Hydration competition

With the existence of THF, H2 can be enclathrated in the hydrate cages depending on the composition of gas mixtures. In the H2 + CO2 + THF mixed hydrate, it is likely that THF occupies all of the H-cage while H2 and CO2 can occupy the D-cage of s-ll hydrate competitively. 

Although there are no new methane VPO competitive processes, current technology may be usehil for the production of impure methanol in remote areas for use as a hydrate inhibitor in natural gas pipelines (119,120). 

Sulfate radical anion may be converted to the hydroxyl radical in aqueous solution. Evidence for this pathway under polymerization conditions is the formation of a proportion of hydroxy end groups in some polymerizations. However, the hydrolysis of sulfate radical anion at neutral pi I is slow (k— 107 M"1 s 1) compared with the rale of reaction with most monomers (Ar=l08-109 M 1 s 1, Table 3.7)440 under typical reaction conditions. Thus, hydrolysis should only be competitive with addition when the monomer concentration is very low. The formation of hydroxy end groups in polymerizations initiated by sulfate radical anion can also be accounted for by the hydration of an intermediate radical cation or by the hydrolysis of an initially formed sulfate adduct either during the polymerization or subsequently. 

For many solubilized enzymes the greatest catalytic activity and/or changes in conformation are found at R < 12, namely, when the competition for the water in the system between surfactant head groups and biopolymers is strong. This emphasizes the importance of the hydration water surrounding the biopolymer on its reactivity and conformation [13], It has been reported that enzymes incorporated in the aqueous polar core of the reversed micelles are protected against denaturation and that the distribution of some proteins, such as chymotrypsine, ribonuclease, and cytochrome c, is well described by a Poisson distribution. The protein state and reactivity were found markedly different from those observed in bulk aqueous solution [178,179],... 

McCracken, P. G. Bolton, J. L. Thatcher, G. R. J. Covalent modification of proteins and peptides by the quinone methide from 2-ZerZ-butyl-4,6-dimethylphenol selectivity and reactivity with respect to competitive hydration, j. Org. Chem. 1997, 62, 1820-1825. 

K. Mizutani, T. Electronic and structural requirements for metabolic activation of butylated hydroxytoluene analogs to their quinone methides, intermediates responsible for lung toxicity in mice. Biol. Pharm. Bull. 1997, 20, 571-573. (c) McCracken, P. G. Bolton, J. L. Thatcher, G. R. J. Covalent modification of proteins and peptides by the quinone methide from 2-rm-butyl-4,6-dimethylphenol selectivity and reactivity with respect to competitive hydration. J. Org. Chem. 1997, 62, 1820-1825. (d) Reed, M. Thompson, D. C. Immunochemical visualization and identification of rat liver proteins adducted by 2,6-di- m-butyl-4-methylphenol (BHT). Chem. Res. Toxicol. 1997, 10, 1109-1117. (e) Lewis, M. A. Yoerg, D. G. Bolton, J. L. Thompson, J. Alkylation of 2 -deoxynucleosides and DNA by quinone methides derived from 2,6-di- m-butyl-4-methylphenol. Chem. Res. Toxicol. 1996, 9, 1368-1374. 

Fig. 5.5.15 Spatially resolved 13C DEPT spectra recorded for the competitive etherification and hydration reactions of 2-methyl-2-butene (2M2B) to 2-methoxy-2-methylbutane (tert-amyl methyl ether, TAME) and 2-methyl-butan-2-ol (tert-amyl alcohol, TAOH), respectively. The molar composition of the feed was in the ratio 2 10 1 for 2M2B methanol water. The...

Deoxycytidine (dCyd) (14 in Scheme 2) is also an excellent target for one-electron oxidation reactions mediated by triplet excited menadione. On the basis of extensive identification of dCyd photooxidation products, it was concluded that this nucleoside decomposes by competitive hydration and deprotonation reactions of cytosine radical cations with yields of 52% and 40%, respectively [53]. It was also found, on the basis of 180 labeling experiments, that hydration of cytosine radical cations (15) predominantly occurs... 

It may be noted that competitive deprotonation of 29 at C-l gives rise to 2-deoxyribonolactone (27) with the concomitant release of free 5-methylcy-tosine as minor processes. Interestingly, competitive hydration of 5-MedCyd radical cations (29) occurs exclusively at C-6 as inferred from labeling experiments with 1802 (36) [61]. Thus, mass spectrometry analysis of the four cis and trans diastereomers of 5-MedCyd glycols 36 showed that incorporation of 1802 takes place exclusively at C-5 of 6-hydroxy-5,6-dihydro-2 -deoxycy-tyd-5-yl radicals (34). 

Relevant kinetic information on two competitive reactions of guanine radical cations within double stranded DNA, namely hydration and hole transfer to another guanine residue, has been examined [13]. Thus, the pseudoorder rate for hydration of guanine radical cations 38 has been estimated to... 

Acids that have weakly nucleophilic anions, e.g. HS04e from dilute aqueous H2S04, are chosen as catalysts, so that their anions will offer little competition to H20 any R0S03H formed will in any case be hydrolysed to ROH under the conditions of the reaction. Rearrangement of the carbocationic intermediate may take place, and electrophilic addition of it to as yet unprotonated alkene is also known (p. 185). The reaction is used on the large scale to convert cracked petroleum alkene fractions to alcohols by vapour phase hydration with steam over heterogeneous acid catalysts. Also under acid catalysis, ROH may be added to alkenes to yield ethers, and RCOzH to yield esters. 

The oxidative formation of p-benzoquinones from anilides such as 7-108 was used for the synthesis of the core scaffold of the natural products elisabethin A (7-106) and pseudopterosin A aglycone (7-107) (Scheme 7.30). Exposure of anilide 7-108 to DMP [53] led to the formation of the o-imidoquinone 7-109, which underwent an intramolecular Diels-Alder reaction to give 7-110 in 28% yield after hydration. In a competitive pathway, the p-quinone 7-111 is also formed from 7-108, which on heating in toluene again underwent an intramolecular Diels-Alder reaction to give cycloadduct 7-112 in 25% overall yield. Hydrolysis of 7-112 furnished the carbocyclic skeleton 7-113 of elisabethin A (7-106). 

The formation of hydrated electrons by the photolysis of halide ions in solution may be envisaged in two steps. The first step is the CTTS absorption leading to (X -). The second step is a slow, thermal process releasing the electron in competition with degradation and recapture. In the presence of acid and alcohol, photolysis of halide solutions generates H2 with a yield that increases both with acid and alcohol concentrations (seejortner et al., 1962, 1963, 1964). At 25°, the limiting quantum yields are 0.98 for Cl- at 185 nm, 0.6 and 0.5 for Brat 185 and 229 nm, respectively, and 0.3 and 0.25 for I- at 254 and 229 nm, respectively. Since most of these yields are less than 1, the direct reaction of HsO and (Xaq-) is ruled out. Instead, it is proposed that eh is produced from the... 

The detailed mechanism of inhibition of TEM-2 (class A) enzyme with clavulanate has been established (Scheme 1) [23,24], The inhibition is a consequence of the instability of the acyl enzyme formed between the /1-lactam of clavulanate and the active site Ser-70 of the enzyme. In competition with deacylation, the clavulanate acyl-enzyme complex A undergoes an intramolecular fragmentation. This fragmentation initially provides the new acyl enzyme species B, which is at once capable of further reaction, including tautomeriza-tion to an entity C that is much less chemically reactive to deacylation. This species C then undergoes decarboxylation to give another key intermediate enamine D, which is in equilibrium with imine E. The imine E either forms stable cross-linked vinyl ether F, by interacting with Ser-130 or is converted to the hydrated aldehyde G to complete the inactivation. 

Fig. 15 Spatially resolved 13C DEPT-MRI spectra recorded for the competitive etherification and hydration reactions of 2M2B to TAME and TAOH, respectively. Spectra recorded at 6 positions along the length of the bed are shown, at 2.5 mm intervals. The entrance to the bed is at 0 mm. 

Individually, the silk molecules will adopt a shape and conformation dictated by the competition to hydrate polar and nonpolar moieties (Hossain et al., 2003 Jin and Kaplan, 2003). In addition, to achieve fiber formation and optimal axial stiffness, the system must organize and lock the molecules in their extended configuration (Donald and Windle, 1992)... 

In summary, the physiological control of silk protein conversion shows an ingenious balance of activating and inhibiting mechanisms that are dependent on composition and sequence arrangement (Krejchi et al., 1994). Denaturing effects observed in silks appear to be identical to those found in amyloid-forming proteins, and they principally alter the competitive outcome of the hydration of nonpolar and polar residues (Anfinsen, 1973 Dill, 1990 Dobson and Karplus, 1999 Kauzmann, 1959). The key differences to amyloids may lie in the hierarchical level of the structures (Muthukumar et al., 1997) involved in the assembly of silks compared to amyloids. 

The various rearrangement reactions discussed above do not involve hydration and would seem, thus, to fall outside the scope of this work. However, they are of relevance, being competitive with the addition of nucleophiles and, particularly, with enzymatic hydration. As such, they should be taken into account in the interpretation of metabolic and toxicological results. 

An unusual case of intramolecular competition (chemoselectivity, see Chapt. 1 in [la]) between ester and oxirane occurs in the detoxification of (oxiran-2-yl)methyl 2-ethyl-2,5-dimethylhexanoate (10.49), one of the most abundant isomers of an epoxy resin. The compound is chemically very stable, i.e., resistant to aqueous hydrolysis, but is rapidly hydrolyzed in cytosolic and microsomal preparations by epoxide hydrolase and carboxylesterase, which attack the epoxide and ester groups, respectively [129], The rate of overall enzymatic hydrolysis was species dependent, decreasing in the order mouse > rat > human, but was relatively fast in all tissues examined (lung and skin as portals of entry, and liver as a further barrier). In mouse and rat lung microsomes, ester hydrolysis was 3-4 times faster than epoxide hydration, whereas the opposite was true in human lung microsomes. 

The ion-water interactions are very strong Coulomb forces. As the hydrated ion approaches the solution/metal interface, the ion could be adsorbed on the metal surface. This adsorption may be accompanied by a partial loss of coordination shell water molecules, or the ion could keep its coordination shell upon adsorption. The behavior will be determined by the competition between the ion-water interactions and the ion-metal interactions. In some cases, a partial eharge transfer between the ion and the metal results in a strong bond, and we term this process chemisorption, in contrast to physisorption, which is much weaker and does not result in substantial modification of the ion s electronic structure. In some cases, one of the coordination shell molecules may be an adsorbed water molecule. hi this case, the ion does not lose part of the coordination shell, but some reorganization of the coordination shell molecules may occur in order to satisfy the constraint imposed by the metal surface, especially when it is charged. 

It was postulated that the differences in enzyme activity observed primarily result from interactions between enzyme-bound water and solvent, rather than enzyme and solvent. As enzyme-associated water is noncovalently attached, with some molecules more tightly bound than others, enzyme hydration is a dynamic process for which there will be competition between enzyme and solvent. Solvents of greater hydrophihcity will strip more water from the enzyme, decreasing enzyme mobility and ultimately resulting in reversible enzyme deactivation. Each enzyme, having a unique sequence (and in some cases covalently or noncovalently attached cofactors and/or carbohydrates), will also have different affinities for water, so that in the case of PPL the enzyme is sufficiently hydrophilic to retain water in all but the most hydrophilic solvents. 

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