Harmonic and anharmonic dynamics of Fe-CO and Fe-O(2) in heme models

Common Reactivity and Properties of Heme Peroxidases: A DFT Study of Their Origin

Electronic structure calculations using the density-functional theory (DFT) have been performed to analyse the effect of water molecules and protonation on the heme group of peroxidases in different redox (ferric, ferrous, compounds I and II) and spin states. Shared geometries, spectroscopic properties at the Soret region, and the thermodynamics of peroxidases are discussed. B3LYP and M06-2X density functionals with different basis sets were employed on a common molecular model of the active site (Fe-centred porphine and proximal imidazole). Computed Gibbs free energies indicate that the corresponding aquo complexes are not thermodynamically stable, supporting the five-coordinate Fe(III) centre in native ferric peroxidases, with a water molecule located at a non-bonding distance. Protonation of the ferryl oxygen of compound II is discussed in terms of thermodynamics, Fe-O bond distances, and redox properties. It is demonstrated that this protonation is necessary to account for the experimental data, and computed Gibbs free energies reveal pK_a values of compound II about 8.5-9.0. Computation indicates that the general oxidative properties of peroxidase intermediates, as well as their reactivity towards water and protons and Soret bands, are mainly controlled by the iron porphyrin and its proximal histidine ligand.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Ultrafast carbon monoxide photolysis and heme spin-crossover in myoglobin via nonadiabatic quantum dynamics

Light absorption of myoglobin triggers diatomic ligand photolysis and a spin crossover transition of iron(II) that initiate protein conformational change. The photolysis and spin crossover reactions happen concurrently on a femtosecond timescale. The microscopic origin of these reactions remains controversial. Here, we apply quantum wavepacket dynamics to elucidate the ultrafast photochemical mechanism for a heme-carbon monoxide (heme-CO) complex. We observe coherent oscillations of the Fe-CO bond distance with a period of 42 fs and an amplitude of ∼1 Å. These nuclear motions induce pronounced geometric reorganization, which makes the CO dissociation irreversible. The reaction is initially dominated by symmetry breaking vibrations inducing an electron transfer from porphyrin to iron. Subsequently, the wavepacket relaxes to the triplet manifold in ∼75 fs and to the quintet manifold in ∼430 fs. Our results highlight the central role of nuclear vibrations at the origin of the ultrafast photodynamics of organometallic complexes.

Molecular basis for catalysis and substrate-mediated cellular stabilization of human tryptophan 2,3-dioxygenase

Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) play a central role in tryptophan metabolism and are involved in many cellular and disease processes. Here we report the crystal structure of human TDO (hTDO) in a ternary complex with the substrates L-Trp and O₂ and in a binary complex with the product N-formylkynurenine (NFK), defining for the first time the binding modes of both substrates and the product of this enzyme. The structure indicates that the dioxygenation reaction is initiated by a direct attack of O₂ on the C₂ atom of the L-Trp indole ring. The structure also reveals an exo binding site for L-Trp, located ~42 Å from the active site and formed by residues conserved among tryptophan-auxotrophic TDOs. Biochemical and cellular studies indicate that Trp binding at this exo site does not affect enzyme catalysis but instead it retards the degradation of hTDO through the ubiquitin-dependent proteasomal pathway. This exo site may therefore provide a novel L-Trp-mediated regulation mechanism for cellular degradation of hTDO, which may have important implications in human diseases.

Ultrafast Dynamics of Carboxy-Hemoglobin: Two-Dimensional Infrared Spectroscopy Experiments and Simulations

This Letter presents a comparison between experimental and simulated 2D mid-infrared spectra of carboxy-hemoglobin in the spectral region of the carbon monoxide stretching mode. The simulations rely on a fluctuating potential energy surface that includes both the effect of heme and the protein surroundings computed from molecular dynamics simulations. A very good agreement between theory and experiment is obtained with no adjustable parameters. The simulations show that the effect of the distal histidine through the hydrogen bond is strong and is directly responsible for the slow decay of the frequency-frequency correlation function on a 10 ps time scale. This study confirms that fluctuations in carboxy-hemoglobin are more inhomogeneous than those in the more frequently studied carboxy-myoglobin. The comparison between simulations and experiments brings valuable information on the complex relation between protein structure and spectral diffusion.

Transition metal complexes and the activation of dioxygen

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.

Correlated ligand dynamics in oxyiron picket fence porphyrins: structural and Mössbauer investigations

Disorder in the position of the dioxygen ligand is a well-known problem in dioxygen complexes and, in particular, those of picket fence porphyrin species. The dynamics of Fe-O2 rotation and tert-butyl motion in three different picket fence porphyrin derivatives has been studied by a combination of multitemperature X-ray structural studies and Mössbauer spectroscopy. Structural studies show that the motions of the dioxygen ligand also require motions of the protecting pickets of the ligand binding pocket. The two motions appear to be correlated, and the temperature-dependent change in the O2 occupancies cannot be governed by a simple Boltzmann distribution. The three [Fe(TpivPP)(RIm)(O2)] derivatives studied have RIm = 1-methyl-, 1-ethyl-, or 2-methylimidazole. In all three species there is a preferred orientation of the Fe-O2 moiety with respect to the trans imidazole ligand and the population of this orientation increases with decreasing temperature. In the 1-MeIm and 1-EtIm species the Fe-O2 unit is approximately perpendicular to the imidazole plane, whereas in the 2-MeHIm species the Fe-O2 unit is approximately parallel. This reflects the low energy required for rotation of the Fe-O2 unit and the small energy differences in populating the possible pocket quadrants. All dioxygen complexes have a crystallographically required 2-fold axis of symmetry that limits the accuracy of the determined Fe-O2 geometry. However, the 80 K structure of the 2-MeHIm derivative allowed for resolution of the two bonded oxygen atom positions and provided the best geometric description for the Fe-O2 unit. The values determined are Fe-O = 1.811(5) Å, Fe-O-O = 118.2(9)°, O-O = 1.281(12) Å, and an off-axis tilt of 6.2°. Demonstration of the off-axis tilt is a first. We present detailed temperature-dependent simulations of the Mössbauer spectra that model the changing value of the quadrupole splitting and line widths. Residuals to fits are poorer at higher temperature. We believe that this is consistent with the idea that population of the two conformers is related to the concomitant motions of both Fe-O2 rotations and motions of the protecting tert-butyl pickets.

Effects of local protein environment on the binding of diatomic molecules to heme in myoglobins. DFT and dispersion-corrected DFT studies

The heme-AB binding energies (AB = CO, O2) in a wild-type myoglobin (Mb) and two mutants (H64L, V68N) of Mb have been investigated in detail with both DFT and dispersion-corrected DFT methods, where H64L and V68N represent two different, opposite situations. Several dispersion correction approaches were tested in the calculations. The effects of the local protein environment were accounted for by including the five nearest surrounding residues in the calculated systems. The specific role of histidine-64 in the distal pocket was examined in more detail in this study than in other studies in the literature. Although the present calculated results do not change the previous conclusion that the hydrogen bonding by the distal histidine-64 residue plays a major role in the O2/CO discrimination by Mb, more details about the interaction between the protein environment and the bound ligand have been revealed in this study by comparing the binding energies of AB to a porphyrin and the various myoglobins. The changes in the experimental binding energies from one system to another are well reproduced by the calculations. Without constraints on the residues in geometry optimization, the dispersion correction is necessary, since it improves the calculated structures and energetic results significantly.

New perspectives on iron-ligand vibrations of oxyheme complexes

We report our studies of the vibrational dynamics of iron for three imidazole-ligated oxyheme derivatives that mimic the active sites of histidine-ligated heme proteins complexed with dioxygen. The experimental vibrational data are obtained from nuclear resonance vibrational spectroscopy (NRVS) measurements conducted on both powder samples and oriented single crystals, and which includes several in-plane (ip) and out-of-plane (oop) measurements. Vibrational spectral assignments have been made through a combination of the oriented sample spectra and predictions based on density functional theory (DFT) calculations. The two Fe-O(2) modes that have been previously observed by resonance Raman spectroscopy in heme proteins are clearly shown to be very strongly mixed and are not simply either a bending or stretching mode. In addition, a third Fe-O(2) mode, not previously reported, has been identified. The long-sought Fe-Im stretch, not observed in resonance Raman spectra, has been identified and compared with the frequencies observed for the analogous CO and NO species. The studies also suggest that the in-plane iron motion is anisotropic and is controlled by the orientation of the Fe-O(2) group and not sensitive to the in-plane Fe-N(p) bonds and/or imidazole orientations.

Quantitative vibrational dynamics of iron in carbonyl porphyrins

We use nuclear resonance vibrational spectroscopy and computational predictions based on density functional theory (DFT) to explore the vibrational dynamics of (57)Fe in porphyrins that mimic the active sites of histidine-ligated heme proteins complexed with carbon monoxide. Nuclear resonance vibrational spectroscopy yields the complete vibrational spectrum of a Mössbauer isotope, and provides a valuable probe that is not only selective for protein active sites but quantifies the mean-squared amplitude and direction of the motion of the probe nucleus, in addition to vibrational frequencies. Quantitative comparison of the experimental results with DFT calculations provides a detailed, rigorous test of the vibrational predictions, which in turn provide a reliable description of the observed vibrational features. In addition to the well-studied stretching vibration of the Fe-CO bond, vibrations involving the Fe-imidazole bond, and the Fe-N(pyr) bonds to the pyrrole nitrogens of the porphyrin contribute prominently to the observed experimental signal. All of these frequencies show structural sensitivity to the corresponding bond lengths, but previous studies have failed to identify the latter vibrations, presumably because the coupling to the electronic excitation is too small in resonance Raman measurements. We also observe the FeCO bending vibrations, which are not Raman active for these unhindered model compounds. The observed Fe amplitude is strongly inconsistent with three-body oscillator descriptions of the FeCO fragment, but agrees quantitatively with DFT predictions. Over the past decade, quantum chemical calculations have suggested revised estimates of the importance of steric distortion of the bound CO in preventing poisoning of heme proteins by carbon monoxide. Quantitative agreement with the predicted frequency, amplitude, and direction of Fe motion for the FeCO bending vibrations provides direct experimental support for the quantum chemical description of the energetics of the FeCO unit.

Protonated Heme

The ions formally corresponding to protonated heme [Fe(II)-hemeH](+) have been obtained by collision-induced dissociation from the electrospray ionization of microperoxidase (MP11) and their gas-phase chemistry has been studied by FTICR mass spectrometry. H/D-exchange reactions, used as a tool to gain information on the protonation sites in polyfunctional molecules, show that labile hydrogens pertain to the propionyl substituents at the periphery of the protoporphyrin IX. Several conceivable isomers for protonated heme have been evaluated by density functional theory. The most stable among the species investigated is the one corresponding to protonation at the beta carbon atom of a vinyl group, yielding a proton affinity (PA) value for [Fe(II)-heme] of 1220 kJ mol(-1). This high PA is consistent with the inertness of the hydrogen atoms at the protonation site towards H/D exchange with ND(3) and CD(3)CO(2)D. Peculiar features of this [Fe(II)-hemeH](+) isomer emerge by analysis of its electronic structure, showing that the vinyl group undergoing formal protonation has gained significant radical character due to electron transfer from the metal center. As a consequence, the iron atom acquires partial iron(III) character and none of the two formal descriptions [Fe(II)-hemeH(+)] and [Fe(III)-hemeH(.)](+) alone may adequately illustrate the protonated heme ion. In agreement with this description, the reactivity of protonated heme presents dual facets, resembling iron(III) in some aspects and iron(II) in others. On the one hand, protonated heme behaves like [Fe(III)-heme](+) ions in H/D-exchange reactions. On the other, it shows markedly decreased reactivity towards the addition of ligands with the notable exception of NO, in line with the high affinity shown by iron(II) complexes towards this molecule, NO, of key biological role.

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.

Spin-Forbidden Ligand Binding to the Ferrous−Heme Group: Ab Initio and DFT Studies

The potential energy surfaces (PESs) and associated energy barriers that characterize the spin-forbidden recombination reactions of the gas-phase ferrous deoxy-heme group with CO, NO, and H2O ligands have been calculated using density functional theory (DFT). The bond energy for binding of O2 has also been calculated. Extensive large basis set CCSD(T) calculations on two small models of the heme group have been used to calibrate the accuracy of different DFT functionals for treating these systems. Pure functionals are shown to overestimate the stability of the low-spin forms of the deoxy-heme model, and to overestimate the binding energy of H2O and CO, whereas hybrid functionals such as B3PW91 and B3LYP yield accurate results. Accordingly, the latter functionals have been used to explore the PESs for binding. CO binding is found to involve a significant barrier of ca. 3 kcal mol-1 due to the need to change from the deoxy-heme quintet ground state to the bound singlet state. Binding of water does not involve a barrier, but the resulting bond is weak and may be further weakened in the protein environment, which should explain why water binding is not usually observed in heme proteins such as myoglobin. NO binding involves a low barrier, which is consistent with observed rapid geminate recombination. The calculated bond energies are in good agreement with previous reported values and in fair agreement with experiment for CO and O2. The value for NO is significantly lower than the experimentally derived bond energy, suggesting that B3LYP is less accurate in this case.

Structure and dynamics of dioxygen bound to cobalt and iron heme

In this study we use ab initio molecular dynamics simulations to analyze the structure and dynamics of the oxygen ligand in models of the oxymyoglobin active site and its cobalt-substituted analog. Our calculations are performed for iron-porphyrin and cobalt-porphyrin complexes with imidazole and oxygen as axial ligands, and we investigate the effect of the distal histidine in the structure and dynamics of the metal-oxygen unit (MeO(2), Me = Fe, Co). We find that the interaction between the distal histidine and the oxygen ligand is stronger for the cobalt complex than for the iron one, consistent with the superoxide ion character of the bound O(2). The dynamics of the O(2) ligand can be described as oscillations of the O-O axis projection on the porphyrin plane within a porphyrin quadrant combined with frequent jumps from one quadrant to another. However, the ligand motion is significantly faster for CoO(2) compared to FeO(2). As a result, the iron complex shows localized ligand sites, whereas for cobalt several configurations are possible. This gives support to the highly dynamic motion of the oxygen ligand found in several experiments on cobalt oxymyoglobin and model complexes and underlines the higher mobility of the CoO(2) fragment compared to FeO(2).

Modeling heme proteins using atomistic simulations

Heme proteins are found in all living organisms, and perform a wide variety of tasks ranging from electron transport, to the oxidation of organic compounds, to the sensing and transport of small molecules. In this work we review the application of classical and quantum-mechanical atomistic simulation tools to the investigation of several relevant issues in heme proteins chemistry: (i) conformational analysis, ligand migration, and solvation effects studied using classical molecular dynamics simulations; (ii) electronic structure and spin state energetics of the active sites explored using quantum-mechanics (QM) methods; (iii) the interaction of heme proteins with small ligands studied through hybrid quantum mechanics-molecular mechanics (QM-MM) techniques; (iv) and finally chemical reactivity and catalysis tackled by a combination of quantum and classical tools.

Laser control of vibrational excitation in carboxyhemoglobin: A quantum wave packet study

A coherent control algorithm is applied to obtain complex-shaped infrared laser pulses for the selective vibrational excitation of carbon monoxide at the active site of carbonmonoxyhemoglobin, modeled by the six-coordinated iron-porphyrin-imidazole-CO complex. The influence of the distal histidine is taken into account by an additional imidazole molecule. Density-functional theory is employed to calculate a multidimensional ground-state potential energy surface, and the vibrational dynamics as well as the laser interaction is described by quantum wave-packet calculations. At each instant in time, the optimal electric field is calculated and used for the subsequent quantum dynamics. The results presented show that the control scheme is applicable to complex systems and that it yields laser pulses with complex time-frequency structures, which, nevertheless, have a clear physical interpretation.

Quantitative vibrational dynamics of iron in nitrosyl porphyrins

We use quantitative experimental and theoretical approaches to characterize the vibrational dynamics of the Fe atom in porphyrins designed to model heme protein active sites. Nuclear resonance vibrational spectroscopy (NRVS) yields frequencies, amplitudes, and directions for 57Fe vibrations in a series of ferrous nitrosyl porphyrins, which provide a benchmark for evaluation of quantum chemical vibrational calculations. Detailed normal mode predictions result from DFT calculations on ferrous nitrosyl tetraphenylporphyrin Fe(TPP)(NO), its cation [Fe(TPP)(NO)]+, and ferrous nitrosyl porphine Fe(P)(NO). Differing functionals lead to significant variability in the predicted Fe-NO bond length and frequency for Fe(TPP)(NO). Otherwise, quantitative comparison of calculated and measured Fe dynamics on an absolute scale reveals good overall agreement, suggesting that DFT calculations provide a reliable guide to the character of observed Fe vibrational modes. These include a series of modes involving Fe motion in the plane of the porphyrin, which are rarely identified using infrared and Raman spectroscopies. The NO binding geometry breaks the four-fold symmetry of the Fe environment, and the resulting frequency splittings of the in-plane modes predicted for Fe(TPP)(NO) agree with observations. In contrast to expectations of a simple three-body model, mode energy remains localized on the FeNO fragment for only two modes, an N-O stretch and a mode with mixed Fe-NO stretch and FeNO bend character. Bending of the FeNO unit also contributes to several of the in-plane modes, but no primary FeNO bending mode is identified for Fe(TPP)(NO). Vibrations associated with hindered rotation of the NO and heme doming are predicted at low frequencies, where Fe motion perpendicular to the heme is identified experimentally at 73 and 128 cm-1. Identification of the latter two modes is a crucial first step toward quantifying the reactive energetics of Fe porphyrins and heme proteins.

On the role of the axial ligand in heme proteins: a theoretical study

We present a systematic investigation of how the axial ligand in heme proteins influences the geometry, electronic structure, and spin states of the active site, and the energies of the reaction cycles. Using the density functional B3LYP method and medium-sized basis sets, we have compared models with His, His+Asp, Cys, Tyr, and Tyr+Arg as found in myoglobin and hemoglobin, peroxidases, cytochrome P450, and heme catalases, respectively. We have studied 12 reactants and intermediates of the reaction cycles of these enzymes, including complexes with H(2)O, OH(-), O(2-), CH(3)OH, O(2), H(2)O(2), and HO(2)(-) in various formal oxidation states of the iron ion (II to V). The results show that His gives ~0.6 V higher reduction potentials than the other ligands. In particular, it is harder to reduce and protonate the O(2) complex with His than with the other ligands, in accordance with the O(2) carrier function of globins and the oxidative chemistry of the other proteins. For most properties, the trend Cys<Tyr<Tyr+Arg<His+Asp<His is found, reflecting the donor capacity of the various ligands. Thus, it is easier to reduce compound I with a His+Asp ligand than with a Cys ligand, in accordance with the one-electron chemistry of peroxidases and the hydroxylation reactions of cytochromes P450. However, the Tyr complexes have an unusually low affinity for all neutral ligands, giving them a slightly enhanced driving force in the oxidation of H(2)O(2) by compound I.

Protonation state of the equatorial ligands and dynamics of the OH...O units in a cobaloxime biomimetic

The protonation state of the dimethylglyoxime ligands in the B(12) coenzyme biomimetic [Co(CCl=CHCl)(dmgH)(2)(py)].CHCl(3) was investigated by using first-principles molecular dynamics. Our simulations at 173 and 300 K reveal that one of the oxime protons remains bonded to a nitroxyl group, while the proton of the second NO...H-ON unit is essentially shared with similar probability between the two oxygen atoms. This reconciles the results of the experimental determination (Jones, P. G.; Yang, L.; Steinborn, D. Acta Cryst. 1996, C52, 2399), showing all N-O distances as equivalent, with the commonly accepted rule that the protonation state of the dimethylglyoxime ligands can be identified by the different N-O distances. Further aspects of the dynamics of the OH...O units, in relation to the occurrence of weak CH...O intermolecular interactions, are analyzed.

Ab initio molecular dynamics with density functional theory

Recent applications of density functional theory base ab initio molecular dynamics in chemical relevant systems are reviewed. The emphasis is on the dynamical aspect in the study of structures, reaction mechanisms, and electronic properties in both the molecular and condensed phases. Examples were chosen from fluxional molecules, solution reactions, and biological systems to illustrate the broad potential applications and unique information that can be obtained from ab initio molecular dynamics calculations. Recent advances in the development of efficient numerical algorithms for the prediction of spectroscopic properties are highlighted.

Heme-based sensors: theoretical modeling of heme-ligand-protein interactions

Heme proteins are uniquely adapted to bind the important diatomic molecules O(2), NO and CO. An increasing number of heme proteins are being discovered that sense these molecules and thereby regulate a variety of biochemical responses. The interactions of diatomic molecules with heme, and with the surrounding protein, are therefore of great interest. Recent theoretical modeling, using density functional theory, captures many features of these interactions, as exemplified by the well-characterized heme protein myoglobin. Important details, however, especially the mutual influence of the bound diatomic molecule and the proximal ligand, remain to be clarified.

Influence of the heme pocket conformation on the structure and vibrations of the Fe-CO bond in myoglobin: a QM/MM density functional study

The influence of the distal pocket conformation on the structure and vibrations of the heme-CO bond in carbonmonoxy myoglobin (MbCO) is investigated by means of hybrid QM/MM calculations based on density functional theory combined with a classical force field. It is shown that the heme-CO structure (QM treated) is quite rigid and not influenced by the distal pocket conformation (MM treated). This excludes any relation between FeCO distortions and the different CO absorptions observed in the infrared spectra of MbCO (A states). In contrast, both the CO stretch frequency and the strength of the CO...His64 interaction are very dependent on the orientation and tautomerization state of His64. Our calculations indicate that the CO...N(epsilon) type of approach does not contribute to the A states, whereas the CO...H-N(epsilon) interaction is the origin of the A(1) and A(3) states, the His64 residue being protonated at N(epsilon). The strength of the CO...His64 interaction is quantified, in comparison with the analogous O(2)...His64 interaction and with the observed changes in the CO stretch frequency. Additional aspects of the CO...His64 interaction and its biological implications are discussed.

Structural and electronic properties of Co-corrole, Co-corrin, and Co-porphyrin

A quantitative study of the structure and electronic properties of Co-corrole, Co-corrin, and Co-porphyrin, using density functional theory, is reported. The structure of each macrocycle is optimized, with no symmetry constraints, by considering different spin states. The ground-state structures and spin states (S = 1 for Co-corrole, S = 0 for Co-corrin and S = 1/2 for Co-porphyrin) are in good agreement with the experimental data available. The trends in the sizes of the coordination cavities upon varying the inner metal atom and/or the macrocycle are analyzed and compared with those for the Fe-porphyrin we studied previously. Our results reveal that most of the distortion of the Co-corrin core in the B12 coenzyme is due to the inherent properties of Co-corrin. Quite different behaviors are found between corrinoids and porphyrins upon varying the spin state. While an increase in the metal-nitrogen (M-N) distance with spin state occurs in the porphyrins, the corrinoids show little variation in the M-N distance and, in some cases, undergo small structural changes in the ring structure. These results aid in understanding the often discussed question of why nature has chosen corrin/porphyrin for carrying out the particular biological functions identified in the B12 coenzyme.