Water Addition Reactions Research Articles

Exploring the abundance of oleate hydratases in the genus Rhodococcus\u2014discovery of novel enzymes with complementary substrate scope

Oleate hydratases (Ohys, EC 4.2.1.53) are a class of enzymes capable of selective water addition reactions to a broad range of unsaturated fatty acids leading to the respective chiral alcohols. Much research was dedicated to improving the applications of existing Ohys as well as to the identification of undescribed Ohys with potentially novel properties. This study focuses on the latter by exploring the genus Rhodococcus for its plenitude of oleate hydratases. Three different Rhodococcus clades showed the presence of oleate hydratases whereby each clade was represented by a specific oleate hydratase family (HFam). Phylogenetic and sequence analyses revealed HFam-specific patterns amongst conserved amino acids. Oleate hydratases from two Rhodococcus strains (HFam 2 and 3) were heterologously expressed in Escherichia coli and their substrate scope investigated. Here, both enzymes showed a complementary behaviour towards sterically demanding and multiple unsaturated fatty acids. Furthermore, this study includes the characterisation of the newly discovered Rhodococcus pyridinivorans Ohy. The steady-state kinetics of R. pyridinivorans Ohy was measured using a novel coupled assay based on the alcohol dehydrogenase and NAD+-dependent oxidation of 10-hydroxystearic acid.

Lignin-Biosynthetic Study: Reactivity of Quinone Methides in the Diastereopreferential Formation of p-Hydroxyphenyl- and Guaiacyl-Type β- O-4 Structures.

p-Quinone methides are involved in lignin biosynthesis as transient intermediates, and the aromatization step has a great impact on the chemical structure of the resulting lignin. A series of quinone methides (QMs) were synthesized and allowed to react with water in pH 3-7 buffers at 25 °C to mimic the formation of p-hydroxyphenyl- and guaiacyl-type (H- and G-type, respectively) β- O-4 structures in gymnosperm-plant cell walls. Water addition occurred in 3-methoxy-substituted QMs (G-type QMs) with half-lives of 1.4-15 min. In contrast, nonsubstituted QMs (H-type QMs) were very labile; they were aromatized to β- O-4 products with half-lives of only 10-40 s. The rapid aromatization in H-type QMs may provide an advantage over G-type species for efficiently driving the lignin-polymerization cycle, which possibly contributes to the development of highly lignified compression wood. In the water-addition reaction, the threo isomers of the β- O-4 products were stereopreferentially formed more than the erythro isomers from both G- and H-type QMs ( erythro/ threo ratios of 24:76 and 50:50, respectively). The proportion of erythro isomers was higher at lower-pH conditions. This pH-dependent trend agrees with findings from a previous study on 3,5-dimethoxy-substituted (syringyl-type, S-type) QMs; thus, this pH-dependent trend is common in H-, G-, and S-type lignin-related QMs. Higher threo-selectivity was obtained by changing the β-etherified aromatic rings from G- to H-type. A similar but weaker effect was also observed by changing the QM moiety from G- to H-type.

Hydrogen evolution from water reactions with molybdenum sulfide cluster anions

A study of reactions between a distribution of 98MoxSy- clusters ranging from sub-sulfide (x ∼ y) to hypersulfide (y > 2x) is presented, compared with previous studies on homologous MoxOz- clusters, and analyzed with supporting computational studies of the reaction free energy pathways, giving insights into the molecular scale interactions involved in the electro- or photocatalytically driven hydrogen evolution reaction. First, reactions between MoxSy- clusters with water are approximately an order of magnitude slower than reactions between oxides with water, an effect attributed to longer-range repulsion between the cluster anions and water. However, sequential oxidation of the MoxSy- clusters, which generates oxosulfides and H2, is observed for several species. Evidence of oxidation of several hypersulfides with H2 evolution is also observed, indicating that oxidation of S2 groups is also possible. Based on the computational results on several of the bimetallic species, oxidation of the subsulfide Mo2S2− and Mo2S3− clusters is barrierless, while oxidation of the nascent Mo2S2O- cluster formed by oxidation of Mo2S2− is low-barrier. The stoichiometric Mo2S4- + H2O free energy path is predicted to have a modest barrier to the dissociative addition of water, which is consistent with the experimental observation of both oxidation (with H2 production) and water addition reactions. Calculations on the reaction free energy pathways of the oxosulfide species suggest that the formation of two dihydroxide groups, rather than a thiol and hydroxide group, is favored. Oxidation of the oxides is more thermodynamically favored than oxidation of sulfide analogs.

Formation of glycine from HCN and H2O: A computational mechanistic study

The potential energy surfaces for the formation of glycine from HCN and H2O were determined from CBS-QB3 calculations. After the formation of a HCN trimer, amino malononitrile, amino malononitrile monoamide (3) was formed by a water addition reaction. Two pathways were found for the subsequent reaction, 3+2H2O→glycine+HNCO+NH3. One pathway involving an amino ketone was much more favored than the other pathway involving glycinamide. Addition of a water molecule as a catalyst greatly enhanced steps occurring by hydrogen rearrangement.

Role of weakly bound complexes in temperature-dependence and relative rates of M(x)O(y)(-) + H2O (M = Mo, W) reactions.

Results of a systematic comparison of the MoxOy (-) + H2O and WxOy (-) + H2O reaction rate coefficients are reported and compared to previous experimental and computational studies on these reactions. WxOy (-) clusters undergo more direct oxidation by water to yield WxOy+1 (-) + H2, while for MoxOy (-) clusters, production of MoxOyH2 (-) (trapped intermediates in the oxidation reaction) is comparatively more prevalent. However, MoxOy (-) clusters generally have higher rate coefficients than analogous WxOy (-) clusters if MoxOy+1H2 (-) formation is included. Results of calculations on the M2Oy (-) + H2O (M = Mo, W; y = 4, 5) reaction entrance channel are reported. They include charge-dipole complexes formed from long-range interactions, and the requisite conversion to a Lewis acid-base complex that leads to MxOy+1H2 (-) formation. The results predict that the Lewis acid-base complex is more strongly bound for MoxOy (-) clusters than for WxOy (-) clusters. The calculated free energies along this portion of the reaction path are also consistent with the modest anti-Arrhenius temperature dependence measured for most MoxOy (-) + H2O reactions, and the WxOy (-) + H2O reaction rate coefficients generally being constant over the temperature range sampled in this study. For clusters that exhibit evidence of both water addition and oxidation reactions, increasing the temperature increases the branching ratio toward oxidation for both species. A more direct reaction path to H2 production may therefore become accessible at modest temperatures for certain cluster stoichiometries and structures.

Dissociation of gas-phase, doubly-charged uranyl-acetone complexes by collisional activation and infrared photodissociation

Past studies of fragmentation reactions of doubly-charged uranyl (UO22+) complexes have been impeded by very rapid water addition reactions that cause H2O adducts to dominate product ion spectra. The fragmentation of uranyl-acetone (aco) complexes ([UO2(aco)n]2+, n=1–5), generated by electrospray ionization, is revisited here using: (a) collisional activation in a linear ion trap (LIT) mass spectrometer in which the level of background H2O is significantly lower, and (b) infrared multiple-photon photodissociation (IRMPD, 10.6μm) in the LIT and a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. Lower levels of adventitious H2O in the LIT provided access to fragmentation of [UO2(aco)n]2+, n=1–5. For n=3–5, direct elimination of aco ligands is the favored fragmentation pathway. For n=1 and 2, charge reduction reactions are dominant. For [UO2(aco)2]2+, the most abundant product ion is [UO2(aco)]+, while UO2+ is observed following collision-induced dissociation (CID) of [UO2(aco)]2+. Minor peaks corresponding to ligated [UO2OH]+ are also observed. The IRMPD experiments in the FT-ICR yielded highly accurate mass measurements that confirm composition assignments, and shed light on dissociation reactions in a gas-phase environment that is entirely free of adventitious H2O. For [UO2(aco)n]2+, n=3–5, the primary photodissociation channel is direct aco elimination, along with charge-reduction pathways that involve intra-complex proton transfer and formation of species that contain enolate ligands. Similar pathways are observed for IRMPD measurements in the LIT.

ChemInform Abstract: CO Extrusion in Homogeneous Gold Catalysis: Reactivity of Gold Acyl Species Generated Through Water Addition to Gold Vinylidenes.

AbstractA novel reactivity of gold vinylidenes is discovered through the addition reaction of water to gold vinylidene intermediates resulting in CO‐extrusion and cyclization to indenes.

A One‐Pot Oxidation/Enantioselective Oxa‐Michael Cascade

AbstractA laccase/(2,2,6,6‐tetramethylpiperidin‐1‐yl)oxy (TEMPO) mediated oxidation was combined with an aqueous, enantioselective copper‐catalyzed Michael addition reaction of water in one pot. The copper catalyst was also immobilized onto DNA to induce enantioselectivity in the reaction. Low conversions were observed when the reactions were performed simultaneously, caused by an undesired reaction of an oxidised TEMPO intermediate. We increased the conversions by using a stepwise approach. Thus, after completion of the oxidation, the first reaction was stopped by inhibiting the enzyme with HCO2K and reducing the reactive TEMPO intermediate. Next, the Michael addition reaction was started by adding the Cu catalyst. By applying this strategy, an efficient two‐step one‐pot sequence, proceeding with 20 % ee, was realized. The yield and ee of the second reaction were not affected by the oxidation reaction.

Mechanisms for the Reactions of Group 10 Transition Metal Complexes with Metal–Group 14 Element Bonds, Bbt(Br)E═M(PCy3)2(E = C, Si, Ge, Sn, Pb; M = Pd and Pt)

The electronic structures of the Bbt(Br)E═M(PCy(3))(2) (E = C, Si, Ge, Sn, Pb and M = Pt, Pd) complexes and their potential energy surfaces for the formation and water addition reactions were studied using density functional theory (B3LYP/LANL2DZ). The theoretical evidence suggests that the bonding character of the E═M double bond between the six valence-electron Bbt(Br)E: species and the 14 valence-electron (PCy(3))(2)M complexes has a predominantly high s-character. That is, on the basis of the NBO, this theoretical study indicates that the σ-donation from the E element to the M atom prevails. Also, theoretical computations suggest that the relative reactivity decreases in the order: Bbt(Br)C═M(PCy(3))(2) > Bbt(Br)Si═M(PCy(3))(2) > Bbt(Br)Ge═M(PCy(3))(2) > Bbt(Br)Sn═M(PCy(3))(2) > Bbt(Br)Pb═M(PCy(3))(2), irrespective of whether M = Pt or M = Pd is chosen. Namely, the greater the atomic weight of the group 14 atom (E), the larger is the atomic radius of E and the more stable is its Bbt(Br)E═M(PCy(3))(2) doubly bonded species toward chemical reactions. The computational results show good agreement with the available experimental observations. The theoretical results obtained in this work allow a number of predictions to be made.

The nature of the styrylindolium dye: transformations among its monomer, aggregates and water adducts

The optical properties of styrylindolium dyes are strongly influenced by the external environment. After a detailed investigation by electronic absorption spectroscopy, we found that five states of 2-methoxylstyryl-indolium dye viz. monomer, D-aggregate, H-aggregate, J-aggregate and water adducts can be observed under different conditions. The spectroscopic studies revealed that the conversion rate of the dye among its monomer and the other aggregates is slow due to the steric hindrance of the 3,3-dimethyls. Meanwhile, it could form water adducts by the reaction of the water residue in solvent with the charged electron-deficient CN double bond of the dye. The formation of water adducts is also an important factor that influences the optical properties of the dye because the water addition reaction breaks the dye's π-conjugate. Generally, the dye exists mainly in the monomeric form in freshly prepared solutions. A small portion of the D-aggregate, H-aggregate and J-aggregate can be observed in some cases. In equilibrium solutions, however, significant changes are observed in most solvents with different polarities. The J-aggregate is an unstable state which is irreversibly converted to other species. The monomer converts reversibly among the D-aggregate, H-aggregate and water adducts in the equilibrium state. At the same time, the water adducts are sensitive to the external environment and their content can be modulated by temperature and pH conditions. The dye's prominent optical signal changes arising from pH- and thermal dependence of water addition or the disassociation of the D-aggregate led to potential applications in the field of thermoresponsive probes under given conditions.

One-pot facile synthesis of PEGylated Au nanoparticles in an aqueous media

We describe a facile protocol for the synthesis of PEGylated Au nanoparticles by simply mixing aqueous solutions of HAuCl4 and oligo(ethylene glycol) ethyl ether methacrylate. This method was applied to generate uniform multiply-twinned Au nanostructures of ∼21 nm in diameter with high yields. Our proposed mechanism indicates that the generation of primary alcohol intermediates from the nucleophilic addition reaction of water (nucleophile) with AuIII–vinyl complex is responsible for the reduction of gold ions. This protocol was also used to synthesize Ag nanoparticles and small aggregates of Pd nanoparticles. Due to the exclusion of sophisticated synthesis of PEG containing stabilizers, additional surfactants, or reducing agents, this approach provides a remarkably simple, versatile, and environmentally benign protocol to prepare PEGylated noble-metal nanocrystals. A comparative BSA adsorption study proved the lack of non-specific binding, a common obstacle in designing biocompatible nanoparticles.

Mechanism of the N-protecting group dependent annulations of 3-aryloxy alkynyl indoles under gold catalysis: a computational study

The mechanism of the gold-catalyzed annulations of 3-aryloxy alkynyl indoles developed by Tu et al. was studied by DFT calculations. It was found that both indole derivatives of electron-donating and electron-withdrawing protective groups would first undergo the 5-exo-dig cyclization simultaneously upon activation by cationic [PR(3)Au(+)] species. However, divergent reactivity of the resulting spirocyclic intermediate in competitive 1,2-alkenyl migration and nucleophilic water addition reactions towards C3 was predicted. When protected by electron-donating group, the 1,2-alkenyl migration occurs to generate a tricyclic intermediate, from which an aromatic Claisen rearrangement/nucleophilic addition sequence results in the observed 1,2-phenoxy migration. In case of electron-withdrawing group, the 1,2-alkenyl migration would be unfavorable. Instead, the nucleophilic addition of water oxygen to C3 is more facile, and leads to the hemiketal intermediate. The possible roles of water-cluster and OTf anion as proton shuttles in both reactions were also evaluated.

The role of basis set superposition error in water addition reactions to Ln(III) cations

AbstractOur goal in this work was to characterize the importance of basis set superposition error (BSSE) in the reaction energetics of water addition to highly charged metal ions that exhibit strong ion‐dipole interactions with water. The gas phase water addition reactions, M(H2O) + H2O → M(H2O) [M = La(III), Lu(III); N = 0–8] have been studied, with a particular emphasis on a posteriori methods for calculating BSSE and its constituent energetic components as a function of M(H2O) cluster size and water basis set. Because of accumulation of BSSE within the subclusters in the reaction series, the successive reaction counterpoise method for determining the BSSE correction interaction energies is advocated. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

DFT Study on the Nucleophilic Addition Reaction of Water and Ammonia to the Thymine Radical Cation

The nucleophilic addition reactions of water and ammonia molecules toward the C5-C6 double bond of thymine radical cations were investigated using density functional theory. We predicted that the nucleophilic addition favored the C5-site of thymine radical cations, in contrast to the previous experimental observations in bulk solution where the addition product to the C6-site was dominant. Considering the molecular orbital factors, we estimated the relative reactivity of the C5- and C6-sites of thymine radical cations for the nucleophilic addition of ammonia. We found that the C5 was more reactive than the C6 for the small-size clusters of Thy1(NH3)n+, n = 0-2, in the gas phase and even in aqueous solution, though the difference in the reactivity between the two sites became smaller as the number of ammonia molecules increased. This variation of the reactivity was attributed to the electron density redistribution within the thymine radical cations induced by the ammonia molecules as a nucleophile. We suggest that the dominance of the C6-addition product in bulk solution is mainly due to the higher stability of the C6-addition product by solvation, rather than to the higher reactivity of the C6-site for the nucleophilic addition.

Degradation of rapamycin and its ring-opened isomer: Role of base catalysis

Rapamycin is a natural macrolide immunosuppressant with a distinct mechanism of action. Quantitative analysis of rapamycin poses many challenges associated with facile degradation and the multitude of isomeric forms. The primary goal of this study was to compare degradation of rapamycin and its ring-opened isomer, secorapamycin, in aqueous solution under identical conditions. Reaction kinetics and mechanisms were studied in 30/70 vol/vol acetonitrile-water mixtures containing either MeCOONH4 (apparent pH 7.3) or NaOH (apparent pH 12.2). Degradation kinetics was well described by the first-order rate law. For rapamycin in 237 and 23.7 mM solutions of MeCOONH4, apparent half-lives of 200 h and 890 h were obtained. When compared to the latter value, the rapamycin half-life was reduced by 3 orders of magnitude in the pH 12.2 solution. Under all conditions studied, secorapamycin degradation was significantly slower than that of the parent compound. Both specific and general base catalysis was observed for reactions of rapamycin and secorapamycin. Two primary products of rapamycin degradation were identified as individual isomers of secorapamycin and a hydroxy acid formed via lactone hydrolysis. No evidence for the interconversion between the products was obtained. In highly basic solutions, both products undergo fragmentation and water addition reactions.

New Reactions of 1‐Alkynes Catalyzed by Transition Metal Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

New reactions of 1-alkynes catalyzed by transition metal complexes.

Recently discovered catalytic reactions with ruthenium and lanthanide metal complexes have extended the scope of 1-alkynes as useful reagents. The specific formation of aryl-substituted (Z)-1,3-enzymes via the dimerization of HC(triple bond) CR(1) (R(1) = aryl) has been attained using dimeric lanthanide complexes, the catalytic activity of which appears to be unaffected by time. The dimerization of HC(triple bond) CR(2) (R(2) = t-Bu, SiMe(3)) catalyzed by Ru(cod)(cot)/PR(3) or RuH(2)(PPh(3))(3) produces a good yield of butatrienes (Z)R(2)CH=C=C=CHR(2) with a high degree of selectivity. Under certain conditions, HC(triple bond) C=SiMe(3) dimerizes to yield exclusively (Z)-M(3)Si-C(triple bond) C-CH=CH-SiMe(3). The hydration of HC(triple bond)CR(3) (R(3) = alkyl, aryl) catalyzed by RuCl(2)/PR'(3) or CpRuCl(PR"(3))(2) has realized the first example of anti-Markovnikov regioselectivity in an addition reaction of water that produces aldehydes R(3)CH(2)bond;CHO. The application of this reaction to propargylic alcohols has lead to their formal isomerization to alpha,beta-unsaturated aldehydes. In contrast, the addition of amines R(4)bond;NH(2) (R(4) = aryl) to HCtbond;CR(5) (R(5) = alkyl, aryl) conforms to Markovnikov's rule to produce ketimines R(5)bond;(C=NR(4))bond;CH(3) when catalyzed by a Ru(3)(CO)(12)/additive. Since the reaction can be performed in air without the need for any solvents, it enables the practical synthesis of aromatic ketimines, which are difficult to prepare by conventional methods. The synthesis of indoles using deactivated anilines is one practical application of this reaction. The mechanisms of some of these reactions have been analyzed in detail with the aid of theoretical calculations.

Reaction enthalpies for the hydrolysis of 3p, 4p and early 3d oxide bridges – how unique are the oligophosphates?

The hydrolysis of M–O–M bridges in binuclear oxyhydroxides of the 3p, 4p and early 3d elements is investigated by quantum chemistry. The reaction enthalpies for the water addition reactions are compared. Hydrolysis of oxygen bridges is found endothermic for the Al–P, Ga–As and Sc–Mn species, but exothermic for the S, Se, Cl and Br species. The hydrolysis of pyrophosphoric acid is unique in displaying the reaction enthalpy, which is closest to zero. This fact is taken to explain why the oligophosphates are utilised in controlling cellular reactions, as the hydrolysis energetics rely entirely on local conditions.

Importance of frontier orbital interactions in addition reaction of water to disilene

AbstractAddition mechanisms of water to disilene were investigated in detail using the high level ab initio molecular orbital (MO) theory. We have found that the search for transition states by ab initio MO calculations combined with the frontier MO theory provides a good perspective view of the mechanisms of the reaction of disilenes and water. The initial electrophilic and nucleophilic interactions of the water with disilene give the corresponding weak complexes, CE and CN, which leads to the next nucleophilic and electrophilic attack of the water part, respectively. The latter pathway requires an antarafacial approach, while the former pathway proceeds via suprafacial addition. Whereas in the model disilene–water addition reaction, the pathway from CE is more favorable than the pathway from CN, the stereochemical ambiguity of the addition reactions found experimentally may be explained by the competition of the two pathways. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 192–197, 2001

Chemical Simulation of Biogenesis of the 2,4,5-Trihydroxyphenylalanine Quinone Cofactor of Copper Amine Oxidases: Mechanistic Distinctions Point toward a Unique Role of the Active Site in the o-Quinone Water Addition Step

The biogenesis of the 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor from tyrosine at the active site of copper amine oxidases is believed to proceed along a pathway that includes a conjugate addition of water to the corresponding o-quinone intermediate, followed by autoxidation of the resulting benzenetriol to the hydroxyquinone cofactor. The water addition reaction has been presumed to occur not only in previous model studies reported for cofactor biogenesis starting with either catechol or o-quinone, but also for generation of the neurotoxin 6-hydroxydopamine during autoxidation of dopamine. We here report the surprising finding that water addition does not occur under solution chemistry conditions. The production of hydroxyquinone from catechol arises instead from reaction of the o-quinone with H2O2 generated during autoxidation of catechol. When starting with the o-quinone itself, production of hydroxyquinone still arises from autoxidation of the catechol, generated either by reduction of the o...