Ferric superoxide and ferric hydroxide are used in the catalytic mechanism of hydroxyethylphosphonate dioxygenase: a density functional theory investigation

Hydroxyethylphosphonate dioxygenase (HEPD) is a mononuclear nonheme iron enzyme that utilizes an O(2) molecule to cleave a C-C bond in 2-hydroxyethylphosphonate and produce hydroxymethylphosphonate (HMP) and formic acid. Density functional theory calculations were performed on an enzyme active-site model of HEPD to understand its catalytic mechanism. The reaction starts with H-abstraction from the C2 position of 2-HEP by a ferric superoxide-type (Fe(III)-OO(?-)) intermediate, in a similar manner to the H-abstraction in the reaction of the dinuclear iron enzyme myo-inositol oxygenase. The resultant Fe(II)-OOH intermediate may follow either a hydroperoxylation or hydroxylation pathway, the former process being energetically more favorable. In the hydroperoxylation pathway, a ferrous-alkylhydroperoxo intermediate is formed, and then its O-O bond is homolytically cleaved to yield a complex of ferric hydroxide with a gem-diol radical. Subsequent C-C bond cleavage within the gem-diol leads to formation of an R-CH(2)(?) species and one of the two products (i.e., formic acid). The R-CH(2)(?) then intramolecularly forms a C-O bond with the ferric hydroxide to provide the other product, HMP. The overall reaction pathway does not require the use of a high-valent ferryl intermediate but does require ferric superoxide and ferric hydroxide intermediates.     

Ab initio studies on Al(+)(H(2)O)(n), HAlOH(+)(H(2)O)(n-1), and the size-dependent H(2) elimination reaction

We report computational studies on Al(+)(H(2)O)(n), and HAlOH(+)(H(2)O)(n-1), n = 6-14, by the density functional theory based ab initio molecular dynamics method, employing a planewave basis set with pseudopotentials, and also by conventional methods with Gaussian basis sets. The mechanism for the intracluster H(2) elimination reaction is explored. First, a new size-dependent insertion reaction for the transformation of Al(+)(H(2)O)(n), into HAlOH(+)(H(2)O)(n-1) is discovered for n > or = 8. This is because of the presence of a fairly stable six-water-ring structure in Al(+)(H(2)O)(n) with 12 members, including the Al(+). This structure promotes acidic dissociation and, for n > or = 8, leads to the insertion reaction. Gaussian based BPW91 and MP2 calculations with 6-31G* and 6-31G** basis sets confirmed the existence of such structures and located the transition structures for the insertion reaction. The calculated transition barrier is 10.0 kcal/mol for n = 9 and 7.1 kcal/mol for n = 8 at the MP2/6-31G** level, with zero-point energy corrections. Second, the experimentally observed size-dependent H(2) elimination reaction is related to the conformation of HAlOH(+)(H(2)O)(n-1), instead of Al(+)(H(2)O)(n). As n increases from 6 to 14, the structure of the HAlOH(+)(H(2)O)(n-1) cluster changes into a caged structure, with the Al-H bond buried inside, and protons produced in acidic dissociation could then travel through the H(2)O network to the vicinity of the Al-H bond and react with the hydride H to produce H(2). The structural transformation is completed at n = 13, coincident approximately with the onset of the H(2) elimination reaction. From constrained ab initio MD simulations, we estimated the free energy barrier for the H(2) elimination reaction to be 0.7 eV (16 kcal/mol) at n = 13, 1.5 eV (35 kcal/mol) at n = 12, and 4.5 eV (100 kcal/mol) at n = 8. The existence of transition structures for the H(2) elimination has also been verified by ab initio calculations at the MP2/6-31G** level. Finally, the switch-off of the H(2) elimination for n > 24 is explored and attributed to the diffusion of protons through enlarged hydrogen bonded H(2)O networks, which reduces the probability of finding a proton near the Al-H bond.     

How Chorismatases Regulate Distinct Reaction Channels in a Single Conserved Active Pocket: Mechanistic Analysis with QM/MM (ONIOM) Investigations

The FkbO and Hyg5 subfamilies of chorismatases share the same active-site architectures, but perform distinct reaction mechanisms, that is, FkbO employs a hydrolysis reaction whereas Hyg5 proceeds through an intramolecular mechanism. Despite extensive research efforts, the detailed mechanism of the product selectivity in chorismatases need to be further unmasked. In this study, the effects of the A/G residue group (A244^FkbO/G240^Hyg5) and the V/Q residue group (V209^FkbO/Q201^Hyg5) on the catalytic mechanisms are investigated by employing molecular dynamics simulations and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations of the two wild-type models (FkbO/CHO and Hyg5/CHO; CHO=chorismate) and four mutants models (A244G-FkbO/CHO and G240A-Hyg5/CHO; V209Q-FkbO/CHO and Q201V-Hyg5/CHO). Our results showed that the A/G residue group mentioned by previous works would cause changes in the binding states of the substrate and the orientation of the catalytic glutamate, but only these changes affect the product selectivity in chorismatases limitedly. Interestingly, the distal V/Q residue group, which determines the internal water self-regulating ability at the active site, has significant impact on the selectivity of the catalytic mechanisms. The V/Q residue group is suggested to be an important factor to control the catalytic activities in chorismatases. The results are consistent with biochemical and structural experiments, providing novel insight into the mechanism of product selectivity in chorismatases.     

A quantum mechanical investigation of possible mechanisms for the nucleotidyl transfer reaction catalyzed by DNA polymerase beta

Several quantum mechanical (QM) and hybrid quantum/molecular mechanical (QM/MM) studies have been employed recently to analyze the nucleotidyl transfer reaction in DNA polymerase beta (pol beta). Our examination reveals strong dependence of the reported mechanism on the initial molecular model. Thus, we explore here several model systems by QM methods to investigate pol beta's possible pathway variations. Although our most favorable pathway involves a direct proton transfer from O3'(primer) to O2alpha(Palpha), we also discuss other initial proton-transfer steps--to an adjacent water, to triphosphate, or to aspartic units--and the stabilizing effect of crystallographic water molecules in the active site. Our favored reaction route has an energetically undemanding initial step of less than 1.0 kcal/mol (at the B3LYP/6-31G(d,p) level), and involves a slight rearrangement in the geometry of the active site. This is followed by two major steps: (1) direct proton transfer from O3'(primer) to O2alpha(Palpha) leading to the formation of a pentavalent, trigonal bipyramidal Palpha center, via an associative mechanism, at a cost of about 28 kcal/mol, and (2) breakage of the triphosphate unit (exothermic process, approximately 22 kcal/mol) that results in the full transfer of the nucleotide to the DNA and the formation of pyrophosphate. These energy values are expected to be lower in the physical system when full protein effects are incorporated. We also discuss variations from this dominant pathway, and their impact on the overall repair process. Our calculated barrier for the chemical reaction clearly indicates that chemistry is rate-limiting overall for correct nucleotide insertion in pol beta, in accord with other studies. Protonation studies on relevant intermediates suggest that, although protonation at a single aspartic residue may occur, the addition of a second proton to the system significantly disturbs the active site. We conclude that the active site rearrangement step necessary to attain a reaction-competent geometry is essential and closely related to the "pre-chemistry" avenue described recently as a key step in the overall kinetic cycle of DNA polymerases. Thus, our work emphasizes the many possible ways for DNA polymerase beta's chemical reaction to occur, determined by the active site environment and initial models.     

Ab initio study of the atmospheric oxidation of CS2.

The reactions of OH with CS2, OCS, and 3SO and of 3O2 with CS2, SCSOH, and HOSO have been studied by optimizing minima and transition states with B3LYP/6-31+G(d) and carrying out higher-level ab initio calculations on fixed geometries. The combined calculations provide valuable insight into the mechanism for the atmospheric oxidation of CS2. The initial step is the formation of the SCSOH complex (1) which readily adds molecular oxygen to form the SC(OO)SOH complex (8). A key step is the oxygen atom transfer to the sulfur bearing the hydroxyl group which leads directly to OCS plus HOSO. The HOSO + 3O2 reaction has a near zero calculated activation barrier so generation of O2H + SO2 should proceed readily in the atmosphere.     

Quantum mechanical/molecular mechanical study of mechanisms of heme degradation by the enzyme heme oxygenase: the strategic function of the water cluster

Heme degradation by heme oxygenase (HO) enzymes is important in maintaining iron homeostasis and prevention of oxidative stress, etc. In response to mechanistic uncertainties, we performed quantum mechanical/molecular mechanical investigations of the heme hydroxylation by HO, in the native route and with the oxygen surrogate donor H2O2. It is demonstrated that H2O2 cannot be deprotonated to yield Fe(III)OOH, and hence the surrogate reaction starts from the FeHOOH complex. The calculations show that, when starting from either Fe(III)OOH or Fe(III)HOOH, the fully concerted mechanism involving O-O bond breakage and O-C(meso) bond formation is highly disfavored. The low-energy mechanism involves a nonsynchronous, effectively concerted pathway, in which the active species undergoes first O-O bond homolysis followed by a barrier-free (small with Fe(III)HOOH) hydroxyl radical attack on the meso position of the porphyrin. During the reaction of Fe(III)HOOH, formation of the Por+*FeIV=O species, compound I, competes with heme hydroxylation, thereby reducing the efficiency of the surrogate route. All these conclusions are in accord with experimental findings (Chu, G. C.; Katakura, K.; Zhang, X.; Yoshida, T.; Ikeda-Saito, M. J. Biol. Chem. 1999, 274, 21319). The study highlights the role of the water cluster in the distal pocket in creating "function" for the enzyme; this cluster affects the O-O cleavage and the O-Cmeso formation, but more so it is responsible for the orientation of the hydroxyl radical and for the observed alpha-meso regioselectivity of hydroxylation (Ortiz de Montellano, P. R. Acc. Chem. Res. 1998, 31, 543). Differences/similarities with P450 and HRP are discussed.     

The fragmentation-recombination mechanism of the enzyme glutamate mutase studied by QM/MM simulations

The radical mechanism of the conversion of glutamate to methylaspartate catalyzed by glutamate mutase is studied with quantum mechanical/molecular mechanical (QM/MM) simulations based on density functional theory (DFT/MM). The hydrogen transfer between the substrate and the cofactor is found to be rate limiting with a barrier of 101.1 kJ mol(-1). A careful comparison to the uncatalyzed reaction in water is performed. The protein influences the reaction predominantly electrostatically and to a lesser degree sterically. Our calculations shed light on the atomistic details of the reaction mechanism. The well-known arginine claw and Glu 171 ( Clostridium cochlearium notation) are found to have the strongest influence on the reaction. However, a catalytic role of Glu 214, Lys 322, Gln 147, Glu 330, Lys 326, and Met 294 is found as well. The arginine claw keeps the intermediates in place and is probably responsible for the enantioselectivity. Glu 171 temporarily accepts a proton from the glutamyl radical intermediate and donates it back at the end of the reaction. We relate our results to experimental data when available. Our simulations lead to further understanding of how glutamate mutase catalyzes the carbon skeleton rearrangement of glutamate.     

Pyrazolate-based cobalt(II)-containing metal-organic frameworks in heterogeneous catalytic oxidation reactions: elucidating the role of entatic states for biomimetic oxidation processes

Crystal structures of two metal-organic frameworks (MFU-1 and MFU-2) are presented, both of which contain redox-active Co(II) centres coordinated by linear 1,4-bis[(3,5-dimethyl)pyrazol-4-yl] ligands. In contrast to many MOFs reported previously, these compounds show excellent stability against hydrolytic decomposition. Catalytic turnover is achieved in oxidation reactions by employing tert-butyl hydroperoxide and the solid catalysts are easily recovered from the reaction mixture. Whereas heterogeneous catalysis is unambiguously demonstrated for MFU-1, MFU-2 shows catalytic activity due to slow metal leaching, emphasising the need for a deeper understanding of structure-reactivity relationships in the future design of redox-active metal-organic frameworks. Mechanistic details for oxidation reactions employing tert-butyl hydroperoxide are studied by UV/Vis and IR spectroscopy and XRPD measurements. The catalytic process accompanying changes of redox states and structural changes were investigated by means of cobalt K-edge X-ray absorption spectroscopy. To probe the putative binding modes of molecular oxygen, the isosteric heats of adsorption of O(2) were determined and compared with models from DFT calculations. The stabilities of the frameworks in an oxygen atmosphere as a reactive gas were examined by temperature-programmed oxidation (TPO). Solution impregnation of MFU-1 with a co-catalyst (N-hydroxyphthalimide) led to NHPI@MFU-1, which oxidised a range of organic substrates under ambient conditions by employing molecular oxygen from air. The catalytic reaction involved a biomimetic reaction cascade based on free radicals. The concept of an entatic state of the cobalt centres is proposed and its relevance for sustained catalytic activity is briefly discussed.     

基于ReaxFF力场的对硝基苯酚臭氧氧化分子动力学模拟

本文采用基于ReaxFF反应力场的分子动力学方法(ReaxFF MD),利用自主研发的国际首个基于GPU加速的ReaxFF MD程序系统GMD-Reax和独特的化学反应分析工具VARx MD,探索臭氧氧化对硝基苯酚的反应机理。通过模拟考察了300 K恒温条件下臭氧氧化水中对硝基苯酚的行为,获得了酚结构开环、CO_2生成、主要自由基(·OH、·O_2、·O)及团簇型自由基的数量演变趋势,并可定性描述六元环开环和CO_2生成均遵循伪一级反应动力学规律。反应机理分析表明酚类分子在水溶液中被臭氧氧化的路径主要经过攫氢、六元环开环、碳链的氧化分解三个阶段,也揭示了自由基和团簇型自由基在臭氧降解对硝基苯酚时所发挥的重要作用。本工作是应用ReaxFF MD分子模拟方法对常温水环境下臭氧降解酚类污染物反应机理研究的一个尝试,可为深入认识该机理及相关的实验、理论研究提供一定的参考。     

Oxidation of methionine residues in aqueous solutions: free methionine and methionine in granulocyte colony-stimulating factor

The free energy barriers and a mechanism of the oxidation of the amino acid methionine in water and in granulocyte colony-stimulating factor (G-CSF) are analyzed via combined quantum mechanical and molecular mechanical (QM/MM) methods, constrained molecular dynamics, and committor probability calculations. The computed free energy barrier of free methionine amino acid is very close to the measured value (14.7 +/- 1.2 versus 15.5 +/- 0.02 kcal/mol). The reaction coordinate was found to be the difference between the O-O bond of H2O2 and the S-O bond, where the S is the sulfur atom of the methionine residue. It was confirmed by computing the committor probability distribution and the distribution of constrained forces that this coordinate is not coupled to the activation of other degrees of freedom. The computed free energies of the oxidation of methionine residues in G-CSF indicate that the protein environment has insignificant effects on the reaction barriers of oxidation. This result further validates our proposal that the access of solvent to methionine sites, as measured by the two-shell water coordination number, governs the kinetics of the oxidation reaction of methionine groups in a protein molecule. We also found that the number of hydrogen bonds between the distal oxygen of H2O2 and the water molecules near the methionine increases along the reaction coordinate as oxidation progresses, indicating that the charge separation developed during the oxidation by H2O2 is stabilized by specific interactions with water molecules, such as hydrogen bonding.     

Balancing kinetic and thermodynamic control: the mechanism of carbocation cyclization by squalene cyclase

Molecular dynamics simulations with a combined quantum mechanical and molecular mechanical (QM/MM) potential have been carried out to investigate the squalene-to-hopene carbocation cyclization mechanism in squalene-hopene cyclase (SHC). The present study is based on free energy simulations by constructing the free energy surface for the cyclization steps along the reaction pathway. The picture that emerges for the carbocation cyclization cascade is a delicate balance of thermodynamic and kinetic control that ultimately favors the formation of the final hopanoids carbon skeleton. A key finding is that the five- to six-membered ring expansion process is not a viable reaction pathway for either C- or D-ring formation in the cyclization reaction. The only significant intermediate is the A/B-bicyclic cyclohexyl cation (III), from which two asynchronous concerted reaction pathways lead to, respectively, the 6,6,6,5-tetracyclic carbon skeleton and the 6,6,6,6,5-pentacyclic hopanoids. Experimentally, these two products are observed to have 1% and 99% yields, respectively, in the wild-type enzyme. We conclude that the product distribution in the wild-type enzyme is dictated by kinetic control of these two reaction pathways.     

Adapting the nudged elastic band method for determining minimum-energy paths of chemical reactions in enzymes

Optimization of reaction paths for enzymatic systems is a challenging problem because such systems have a very large number of degrees of freedom and many of these degrees are flexible. To meet this challenge, an efficient, robust and general approach is presented based on the well-known nudged elastic band reaction path optimization method with the following extensions: (1) soft spectator degrees of freedom are excluded from path definitions by using only inter-atomic distances corresponding to forming/breaking bonds in a reaction; (2) a general transformation of the distances is defined to treat multistep reactions without knowing the partitioning of steps in advance; (3) a multistage strategy, in which path optimizations are carried out for reference systems with gradually decreasing rigidity, is developed to maximize the opportunity of obtaining continuously changing environments along the path. We demonstrate the applicability of the approach using the acylation reaction of type A beta-lactamase as an example. The reaction mechanism investigated involves four elementary reaction steps, eight forming/breaking bonds. We obtained a continuous minimum energy path without any assumption on reaction coordinates, or on the possible sequence or the concertedness of chemical events. We expect our approach to have general applicability in the modeling of enzymatic reactions with quantum mechanical/molecular mechanical models.     

Quantum mechanical/molecular mechanical simulation study of the mechanism of hairpin ribozyme catalysis

The molecular mechanism of hairpin ribozyme catalysis is studied with molecular dynamics simulations using a combined quantum mechanical and molecular mechanical (QM/MM) potential with a recently developed semiempirical AM1/d-PhoT model for phosphoryl transfer reactions. Simulations are used to derive one- and two-dimensional potentials of mean force to examine specific reaction paths and assess the feasibility of proposed general acid and base mechanisms. Density-functional calculations of truncated active site models provide complementary insight to the simulation results. Key factors utilized by the hairpin ribozyme to enhance the rate of transphosphorylation are presented, and the roles of A38 and G8 as general acid and base catalysts are discussed. The computational results are consistent with available experimental data, provide support for a general acid/base mechanism played by functional groups on the nucleobases, and offer important insight into the ability of RNA to act as a catalyst without explicit participation by divalent metal ions.     

Catalytic mechanism and product specificity of the histone lysine methyltransferase SET7/9: an ab initio QM/MM-FE study with multiple initial structures

Histone lysine methylation is emerging as an important mechanism to regulate chromatin structure and gene activity. To provide theoretical understanding of its reaction mechanism and product specificity, ab initio quantum mechanical/molecular mechanical free energy (QM/MM-FE) calculations and molecular dynamics simulations have been carried out to investigate the histone lysine methyltransferase SET7/9. It is found that the methyl-transfer reaction catalyzed by SET7/9 is a typical in-line S(N)2 nucleophilic substitution reaction with a transition state of 70% dissociative character. The calculated average free energy barrier at the MP2(6-31+G) QM/MM level is 20.4 +/- 1.1 kcal/mol, consistent with the activation barrier of 20.9 kcal/mol estimated from the experimental reaction rate. The barrier fluctuation has a strong correlation with the nucleophilic attack distance and angle in the reactant complex. The calculation results show that the product specificity of SET7/9 as a monomethyltransferase is achieved by disrupting the formation of near-attack conformations for the dimethylation reaction.     

Hydrogenation of CO on a silica surface: an embedded cluster approach

The sequential addition of H atoms to CO adsorbed on a siliceous edingtonite surface is studied with an embedded cluster approach, using density functional theory for the quantum mechanical (QM) cluster and a molecular force field for the molecular mechanical (MM) cluster. With this setup, calculated QM/MM adsorption energies are in agreement with previous calculations employing periodic boundary conditions. The catalytic effect of the siliceous edingtonite (100) surface on CO hydrogenation is assessed because of its relevance to astrochemistry. While adsorption of CO on a silanol group on the hydroxylated surface did not reduce the activation energy for the reaction with a H atom, a negatively charged defect on the surface is found to reduce the gas phase barriers for the hydrogenation of both CO and H2C=O. The embedded cluster approach is shown to be a useful and flexible tool for studying reactions on (semi-)ionic surfaces and specific defects thereon. The methodology presented here could easily be applied to study reactions on silica surfaces that are of relevance to other scientific areas, such as biotoxicity of silica dust and geochemistry.     

Photochemically driven shape changes of crystalline organic nanorods

Nanorods composed of 9-tert-butylanthroate (9-TBAE) are synthesized using an Al2O3 template and solvent annealing. The rods consist of micron-scale crystalline domains, and UV light induces a [4 + 4] photodimerization that results in a uniform 15% expansion along the rod axis. This is in contrast to random 9-TBAE crystals, which disintegrate under the same conditions. Transmission electron microscopy, atomic force microscopy, and comparison of the X-ray crystal structures of the monomer and photodimer all provide evidence for a mechanism based on a crystal-to-crystal photoreaction leading to an increase in molecular volume. It is likely that the high surface-to-volume ratio in the nanorods provides a strain relief pathway that is absent in larger crystals. Preliminary attempts to reverse the reaction using shorter wavelength light to photodissociate the dimers were only partly successful. These results suggest that crystalline organic nanostructures may provide an efficient way to transform photochemical energy into mechanical motion on the nanometer scale.     

Computational analysis of the proton translocation from Asp96 to schiff base in bacteriorhodopsin

The potential energy change during the M --> N process in bacteriorhodopsin has been evaluated by ab initio quantum chemical and advanced quantum chemical calculations following molecular dynamics (MD) simulations. Many previous experimental studies have suggested that the proton transfer from Asp96 to the Schiff base occurs under the following two conditions: (1) the hydrogen bond between Thr46 and Asp96 breaks and Thr46 is detached from Asp96 and (2) a stable chain of four water molecules spans an area from Asp96 --> Schiff base. In this work, we successfully reproduced the proton-transfer process occurring under these two conditions by molecular dynamics and quantum chemical calculations. The quantum chemical computation revealed that the proton transfer from Asp96 to Shiff base occurs in two-step reactions via an intermediate in which an H(3)O(+) appears around Ala215. The activation energy for the proton transfer in the first reaction was calculated to be 9.7 kcal/mol, which enables fast and efficient proton pump action. Further QM/MM (quantum mechanical/molecular mechanical) and FMO (fragment molecular orbital) calculations revealed that the potential energy change during the proton transfer is tightly regulated by the composition and the geometry of the surrounding amino acid residues of bacteriorhodopsin. Here, we report in detail the Asp96 --> Schiff base proton translocation mechanism of bacteriorhodopsin. Additionally, we discuss the effectiveness of combining quantum chemical calculations with truncated cluster models followed by advanced quantum chemical calculations applied to a whole protein to elucidate its reaction mechanism.     

乙烯酮自由基(HCCO·)与氧气(O2)反应势能面的理论研究

在G2(B3LYP/MP2/CC)水平上对反应HCCO+O2进行了计算,得到了反应势能面,提出了3种可能的反应机理:(1)四元环反应机理得到产物P1(HCO+CO2);(2)三元环反应机理得到产物P2(CO+HCO2);(3)O—O键断裂反应机理得到产物P3(O+OCC(O)H)和P4(O+CO+HCO).由反应势能面推测产物P1(HCO+CO2)为主要产物,产物P2(CO+HCO2),P3(O+OCC(O)H)和P4(O+CO+HCO)为次要产物.     

四氯化硅催化氢化合成三氯氢硅机理研究

针对四氯化硅催化氢化过程采用第一性原理机理对其进行模拟研究,结果表明:没有催化剂时,SiCl4与H2反应能垒为464.45 kJ/mol,反应能量为74.94 kJ/mol,与热力学计算结果 71.85 kJ/mol一致.负载在HZSM-5分子筛上的氯化钡可催化四氯化硅氢化反应,其最具催化活性表面为(111)面;H2在BaCl2(111)面上表现排斥性;SiCl4表现为吸附性,可在BaCl2(111)表面稳定吸附并生成.SiCl3自由基,过程吸附能为448.33 kJ/mol;在催化剂BaCl2存在条件下,SiCl4与H2反应为自由基反应,反应步骤能垒为400.23 kJ/mol;氢化过程能垒降为184.97kJ/mol;催化氢化反应过程所需能量为64.20 kJ/mol.催化氢化过程反应条件相对无催化剂过程更为温和.     

Experimental and theoretical study of the structure and reactivity of Fe1-2O< or =6- clusters with CO

Synergistic studies employing experiments in the gas phase and theoretical first principles calculations have been carried out to investigate the structure, stability, and reactivity toward CO of iron oxide cluster anions, Fe(x)O(y)- (x = 1-2, y < or = 6). Collision-induced dissociation studies of iron oxide species, employing xenon collision gas, show that FeO3- and FeO2- are the stable building blocks of the larger iron oxide clusters. Theoretical calculations show that the fragmentation patterns leading to the production of O or FeO(n) fragments are governed both by the energetics of the overall process as well as the number of intermediate states and the changes in spin multiplicity. Mass-selected experiments identified oxygen atom transfer to CO as the dominant reaction pathway for most anionic iron oxide clusters. A theoretical analysis of the molecular level pathways has been carried out to highlight the role of energetics as well as the spin states of the intermediates on the oxidation reaction.