Structural models of the bimetallic subunit at the A-cluster of acetyl coenzyme a synthase/CO dehydrogenase: binuclear sulfur-bridged Ni-Cu and Ni-Ni complexes and their reactions with CO

The Ni(II)-dicarboxamido-dithiolato complexes (Et4N)2[Ni(NpPepS)] (1) and (Et4N)2[Ni(PhPepS)] (2) were used as Nid metallosynthons in the construction of higher nuclearity dinuclear Ni-Cu and Ni-Ni species to model the bimetallic Mp-Nid site of the A-cluster of acetyl coenzyme A synthase/CO dehydrogenase (ACS/CODH). Reaction of 1 with [Cu(neo)Cl] and [Ni(terpy)Cl2] in MeCN affords the dinuclear complexes (Et4N)[Cu(neo)Ni(NpPepS)] (3) and [Ni(terpy)Ni(NpPepS)] (4), respectively. Reaction of 2 with [Ni(dppe)Cl2] in MeCN yields [Ni(dppe)Ni(PhPepS)] (6). The Ni-Cu complex 3 exhibits no redox chemistry at the Nid site and no reaction with CO. In contrast, the Nip sites in 4 and 6 are readily reduced (characterized by their Ni(I) EPR spectra) and bind CO, exhibiting nuco bands at 2044 and 1997 cm-1, respectively, indicating terminal CO binding. The present Ni-Ni systems replicate the structural and chemical properties of the A-cluster site in ACS/CODH and support the presence of Ni at Mp in the catalytically active enzyme.     

Synthetic analogues of the active site of the A-cluster of acetyl coenzyme A synthase/CO dehydrogenase: syntheses, structures, and reactions with CO

Two metallosynthons, namely (Et4N)2[Ni(NpPepS)] (1) and (Et4N)2[Ni(PhPepS)] (2) containing carboxamido-N and thiolato-S as donors have been used to model the bimetallic M(p)-Ni(d) subsite of the A-cluster of the enzyme acetyl coenzyme A synthase/CO dehydrogenase. A series of sulfur-bridged Ni/Cu dinuclear and trinuclear complexes (3-10) have been synthesized to explore their redox properties and affinity of the metal centers toward CO. The structures of (Et4N)2[Ni(PhPepS)] (2), (Et4N)[Cu(neo)Ni(NpPepS)] x 0.5 Et2O x 0.5 H2O (3 x 0.5 Et2O x 0.5 H2O), (Et4N)[Cu(neo)Ni(PhPepS)] x H2O (4 x H2O), (Et4N)2[Ni{Ni(NpPepS)}2] x DMF (5 x DMF), (Et4N)2[Ni(DMF)2{Ni(NpPepS)}2] x 3 DMF (6 x 3 DMF), (Et4N)2[Ni(DMF)2{Ni(PhPepS)}2] (8), and [Ni(dppe)Ni(PhPepS)] x CH2Cl2 (10 x CH2Cl2) have been determined by crystallography. The Ni(d) mimics 1 and 2 resist reduction and exhibit no affinity toward CO. In contrast, the sulfur-bridged Ni center (designated Ni(C)) in the trinuclear models 5-8 are amenable to reduction and binds CO in the Ni(I) state. Also, the sulfur-bridged Ni(C) center can be removed from the trimers (5-8) by treatment with 1,10-phenanthroline much like the "labile Ni" from the enzyme. The dinuclear Ni-Ni models 9 and 10 resemble the Ni(p)-Ni(d) subsite of the A-cluster more closely, and only the modeled Ni(p) site of the dimers can be reduced. The Ni(I)-Ni(II) species display EPR spectra typical of a Ni(I) center in distorted trigonal bipyramidal and distorted tetrahedral geometries for 9(red) and 10(red), respectively. Both species bind CO, and the CO-adducts 9(red)-CO and 10(red)-CO display strong nu(co) at 2044 and 1997 cm(-1), respectively. The reduction of 10 is reversible. The CO-affinity of 10 in the reduced state and the nu(co) value of 10(red)-CO closely resemble the CO-bound reduced A-cluster (nu(co) = 1996 cm(-1)).     

Structural and spectroscopic models of the A-cluster of acetyl coenzyme a synthase/carbon monoxide dehydrogenase: Nature's Monsanto acetic acid catalyst

Acetyl coenzyme A synthase/carbon monoxide dehydrogenase (ACS/CODH) is a bifunctional enzyme present in a number of anaerobic bacteria. The enzyme catalyzes two separate reactions namely, the reduction of atmospheric CO₂ to CO (CODH activity at the C-cluster) and the synthesis of acetyl coenzyme A (ACS activity at the A-cluster) from CO, CH₃ from a corrinoid iron-sulfur protein, and the thiol coenzyme A. The structure(s) of the A-cluster of ACS/CODH from Moorella thermoacetica revealed an unprecedented structure with three different metallic subunits linked to each other through bridging Cys-S residues comprising the active site. In these structure(s) a Fe₄S₄ cubane is bridged via Cys-S to a bimetallic metal cluster. This bimetallic cluster contains a four-coordinate Ni, Cu, or Zn as the proximal metal (to the Fe₄S₄ cluster; designated M_p), which in turn is bridged through two Cys-S residues to a terminal square planar Ni(II) (Ni_d, distal to Fe₄S₄) ligated by two deprotonated carboxamido nitrogens from the peptide backbone. It is now established that Ni is required at the M_p site for the ACS activity. Over the past several years modeling efforts by several groups have provided clues towards understanding the intrinsic properties of the unique site in ACS. To date most studies have focused on dinuclear compounds that model the M_p-Ni_d subsite. Synthesis of such models have revealed that the Ni_p sites (a) are readily removed when mixed with 1,10-phenanthroline (phen) and (b) can be reduced to the Ni(I) and/or Ni(0) oxidation state (deduced by EPR or electrochemical studies) and bind CO in terminal fashion with ν_co value similar to the enzyme. In contrast, the presence of Cu(I) centers at these M_p sites do not bind CO and are not removable with phen supporting a non-catalytic role for Cu(I) at the M_p site in the enzyme. The Ni_d site (coordinated by carboxamido-N/thiolato-S) in these models are very stable in the +2 oxidation state and not readily removed upon treatment with phen suggesting that the source of ‘labile Ni’ and the NiFeC signal arises from the presence of Ni at the M_p site in ACS. This review includes the results and implications of the modeling studies reported so far.     

Reduction and methyl transfer kinetics of the alpha subunit from acetyl coenzyme a synthase

Stopped-flow was used to evaluate the methylation and reduction kinetics of the isolated alpha subunit of acetyl-Coenzyme A synthase from Moorella thermoacetica. This catalytically active subunit contains a novel Ni-X-Fe4S4 cluster and a putative unidentified n = 2 redox site called D. The D-site must be reduced for a methyl group to transfer from a corrinoid-iron-sulfur protein, a key step in the catalytic synthesis of acetyl-CoA. The Fe4S4 component of this cluster is also redox active, raising the possibility that it is the D-site or a portion thereof. Results presented demonstrate that the D-site reduces far faster than the Fe4S4 component, effectively eliminating this possibility. Rather, this component may alter catalytically important properties of the Ni center. The D-site is reduced through a pathway that probably does not involve the Fe4S4 component of this active-site cluster.     

Structural analogues of the bimetallic reaction center in acetyl CoA synthase: a Ni--Ni model with bound CO

Models for the active site of the acetyl CoA synthase (ACS) were synthesized by attachment of Cu+ and Ni(0) to nickel diaminodithiolate (S2N2) and diamidodithiolate (S2N2') complexes. The Ni-Ni species form stable CO adducts, i.e., [{(CO)2Ni}{NiS2N2'}]2-, whereas the Cu-NiS2N2 and Cu-NiS2N2' models do not. These results provide supporting evidence for a biological role for reduced nickel in ACS.     

Effect of Zn on acetyl coenzyme a synthase: evidence for a conformational change in the alpha subunit during catalysis

Acetyl coenzyme A synthase (ACS) is an alpha2beta2 tetramer in which the active-site A-cluster, located in the alpha subunits, consists of an Fe4S4 cubane bridged to a {Nip Nid} binuclear site. The alpha subunits exist in two conformations. In the open conformation, Nip is surface-exposed, while the proximal metal is buried in the closed conformation. Nip is labile and can be replaced by Cu. In this study, the effects of Zn are reported. ACS in which Zn replaced Nip was inactive and did not exhibit the so-called NiFeC EPR signal nor the ability to accept a methyl group from the corrinoid-iron-sulfur protein (CoFeSP). Once Zn-bound, it could not be replaced by subsequently adding Ni. The Zn-bound A-cluster cannot be reduced and bound with CO or become methylated, probably because Zn (like Cu) is insufficiently nucleophilic for these functions. Unexpectedly, Zn replaced Nip only while ACS was engaged in catalysis. Under these conditions, replacement occurred with kapp approximately 0.6 min-1. Replacement was blocked by including EDTA in the assay mix. Zn appears to replace Nip when ACS is in an intermediate state (or states) of catalysis but this(these) state(s) must not be present when ACS is reduced in CO alone, or in the presence of CoA, CoFeSP, or reduced methyl viologen. Nip appears susceptible to Zn-attack when the alpha subunit is in the open conformation and protected from attack when it is in the closed conformation. This is the first evidence that the structurally-characterized conformations of the alpha subunit change during catalysis, indicating a mechanistic role for this conformational change.     

Binuclear complexes containing a methylnickel moiety: relevance to organonickel intermediates in acetyl coenzyme A synthase catalysis

A series of binuclear NiNi complexes supported by a single thiolate bridge and containing a methylnickel moiety have been prepared and fully characterized. The complexes represent structural analogues for the proposed organonickel intermediate in the acetyl coenzyme A synthase catalytic cycle. Variable temperature 31P NMR spectroscopy was used to examine dynamic behavior of the thiolate bridging interaction in two of the derivatives. Kinetic analyses, independent exchange and crossover experiments support an intermolecular exchange mechanism. Carbonylation results in thioester formation via a reductive elimination pathway.     

The A-cluster in subunit beta of the acetyl-CoA decarbonylase/synthase complex from Methanosarcina thermophila: Ni and Fe K-edge XANES and EXAFS analyses

The acetyl-CoA decarbonylase/synthase (ACDS) complex catalyzes the cleavage of acetyl-CoA in methanogens that metabolize acetate to CO(2) and CH(4), and also carries out acetyl-CoA synthesis during growth on one-carbon substrates. The ACDS complex contains five subunits, among which beta possesses an Ni-Fe-S active-site metal cluster, the A-cluster, at which reaction with acetyl-CoA takes place, generating an acetyl-enzyme species poised for C-C bond cleavage. We have used Ni and Fe K fluorescence XANES and EXAFS analyses to characterize these metals in the ACDS beta subunit, expressed as a C-terminally shortened form. Fe XANES and EXAFS confirmed the presence of an [Fe(4)S(4)] cluster, with typical Fe-S and Fe-Fe distances of 2.3 and 2.7 A respectively. An Fe:Ni ratio of approximately 2:1 was found by Kalphabeta fluorescence analysis, indicating 2 Ni per [Fe(4)S(4)]. Ni XANES simulations were consistent with two distinct Ni sites in cluster A, and the observed spectrum could be modeled as the sum of separate square planar and tetrahedral Ni sites. Treatment of the beta subunit with Ti(3+) citrate resulted in shifts to lower energy, implying significant reduction of the [Fe(4)S(4)] center, along with conversion of a smaller fraction of Ni(II) to Ni(I). Reaction with CO in the presence of Ti(3+) citrate generated a unique Ni XANES spectrum, while effects on the Fe-edge were not very different from the reaction with Ti(3+) alone. Ni EXAFS revealed an average Ni coordination of 2.5 S at 2.19 A and 1.5 N/O at 1.89 A. A distinct feature at approximately 2.95 A most likely results from Ni-Ni interaction. The methanogen beta subunit A-cluster is proposed to consist of an [Fe(4)S(4)] cluster bridged to an Ni-Ni center with one Ni in square planar geometry coordinated by 2 S + 2 N and the other approximately tetrahedral with 3 S + 1 N/O ligands. The electronic consequences of two distinct Ni geometries are discussed.     

The tunnel of acetyl-coenzyme a synthase/carbon monoxide dehydrogenase regulates delivery of CO to the active site

The effect of [CO] on acetyl-CoA synthesis activity of the isolated alpha subunit of acetyl-coenzyme A synthase/carbon monoxide dehydrogenase from Moorella thermoacetica was determined. In contrast to the complete alpha(2)beta(2) enzyme where multiple CO molecules exhibit strong cooperative inhibition, alpha was weakly inhibited, apparently by a single CO with K(I) = 1.5 +/- 0.5 mM; other parameters include k(cat) = 11 +/- 1 min(-)(1) and K(M) = 30 +/- 10 microM. The alpha subunit lacked the previously described "majority" activity of the complete enzyme but possessed its "residual" activity. The site affording cooperative inhibition may be absent or inoperative in isolated alpha subunits. Ni-activated alpha rapidly and reversibly accepted a methyl group from CH(3)-Co(3+)FeSP affording the equilibrium constant K(MT) = 10 +/- 4, demonstrating the superior nucleophilicity of alpha(red) relative to Co(1+)FeSP. CO inhibited this reaction weakly (K(I) = 540 +/- 190 microM). NiFeC EPR intensity of alpha developed in accordance with an apparent K(d) = 30 microM, suggesting that the state exhibiting this signal is not responsible for inhibiting catalysis or methyl group transfer and that it may be a catalytic intermediate. At higher [CO], signal intensity declined slightly. Attenuation of catalysis, methyl group transfer, and the NiFeC signal might reflect the same weak CO binding process. Three mutant alpha(2)beta(2) proteins designed to block the tunnel between the A- and C-clusters exhibited little/no activity with CO(2) as a substrate and no evidence of cooperative CO inhibition. This suggests that the tunnel was blocked by these mutations and that cooperative CO inhibition is related to tunnel operation. Numerous CO molecules might bind cooperatively to some region associated with the tunnel and institute a conformational change that abolishes the majority activity. Alternatively, crowding of CO in the tunnel may control flow through the tunnel and deliver CO to the A-cluster at the appropriate step of catalysis. Residual activity may involve CO from the solvent binding directly to the A-cluster.     

Cys-Gly-Cys tripeptide complexes of nickel: binuclear analogues for the catalytic site in acetyl coenzyme a synthase

The tripeptide, Ac-CysGlyCys-CONH2, is utilized as a ligand to bind Ni in a fashion identical to that found at the active site of acetyl coenzyme A synthase. The Ni-peptide construct is a suitable metalloligand for the preparation of larger structures formed via bridging Cys side chains. The complexes Ni(CysGlyCys)Ni(dppe) and Ni(CysGlyCys)Ni(depe) serve as close structural representations for the binuclear subcluster, exhibiting electrochemical properties that demonstrate facile access to the reduced mixed valent Ni(II)Ni(I) state, which binds CO.     

Mossbauer evidence for an exchange-coupled {[Fe4S4]1+ Nip1+} A-cluster in isolated alpha subunits of acetyl-coenzyme A synthase/carbon monoxide dehydrogenase

The active site A-cluster in the alpha subunit of the title enzyme consists of an Fe4S4 cluster coordinated to a [Nip Nid] subcomponent. The cluster must be activated for catalysis using low-potential reductants such as Ti(III) citrate. Relative to the inactive {[Fe4S4]2+ Nip2+ Nid2+} state, the activated state appears to be 2-electrons more reduced, but the location of these electrons within the A-cluster is uncertain, with {[Fe4S4]2+ Nip0 Nid2+} and {[Fe4S4]1+ Nip1+ Nid2+} configurations proposed. Recombinant apo-alpha subunits oligomerize after activation with NiCl2. The dimer fraction, upon reduction with excess Ti(III)citrate, exhibited M?ssbauer spectra consisting of two quadrupole doublets representing 51% and 21% of the Fe, with parameters indicating [Fe4S4]1+ states. Spectra recorded in strong magnetic fields were typical of diamagnetic systems, indicating an exchange-coupled S = 0 {[Fe4S4]1+ Nip1+} state. Additional treatment with CO altered the doublet M?ssbauer parameters, suggesting an interaction with CO, but maintaining the cluster in the {[Fe4S4]1+ Nip1+} state. Reduction with substoichiometric equivalents of Ti(III) citrate afforded an EPR signal typical of Ni1+ ions, with g parallel = 2.10 and g perpendicular = 2.02. Addition of more Ti caused the signal intensity to decline, suggesting that it arises from the semireduced {[Fe4S4]2+ Nip1+} state.     

N2S2Ni metallodithiolate complexes as ligands: structural and aqueous solution quantitative studies of the ability of metal ions to form M-S-Ni bridges to mercapto groups coordinated to nickel(II). implications for acetyl coenzyme A synthase

The nickel(II) complex of an N2S2 ligand, derived from a diazacycle, N,N'-bis(mercaptoethyl)-1,5-diazacycloheptane, (bme-dach)Ni, Ni-1', serves as a metallodithiolate ligand to NiII, CuI, ZnII, Ag, and PbII. The binding ability of the NiN2S2 ligand to the metal ions was established through spectrochemical titrations in aqueous media and compared to classical S-donor ligands. For M = Ni, Zn, Pb, binding constants, log K = ca. 2. were computed for 1:1 Ni-1'/M(solvate) adducts; for Ag+ and Cu+, the 3:2 (Ni-1')3M2 adducts were the first formed products even in water with log beta3,2 values of 26 and >30, respectively. In all cases, the binding ability of Ni-S-R is intermediate between that of a free thiolate and a free thioether. The great specificity for copper over nickel and zinc by N2S2Ni, which serves as a reasonable structural model for the distal nickel of the acetyl CoA synthase active site, relates to biochemical studies of heterogeneity (metal content and type) in various preparations of acetyl CoA synthase enzyme.     

Chemically distinct Ni sites in the A-cluster in subunit beta of the acetyl-CoA decarbonylase/synthase complex from Methanosarcina thermophila: Ni L-edge absorption and X-ray magnetic circular dichroism analyses

The 5-subunit-containing acetyl-CoA decarbonylase/synthase (ACDS) complex plays an important role in methanogenic Archaea that convert acetate to methane, by catalyzing the central reaction of acetate C-C bond cleavage in which acetyl-CoA serves as the acetyl donor substrate reacting at the ACDS beta subunit active site. The properties of Ni in the active site A-cluster in the ACDS beta subunit from Methanosarcina thermophila were investigated. A recombinant, C-terminally truncated form of the beta subunit was employed, which mimics the native subunit previously isolated from the ACDS complex, and contains an A-cluster composed of an [Fe(4)S(4)] center bridged to a binuclear Ni-Ni site. The electronic structures of these two Ni were studied using L-edge absorption and X-ray magnetic circular dichroism (XMCD) spectroscopy. The L-edge absorption data provided evidence for two distinct Ni species in the as-isolated enzyme, one with low-spin Ni(II) and the other with high-spin Ni(II). XMCD spectroscopy confirmed that the species producing the high-spin signal was paramagnetic. Upon treatment with Ti(3+) citrate, an additional Ni species emerged, which was assigned to Ni(I). By contrast, CO treatment of the reduced enzyme converted nearly all of the Ni in the sample to low-spin Ni(II). The results implicate reaction of a high-spin tetrahedral Ni site with CO to form an enzyme-CO adduct transformed to a low-spin Ni(II) state. These findings are discussed in relation to the mechanism of C-C bond activation, in connection with the model of the beta subunit A-cluster developed from companion Ni and Fe K edge, XANES, and EXAFS studies.     

On [Fe4S4]2+ -(mu2-SR)-M II bridge formation in the synthesis of an A-cluster analogue of carbon monoxide dehydrogenase/acetylcoenzyme A synthase

The construction of a synthetic analogue of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme synthase, the site of acetylcoenzyme A formation, requires as a final step the formation of an unsupported [Fe(4)S(4)]-(mu(2)-SR)-Ni(II) bridge to a preformed cluster. Our previous results (Rao, P. V.; Bhaduri, S.; Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 5833) and the work of others have addressed synthesis of dinuclear complexes relevant to the A-cluster. This investigation concentrates on reactions pertinent to bridge formation by examining systems containing dinuclear and mononuclear Ni(II) complexes and the 3:1 site-differentiated clusters [Fe(4)S(4)(LS(3))L'](2-) (L' = TfO(-) (14), SEt (15)). The system 14/[{Ni(L(O)-S(2)N(2))}M(SCH(2)CH(2)PPh(2))](+) results in cleavage of the dinuclear complex and formation of [{Ni(L(O)-S(2)N(2))}Fe(4)S(4)(LS(3))]- (18), in which the Ni(II) complex binds at the unique cluster site with formation of a Ni(mu(2)-SR)(2)Fe bridge rhomb. Cluster 18 and the related species [{Ni(phma)}Fe(4)S(4)(LS(3))](3)- (19) are obtainable by direct reaction of the corresponding cis-planar Ni(II)-S(2)N(2) complexes with 14. The mononuclear complexes [M(pdmt)(SEt)]- (M = Ni(II), Pd(II)) with 14 in acetonitrile or Me(2)SO solution react by thiolate transfer to give 15 and [M(2)(pdmt)(2)]. However, in dichloromethane the Ni(II) reaction product is interpreted as [{Ni(pdmt)(mu(2)-SEt)}Fe(4)S(4)(LS(3))](2-) (20). Reaction of Et(3)NH(+) and 15 affords the double cubane [{Fe(4)S(4)(LS(3))}(2)(mu(2)-SEt)](3-) (21). Cluster 18 contains two mutually supportive Fe-(mu(2)-SR)-Ni(II) bridges, 19 exhibits one strong and one weaker bridge, 20 has one unsupported bridge (inferred from the (1)H NMR spectrum), and 21 has one unsupported Fe-(mu(2)-SR)-Fe bridge. Bridges in 18, 19, and 21 were established by X-ray structures. This work demonstrates that a bridge of the type found in the enzyme A-clusters is achievable by synthesis and implies that more stable, unsupported single thiolate bridges may require reinforcement by an additional covalent linkage between the Fe(4)S(4) and nickel-containing components. (LS(3) = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3-); L(O)-S(2)N(2) = N,N'-diethyl-3,7-diazanonane-1,9-dithiolate(2-); pdmt = pyridine-2,6-methanedithiolate(2-); phma = N,N'-1,2-phenylenebis(2-acetylthio)acetamidate(4-); TfO = triflate.).     

Kinetics of CO insertion and acetyl group transfer steps, and a model of the acetyl-CoA synthase catalytic mechanism

Acetyl-CoA synthase/carbon monoxide dehydrogenase is a Ni-Fe-S-containing enzyme that catalyzes the synthesis of acetyl-CoA from CO, CoA, and a methyl group. The methyl group is transferred onto the enzyme from a corrinoid-iron-sulfur protein (CoFeSP). The kinetics of two steps within the catalytic mechanism were studied using the stopped-flow method, including the insertion of CO into a putative Ni(2+)-CH(3) bond and the transfer of the resulting acetyl group to CoA. Neither step had been studied previously. Reactions were monitored indirectly, starting with the methylated intermediate form of the enzyme. Resulting traces were analyzed by constructing a simple kinetic model describing the catalytic mechanism under reducing conditions. Besides methyl group transfer, CO insertion, and acetyl group transfer, fitting to experimental traces required the inclusion of an inhibitory step in which CO reversibly bound to the form of the enzyme obtained immediately after product release. Global simulation of the reported datasets afforded a consistent set of kinetic parameters. The equilibrium constant for the overall synthesis of acetyl-CoA was estimated and compared to the product of the individual equilibrium constants. Simulations obtained with the model duplicated the essential behavior of the enzyme, in terms of the variation of activity with [CO], and the time-dependent decay of the NiFeC EPR signal upon reaction with CoFeSP. Under standard assay conditions, the model suggests that the vast majority of active enzyme molecules in a population should be in the methylated form, suggesting that the subsequent catalytic step, namely CO insertion, is rate limiting. This conclusion is further supported by a sensitivity analysis showing that the rate is most sensitively affected by a change in the rate coefficient associated with the CO insertion step.     

DFT studies of Pt/Au bimetallic clusters and their interactions with the CO molecule

Density functional theory (DFT) calculations were performed to study Pt/Au clusters of different size, structure, and composition as well as their interactions with a CO molecule. Among the Pt/Au isomers studied here, the planar structure is the most stable structure in many Pt compositions, although three-dimensional structures become more stable with increasing Pt composition. Furthermore, structures with the Pt atoms surrounded by Au atoms are more stable among homotops. However, these conclusions will be altered if ligands are attached to the Pt/Au bimetallic clusters, as evidenced from the results of CO adsorption. When both Au and Pt sites are exposed, CO adsorption at the Pt site is stronger. If only a Au site is available for CO adsorption, the strongest adsorption occurs at approximately 25% Pt composition, which may correlate with the experimentally observed reactivity of the core-shell structured Pt/Au nanoparticles.     

The ultraviolet absorption spectra of the acetyl radical and the kinetics of the CH3 + CO reaction at room temperature

A continuum-absorption spectrum between 200 and 240 nm is assigned to the acetyl radical. Kinetic measurements using molecular modulation spectroscopy show for the reaction CH₃ + CO (+M) → CH₃CO + M the rate constants are (1.8 ± 0.2) × 10^?18 cm³ molecule^?1 s^?1 at 100 Torr and (6 ± 1) × 10^?18 at 750 Torr. The rate constant for acetyl combination 2CH₃CO → (CH₃CO)₂ is (3.0 ± 10) × 10^?11 at 25°C.