Electronic structure and bonding in unligated and ligated FeII porphyrins

Liao, Meng‐Sheng; Scheiner, Steve

doi:10.1063/1.1447902

Cited by 90 publications

(126 citation statements)

References 51 publications

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“…[8,12,13] The relevance of the calculated structures of 3-5 to the corresponding (1-Fe) 2 derivatives is supported by [8] 1) our calculations accurately reproducing the electronic ground states and metric parameters of relevant five-and six-coordinate hemes from the literature and 2) the weak sensitivity of these results to the presence of the peripheral phenyl groups, in accordance with the literature data. [14] The computed structures (Figure 2) are consistent with the effective C 2h symmetry of the dimer/ligand adducts observed by NMR spectroscopy. The FeÀN Im distances, the displacement of the Fe atoms from the porphyrin planes, and the residual doming of the hemes in 3-5 resemble those in corresponding monomeric hemes with a single 2-MeIm ligand.…”

supporting

confidence: 81%

Simple Heme Dimers with Strongly Cooperative Ligand Binding

2007

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We report that the simple stable heme dimers (1-Fe) 2 and (2-Fe) 2 display strongly cooperative ligand binding. For example, (1-Fe) 2 binds a second isocyanide molecule with an approximately 50-fold higher affinity than the first, or with an allosteric interaction energy of DG a = 13 AE 3 kJ mol À1 . The degree of cooperativity is comparable to that of hemoglobin, a prototypical cooperative receptor for small molecules, whereas the mechanism appears to be distinct.With these results, we aim to help address the unmet need for simple, robust, and easily accessible synthetic receptors that bind small molecules (O 2 , NO, CO) cooperatively. Such receptors would be of great value for a number of applications (noncryogenic air separation, [1] chemical O 2 delivery/storage, [2] chemical sensors [3]

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confidence: 81%

Simple Heme Dimers with Strongly Cooperative Ligand Binding

2007

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“…Among these, the B3LYP density functional predicts very closelying quintet and triplet states in deoxyheme models, sometimes with a triplet ground state (14). Recently, DFT was used to compute the electronic spectrum of Fe II -porphine with 2-methylimidazole as the axial ligand, making the quintet state the lowest in energy, but the triplet state only 12 kJ/mol higher (15), in excellent agreement with the experiment. These circumstances indicate that the treatment of spin states in porphyrins is delicate because of the closeness in energy of various spin states.…”

mentioning

confidence: 50%

How O2 Binds to Heme

Jensen

Ryde

2004

Journal of Biological Chemistry

187

189

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We have used density functional methods to calculate fully relaxed potential energy curves of the seven lowest electronic states during the binding of O 2 to a realistic model of ferrous deoxyheme. Beyond a Fe-O distance of ϳ2.5 Å, we find a broad crossing region with five electronic states within 15 kJ/mol. The almost parallel surfaces strongly facilitate spin inversion, which is necessary in the reaction of O 2 with heme (deoxyheme is a quintet and O 2 a triplet, whereas oxyheme is a singlet). Thus, despite a small spin-orbit coupling in heme, the transition probability approaches unity. Using reasonable parameters, we estimate a transition probability of 0.06 -1, which is at least 15 times larger than for the nonbiological Fe-O ؉ system. Spin crossing is anticipated between the singlet ground state of bound oxyheme, the triplet and septet dissociation states, and a quintet intermediate state. The fact that the quintet state is close in energy to the dissociation couple is of biological importance, because it explains how both spin states of O 2 may bind to heme, thereby increasing the overall efficiency of oxygen binding. The activation barrier is estimated to be <15 kJ/mol based on our results and Mö ssbauer experiments. Our results indicate that both the activation energy and the spin-transition probability are tuned by the porphyrin as well as by the choice of the proximal heme ligand, which is a histidine in the globins. Together, they may accelerate O 2 binding to iron by ϳ10 11 compared with the Fe-O ؉ system. A similar near degeneracy between spin states is observed in a ferric deoxyheme model with the histidine ligand hydrogen bonded to a carboxylate group, i.e. a model of heme peroxidases, which bind H 2 O 2 in this oxidation state.All electrons have a spin, which is an intrinsic quantum chemical property that can take only two possible values, normally called ␣ and ␤ (or spin up and down). Almost all normal organic molecules contain an even number of electrons and also an equal number of ␣ and ␤ electrons. They are then said to have paired spin or to be singlets. Molecular oxygen (O 2 ) is a famous exception to this rule. In its ground state, it has two more electrons of one spin state than the other. Thus, it is said to have two unpaired electrons or to be a triplet. The singlet state of O 2 , with all electron spins paired, is ϳ90 kJ/mol higher in energy than the triplet ground state (1).A chemical reaction can normally not change the spin state of an electron. Therefore, reactions between singlet and triplet states are formally spin-forbidden, which means that they are slow. This is the reason why organic matter may exist in an atmosphere containing much O 2 . There is a strong thermodynamic drive of O 2 to oxidize organic matter to H 2 O and CO, but because these products (as well as the organic molecules) are singlets (whereas O 2 is a triplet), this reaction is spin-forbidden and therefore very slow at ambient temperatures. On the other hand, this is a problem when living organisms want to emp...

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“…[46] The performance of different exchange and correlation functionals in the modeling of heme systems was analyzed by Scherlis et al [47] Structure analysis was performed with the variational density matrix method (VMD). [48] …”

Section: Methodsmentioning

confidence: 99%

Protonated Heme

Chiavarino¹,

Crestoni²,

Fornarini³

et al. 2007

Chemistry A European J

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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.

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Electronic structure and bonding in unligated and ligated FeII porphyrins

Cited by 90 publications

References 51 publications

Simple Heme Dimers with Strongly Cooperative Ligand Binding

Simple Heme Dimers with Strongly Cooperative Ligand Binding

How O2 Binds to Heme

Protonated Heme

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