fe(III) spin preferences in these complexes has also been undertaken by computing the electronic properties of the lowest energy Fe(III) spin states. While any detailed interpretation of our results is constrained both by the limited set of well-characterized Fe(III) complexes used in this study and by the complicated dependence of Fe(III) spin preference upon metal-ligand interactions and coordination geometry, these PUHF-INDO/S calculations support the hypothesis that the deprotonated amide nitrogens coordinating the metal stabilize the low-spin Fe(III) ground state seen in NHase. Strong evidence that the sulfur ligands exclusively define the Fe(III) spin state preference by forming metal-ligand bonds with significant covalent character is not provided by these computational studies. This might, however, reflect limitations in modeling these systems at the INDO/S level of theory.     

Computational insight into the electronic structure and absorption spectra of lithium complexes of N-confused tetraphenylporphyrin

The present work is a theoretical investigation on lithium complexes of N-confused tetraphenylporphyrins (aka inverted) employing density functional theory (DFT) and time-dependent DFT, using the B3LYP, CAM-B3LYP, and M06-2X functionals in conjunction with the 6-31G(d,p) basis set. The purpose of the present study is to calculate the electronic structure and the bonding of the complexes to explain the unusual coordination environment in which Li is found experimentally and how the Li binding affects the Q and the Soret bands. The calculations show that, unlike a typical tetrahedral Li(+) cation, this Li forms a typical bond with one N and interacts with the remaining two N atoms, and it is located in the right place to form an agostic-like interaction with the internal C atom. The reaction energy, the enthalpy for the formation of the lithium complexes of N-confused porphyrins, and the effect of solvation are also calculated. The insertion of Li into N-confused porphyrin, in the presence of tetrahydrofuran, is exothermic with a reaction energy calculated to be as high as -72.4 kcal/mol using the lithium bis(trimethylsilyl)amide reagent. Finally, there is agreement in the general shape among the vis-UV spectra determined with different functionals and the experimentally available ones. The calculated geometries are in agreement with crystallographic data, where available.     

Sulfur K-edge XAS and DFT studies on NiII complexes with oxidized thiolate ligands: implications for the roles of oxidized thiolates in the active sites of Fe and Co nitrile hydratase

S K-edge X-ray absorption spectroscopy data on a series of NiII complexes with thiolate (RS-) and oxidized thiolate (RSO2-) ligands are used to quantify Ni-S bond covalency and its change upon ligand oxidation. Analyses of these results using geometry-optimized density functional theory (DFT) calculations suggest that the Ni-S sigma bonds do not weaken on ligand oxidation. Molecular orbital analysis indicates that these oxidized thiolate ligands use filled high-lying S-O pi* orbitals for strong sigma donation. However, the RSO2- ligands are poor pi donors, as the orbital required for pi interaction is used in the S-O sigma-bond formation. The oxidation of the thiolate reduces the repulsion between electrons in the filled Ni t2 orbital and the thiolate out-of-plane pi-donor orbital leading to shorter Ni-S bond length relative to that of the thiolate donor. The insights obtained from these results are relevant to the active sites of Fe- and Co-type nitrile hydratases (Nhase) that also have oxidized thiolate ligands. DFT calculations on models of the active site indicate that whereas the oxidation of these thiolates has a major effect in the axial ligand-binding affinity of the Fe-type Nhase (where there is both sigma and pi donation from the S ligands), it has only a limited effect on the sixth-ligand-binding affinity of the Co-type Nhases (where there is only sigma donation). These oxidized residues may also play a role in substrate binding and proton shuttling at the active site.     

Spectroscopic and computational studies of nitrile hydratase: insights into geometric and electronic structure and the mechanism of amide synthesis

Nitrile hydratases (NHases) are mononuclear nonheme enzymes that catalyze the hydration of nitriles to amides. NHase is unusual in that it utilizes a low-spin (LS) Fe^III center and a unique ligand set comprised of two deprotonated backbone amides, cysteine-based sulfenic acid (RSO(H)) and sulfinic acid (RSO₂^–), and an unmodified cysteine trans to an exogenous ligand site. Electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD) and low-temperature absorption (LT-Abs) spectroscopies are used to determine the geometric and electronic structures of butyrate-bound (NHaseBA) and active (NHaseAq) NHase. These data calibrate DFT models, which are then extended to explore the mechanism of nitrile hydration by NHase. In particular, the nitrile is activated by coordination to the LS Fe^III and the sulfenate group is found to be deprotonated and a significantly better nucleophile than water that can attack the coordinated nitrile to form a cyclic species. Attack at the sulfenate S atom of the cyclic species is favorable and leads to a lower kinetic barrier than attack by water on coordinated, uncyclized nitrile, while attack at the C of the cyclic species is unfavorable. The roles of the unique ligand set and low-spin nature of the NHase active site in function are also explored. It is found that the oxidized thiolate ligands are crucial to maintaining the LS state, which is important in the binding and activation of nitrile susbtrates. The dominant role of the backbone amidate ligands appears to be as a chelate in keeping the sulfenate properly oriented for nucleophilic attack on the coordinated substrate.     

Fe L-edge X-ray absorption spectroscopy of low-spin heme relative to non-heme Fe complexes: delocalization of Fe d-electrons into the porphyrin ligand

Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin pi-->pi(*) transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P-->Fe sigma and pi-donation and Fe-->P pi back-bonding. We find that pi-donation to Fe(III) is much larger than pi back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry.     

ENDOR studies of the ligation and structure of the non-heme iron site in ACC oxidase

Ethylene is a plant hormone involved in all stages of growth and development, including regulation of germination, responses to environmental stress, and fruit ripening. The final step in ethylene biosynthesis, oxidation of 1-aminocyclopropane-1-carboxylic acid (ACC) to yield ethylene, is catalyzed by ACC oxidase (ACCO). In a previous EPR and ENDOR study of the EPR-active Fe(II)-nitrosyl, [FeNO],(7) complex of ACCO, we demonstrated that both the amino and the carboxyl moieties of the inhibitor d,l-alanine, and the substrate ACC by analogy, coordinate to the Fe(II) ion in the Fe(II)-NO-ACC ternary complex. In this report, we use 35 GHz pulsed and CW ENDOR spectroscopy to examine the coordination of Fe by ACCO in more detail. ENDOR data for selectively (15)N-labeled derivatives of substrate-free ACCO-NO (E-NO) and substrate/inhibitor-bound ACCO-NO (E-NO-S) have identified two histidines as protein-derived ligands to Fe; (1,2)H and (17)O ENDOR of samples in D(2)O and H(2)(17)O solvent have confirmed the presence of water in the substrate-free Fe(II) coordination sphere (E-NO). Analysis of orientation-selective (14,15)N and (17)O ENDOR data is interpreted in terms of a structural model of the ACCO active site, both in the presence (E-NO-S) and in the absence (E-NO) of substrate. Evidence is also given that substrate binding dictates the orientation of bound O(2).     

A synthetic analogue of the active site of Fe-containing nitrile hydratase with carboxamido N and thiolato S as donors: synthesis, structure, and reactivities.

As part of our work on models of the iron(III) site of Fe-containing nitrile hydratase, a designed ligand PyPSH(4) with two carboxamide and two thiolate donor groups has been synthesized. Reaction of (Et(4)N)[FeCl(4)] with the deprotonated form of the ligand in DMF affords the mononuclear iron(III) complex (Et(4)N)[Fe(III)(PyPS)] (1) in high yield. The iron(III) center is in a trigonal bipyramidal geometry with two deprotonated carboxamido nitrogens, one pyridine nitrogen, and two thiolato sulfurs as donors. Complex 1 is stable in water and binds a variety of Lewis bases at the sixth site at low temperature to afford green solutions with a band around 700 nm. The iron(III) centers in these six-coordinate species are low-spin and exhibit EPR spectra much like the enzyme. The pK(a) of the water molecule in [Fe(III)(PyPS)(H(2)O)](-) is 6.3 +/- 0.4. The iron(III) site in 1 with ligated carboxamido nitrogens and thiolato sulfurs does not show any affinity toward nitriles. It thus appears that at physiological pH, a metal-bound hydroxide promotes hydration of nitriles nested in close proximity of the iron center in the enzyme. Redox measurements demonstrate that the carboxamido nitrogens prefer Fe(III) to Fe(II) centers. This fact explains the absence of any redox behavior at the iron site in nitrile hydratase. Upon exposure to limited amount of dioxygen, 1 is converted to the bis-sulfinic species. The structure of the more stable O-bonded sulfinato complex (Et(4)N)[Fe(III)(PyP[SO(2)](2))] (2) has been determined. Six-coordinated low-spin cyanide adducts of the S-bonded and the O-bonded sulfinato complexes, namely, Na(2)[Fe(III)(PyP[SO(2)](2))(CN)] (4) and (Et(4)N)(2)[Fe(III)(PyP[SO(2)](2))(CN)] (5), afford green solutions in water and other solvents. The iron(II) complex (Et(4)N)(2)[Fe(II)(PyPS)] (3) has also been isolated and structurally characterized.     

Modulation of the pK(a) of metal-bound water via oxidation of thiolato sulfur in model complexes of Co(III) containing nitrile hydratase: insight into possible effect of cysteine oxidation in Co-nitrile hydratase

The Co(III) complexes of N,N'-bis(2-mercaptophenyl)pyridine-2,6-dicarboxamide (PyPSH(4)), a designed pentadentate ligand with built-in carboxamide and thiolate groups, have been synthesized and studied to gain insight into the role of Cys-S oxidation in Co-containing nitrile hydratase (Co-NHase). Reaction of [Co(NH(3))(5)Cl]Cl(2) with PyPS(4)(-) in DMF affords the thiolato-bridged dimeric Co(III) complex (Et(4)N)(2)[Co(2)(PyPS)(2)] (1). Although the bridged structure is quite robust, reaction of (Et(4)N)(CN) with 1 in acetonitrile affords the monomeric species (Et(4)N)(2)[Co(PyPS)(CN)] (2). Oxidation of 2 with H(2)O(2) in acetonitrile gives rise to a mixture which, upon chromatographic purification, yields K(2)[Co(PyPSO(2)(OSO(2))(CN] (3), a species containing asymmetrically oxidized thiolates. The Co(III) metal center in 3 is coordinated to a S-bound sulfinate and an O-bound sulfonate (OSO(2)) group. Upon oxidation with H(2)O(2), 1 affords an asymmetrically oxidized dimer (Et(4)N)(2)[Co(2)(PyPS(SO(2)))(2)] (4) in which only the terminal thiolates are oxidized to form S-bound sulfinate groups while the bridging thiolates remain unchanged. The thiolato-bridge in 4 is also cleaved upon reaction with (Et(4)N)(CN) in acetonitrile, and one obtains (Et(4)N)(2)[Co(PyPS(SO(2)))(CN)] (5), a species that contains both coordinated thiolate and S-bound sulfinate around Co(III). The structures of 1-4 have been determined. The spectroscopic properties and reactivity of all the complexes have been studied to understand the behavior of the Co(III) site in Co-NHase. Unlike typical Co(III) complexes with bound CN(-) ligands, the Co(III) centers in 2 and 5 are labile and rapidly lose CN(-) in aqueous solutions. Since 3 does not show this lability, it appears that at least one thiolato sulfur donor is required in the first coordination sphere for the Co(III) center in such species to exhibit lability. Both 2 and 5 are converted to the aqua complexes [Co(PyPS)(H(2)O)](-) and [Co(PyPS(SO(2))(H(2)O)](-) in aqueous solutions. The pK(a) values of the bound water in these two species, determined by spectrophotometry, are 8.3 +/- 0.03 and 7.2 +/- 0.06, respectively. Oxidation of the thiolato sulfur (to sulfinate) therefore increases the acidity of the bound water. Since 2 and 5 promote hydrolysis of acetonitrile at pH values above their corresponding pK(a) values, it is also evident that a metal-bound hydroxide is a key player in the mechanism of hydrolysis by these model complexes of Co-NHase. The required presence of a Cys-sulfinic residue and one water molecule at the Co(III) site of Co-NHase as well as the optimal pH of the enzyme near 7 suggests that (i) modulation of the pK(a) of the bound water molecule at the active site of the enzyme could be one role of the oxidized Cys-S residue(s) and (ii) a cobalt-bound hydroxide could be responsible for the hydrolysis of nitriles by Co-NHase.     

Discrimination of mononuclear and dinuclear dinitrosyl iron complexes (DNICs) by S K-edge X-ray absorption spectroscopy: insight into the electronic structure and reactivity of DNICs

In addition to probing the formation of dinitrosyl iron complexes (DNICs) by the characteristic Fe K-edge pre-edge absorption energy ranging from 7113.4 to 7113.8 eV, the distinct S K-edge pre-edge absorption energy and pattern can serve as an efficient tool to unambiguously characterize and discriminate mononuclear DNICs and dinuclear DNICs containing bridged-thiolate and bridged-sulfide ligands. The higher Fe-S bond covalency modulated by the stronger electron-donating thiolates promotes the Fe → NO π-electron back-donation to strengthen the Fe-NO bond and weaken the NO-release ability of the mononuclear DNICs, which is supported by the Raman ν(Fe-NO) stretching frequency. The Fe-S bond covalency of DNICs further rationalizes the binding preference of the {Fe(NO)(2)} motif toward thiolates following the trend of [SEt](-) > [SPh](-) > [SC(7)H(4)SN](-). The relative d-manifold energy derived from S K-edge XAS as well as the Fe K-edge pre-edge energy reveals that the electronic structure of the {Fe(NO)(2)}(9) core of the mononuclear DNICs [(NO)(2)Fe(SR)(2)](-) is best described as {Fe(III)(NO(-))(2)}(9) compared to [{Fe(III)(NO(-))(2)}(9)-{Fe(III)(NO(-))(2)}(9)] for the dinuclear DNICs [Fe(2)(μ-SEt)(μ-S)(NO)(4)](-) and [Fe(2)(μ-S)(2)(NO)(4)](2-).     

Modeling the active site of nitrile hydratase: synthetic strategies to ensure simultaneous coordination of carboxamido-N and thiolato-S to Fe(III) centers

A general strategy for synthesizing Fe(III) complexes of ligands containing carboxamido-N and thiolato-S donors has been described. Reaction of the doubly deprotonated ligand PyPepS2- (where PyPepSH2=N-2-mercaptophenyl-2'-pyridinecarboxamide) with Fe(III) salts in DMF had previously afforded the Fe(III) complex (Et4N)[Fe(PyPepS)2] without any problem(s) associated with autoredox reactions of the thiolate functionality. In the present work, similar reactions with the doubly deprotonated ligand PiPepS2- (where PiPepSH2=2-mercapto-N-pyridin-2-yl-methylbenzamide) with Fe(III) salts, however, fail to afford any Fe(III) complex because of autoredox reactions. The break in the conjugation in the PiPepSH2 ligand frame is the key reason for this difference in behavior between these two very similar ligands. This is demonstrated by the fact that the same reaction with AqPepS2- (where AqPepSH2=2-mercapto-N-quinolin-8-yl-benzamide), another ligand with extended conjugation, affords the Fe(III) complex (Et4N)[Fe(AqPepS)2] without any synthetic complication. It is therefore evident that ligands in which the carboxamide and thiolate functionalities are kept in conjugation could be used to isolate Fe(III) complexes with carboxamido-N and thiolato-S coordination. This finding will be very helpful in future research work in the area of modeling the active site of Fe-containing nitrile hydratase.     

Spectroscopic properties and electronic structure of low-spin Fe(III)-alkylperoxo complexes: homolytic cleavage of the O-O bond

The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-alkylperoxo model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are explored. The vibrational spectra of 1 show three peaks that are assigned to the O-O stretch (796 cm(-1)), the Fe-O stretch (696 cm(-)(1)), and a combined O-C-C/C-C-C bending mode (490 cm(-1)) that is mixed with upsilon(FeO). The corresponding force constants have been determined to be 2.92 mdyn/A for the O-O bond which is small and 3.53 mdyn/A for the Fe-O bond which is large. Complex 1 is characterized by a broad absorption band around 600 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo pi*(upsilon) to a t(2g) d orbital of Fe(III). This metal-ligand pi bond is probed by MCD and resonance Raman spectroscopies which show that the CT state is mixed with a ligand field state (t(2g) --> e(g)) by configuration interaction. This gives rise to two intense transitions under the broad 600 nm envelope with CT character which are manifested by a pseudo-A term in the MCD spectrum and by the shapes of the resonance Raman profiles of the 796, 696, and 490 cm(-1) vibrations. Additional contributions to the Fe-O bond arise from sigma interactions between mainly O-O bonding donor orbitals of the alkylperoxo ligand and an e(g) d orbital of Fe(III), which explains the observed O-O and Fe-O force constants. The observed homolytic cleavage of the O-O bond of 1 is explored with experimentally calibrated density functional (DFT) calculations. The O-O bond homolysis is found to be endothermic by only 15 to 20 kcal/mol due to the fact that the Fe(IV)=O species formed is highly stabilized (for spin states S = 1 and 2) by two strong pi and a strong sigma bond between Fe(IV) and the oxo ligand. This low endothermicity is compensated by the entropy gain upon splitting the O-O bond. In comparison, Cu(II)-alkylperoxo complexes studied before [Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 10177] are much less suited for O-O bond homolysis, because the resulting Cu(III)=O species is less stable. This difference in metal-oxo intermediate stability enables the O-O homolysis in the case of iron but directs the copper complex toward alternative reaction channels.     

Theoretical investigations of the electronic structure and spectroscopy of mononuclear, non-heme [Fe-NO](6) complexes

The unusual metal coordination and spin-state of the Fe(III) center in nitrile hydratase (NHase) has stimulated the synthesis of numerous model complexes in efforts to understand the reactivity and spectroscopic properties of the enzyme. A particular problem has been the development of model Fe(III) complexes that exhibit reversible, photolabile binding to nitric oxide (NO) in a manner similar to that observed for the NHase metal center. We now report a detailed NBO analysis of the ground-state chemical bonding in three [Fe-NO](6) complexes that exhibit different responses to irradiation, together with investigations of their spectroscopic properties using semiempirical INDO/S CI singles calculations. Our computational studies reveal a correlation between the photolability of these complexes and the existence of low-energy transitions that promote an electron into the Fe-NO pi(*) antibonding molecular orbital. In addition to providing detailed insights into how the ligand field influences the spectroscopy of these mononuclear complexes, these studies strengthen our previous conclusions regarding the role of post-translational cysteine modification in modulating the photoreactivity of the inactive, NO complex of NHase.     

A density functional theory calculation of the electronic properties of several high-spin and low-spin iron(II) pyrazolylborate complexes

Density functional theory has been used to study the electronic spin-state properties of low-spin Fe[HB(pz)3]2, high-spin Fe[HB(3-Mepz)3]2, high-spin Fe[HB(3,5-Me 2pz)3]2, and high-spin Fe[HB(3,4,5-Me 3pz)3]2 complexes that exhibit very different iron(II) electronic spin-sate crossover behaviors with changing temperature and pressure. Excellent agreement is obtained between the experimentally observed M?ssbauer-effect quadrupole splittings and isomer shifts of these complexes and those calculated with the B3LYP functional and various different basis sets for both the high-spin and low-spin states of iron(II). The calculations for Fe[HB(pz)3]2 that use the LANL2DZ, 6-31++G(d,p), and 6-311++G(d,p) basis sets for iron all lead to very similar electric field gradients and thus quadrupole splittings. The initial calculations, which were based upon the known X-ray structures, were followed by structural optimization, an optimization that led to small increases in the Fe-N bond distances. Optimization led to at most trivial changes in the intraligand bond distances and angles. The importance of the 3-methyl-H...H-3-methyl nonbonded intramolecular interligand interactions in controlling the minimum Fe-N bond distances and determining the iron(II) spin state both in Fe[HB(3-Mepz)3]2 and in the related methyl-substituted complexes has been identified.