Hydrolysis reaction pathway

Zigmond, 1988). The ATP-hydrolysis that accompanies actin polymerization, ATP —> ADP + Pj, and the subsequent release of the cleaved phosphate (Pj) are believed to act as a clock (Pollard et ah, 1992 Allen et ah, 1996), altering in a time-dependent manner the mechanical properties of the filament and its propensity to depolymerize. Molecular dynamics simulations suggested a so-called back door mechanism for the hydrolysis reaction ATP ADP - - Pj in which ATP enters the actin from one side, ADP leaves from the same side, but Pj leaves from the opposite side, the back door (Wriggers and Schulten, 1997b). This hypothesis can explain the effect of the toxin phalloidin which blocks the exit of the putative back door pathway and, thereby, delays Pi release as observed experimentally (Dancker and Hess, 1990). 

Degradation of a herbicide by abiotic means may be divided into chemical and photochemical pathways. Herbicides are subject to a wide array of chemical hydrolysis reactions with sorption often playing a key role in the process. Chloro-j -triazines are readily degraded by hydrolysis (256). The degradation of many other herbicide classes has been reviewed (257,258). 

Hydrolysis. Esters are cleaved (hydroly2ed) into an acid and an alcohol through the action of water. This hydrolysis is cataly2ed by acids or bases. The mechanistic aspects of ester hydrolysis have received considerable attention and have been reviewed (16). For most esters only two reaction pathways are important. Both mechanisms involve a tetrahedral intermediate and addition-elimination reactions i7i7... 

Acid-catalyzed ester hydrolysis can occur by more than one mechanism, depending on the structure of the ester. The usual pathway, however, is just the reverse of a Fischer esterification reaction (Section 21.3). The ester is first activated toward nucleophilic attack by protonation of the carboxyl oxygen atom, and nucleophilic addition of water then occurs. Transfer of a proton and elimination of alcohol yields the carboxylic acid (Figure 21.8). Because this hydrolysis reaction is the reverse of a Fischer esterification reaction, Figure 21.8 is the reverse of Figure 21.4. 

The two reactants Rj and R2 form an intermediate Ij which reacts to a second intermediate I2 via two reaction pathways, one needing the presence of R2 [48, 108]. A number of additional reactions decrease the selectivity and yield. First, the reactants Rj and R2 are labile to moisture. Hydrolysis results in the formation of the side product reactants S j and 82- By more complex pathways, three other known... 

Different reactions pathways on Rh may explain the intermediate formation of ammonia. NH3 can be obtained via successive reaction steps between adsorbed NHX and dissociated hydrogen species [29]. Alternately, the formation of ammonia may occur via the hydrolysis of isocyanic acid (HNCO) [30]. Isocyanate species are formed by reaction between N and COads on metallic particles. Those species can diffuse onto the support leading to spectator species or alternately react with Hads yielding ultimately HNCO. Previous infrared spectroscopic investigations pointed out that isocyanate species predominantly form over rhodium-based catalysts [31]. 

That the major factor responsible for this shift in reaction pathway is indeed a steric one is demonstrated by the observation that the acids (191) and (192), and their simple esters, undergo ready esterification/hydrolysis by the normal Aac2 mode ... 

As a simple model for the enzyme penicillinase, Tutt and Schwartz (1970, 1971) investigated the effect of cycloheptaamylose on the hydrolysis of a series of penicillins. As illustrated in Scheme III, the alkaline hydrolysis of penicillins is first-order in both substrate and hydroxide ion and proceeds with cleavage of the /3-lactam ring to produce penicilloic acid. In the presence of an excess of cycloheptaamylose, the rate of disappearance of penicillin follows saturation kinetics as the cycloheptaamylose concentration is varied. By analogy to the hydrolysis of the phenyl acetates, this saturation behavior may be explained by inclusion of the penicillin side chain (the R group) within the cycloheptaamylose cavity prior to nucleophilic attack by a cycloheptaamylose alkoxide ion at the /3-lactam carbonyl. The presence of a covalent intermediate on the reaction pathway, although not isolated, was implicated by the observation that the rate of disappearance of penicillin is always greater than the rate of appearance of free penicilloic acid. 

The simple hydrocarbon substrates included ethene, 1,2-propa-diene, propene and cyclopropane (22). Their reactivity with Sm, Yb and Er was surveyed. In contrast to the reactions discussed above, lanthanide metal vapor reactions with these smaller hydrocarbons did not provide soluble products (with the exception of the erbium propene product, Er(C H ) ). Information on reaction pathways had to be obtained primarily by analyzing the products of hydrolysis of the metal vapor reaction product. 

The pyrene-like aromatic chromophore of BaPDE is characterized by a prominent and characteristic absorption spectrum in the A 310-360 nm spectral region, and a fluorescence emission in the X 370-460 nm range. These properties are sensitive to the local microenvironment of the pyrenyl chromophore, and spectroscopic techniques are thus useful in studies of the structures of the DNA adducts and in monitoring the reaction pathways of BaPDE and its hydrolysis products in DNA solutions. 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

Further evidence for the Aa11 mechanism was obtained from a solvent kinetic isotope study. The theoretical kinetic isotope effects for intermediates in the three reaction pathways as derived from fractionation factors are indicated in parentheses in Scheme 6.143,144 For the Aa11 mechanism (pathway (iii)) a solvent KIE (/ch2o A d2o) between 0.48 and 0.33 is predicted while both bimolecular processes (pathways (i) and (ii)) would have greater values of between 0.48 and 0.69. Acid-catalysed hydrolysis of ethylene oxide derivatives and acetals, which follow an A1 mechanism, display KIEs in the region of 0.5 or less while normal acid-catalysed ester hydrolyses (AAc2 mechanism) have values between 0.6 and 0.7.145,146... 

A concept of anion mobility may be considered a useful paradigm for explaining the net retention and loss of cations from soils, and thus exposure pathways. This paradigm relies on the simple fact that total cations must balance total anions in soil solution (or any other solution), and, therefore, total cation leaching can be thought of as a function of total anion leaching. The net production of anions within the soil (e.g., by oxidation or hydrolysis reactions) must result in the net production of cations (normally H+), whereas the net retention of anions (by either absorption or biological uptake) must result in the net retention of cations. 

The same displacement occurs in the hydrolysis of picrylimidazole238 (103). 103 reacts with n-butylamine in water239 to yield picric acid (from the reaction with water) and N-n-butyl-2,4,6-trinitroaniline. The dependence of k0bs values (s 1 moP1 dm3) on pH values indicates the presence (and importance) of equilibrium 27 on the reaction pathway of the... 

Fig. 5.7. The acid-catalyzed hydrolysis of penicillins involves first the formation of an acylium ion (5.22), which, by reacting with H20, yields penicil-loic acids 5.24 (Pathway b). The participation of a neighboring 6-acylamido group increases the rate of hydrolysis. During this intramolecular reaction (Pathway a), oxazolylthiazolidines (5.23) are formed and then give rise to the degradation products penicilloic acids 5.24, penicillenic acids 5.25,...

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

Fig. 11.15. Mechanisms of ring opening of (R)-thiazolidine-4-carboxylic acids (11.113) as derivatives of and chemical delivery systems for l-cysteine (11.114). Activation was shown to be by nonenzymatic, hydrolytic reaction (Pathway a), or by mitochondrial oxidation (Pathway b) to the (R)-4,5-dihydrothiazole-4-carboxylic acid (11.115), followed by a (presumably nonenzymatic) hydrolysis to the IV-acylcysteine, and then by cytosolic hydrolysis to cysteine [138].

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