The Role of Water

On the basis of kinetic data and of the fact that the Diels-Alder reaction is accelerated in aqueous medium by adding salting-out salts (LiCl, NaCl) and retarded by adding salting-in salts (LiC104, guanidinium chloride), Breslow su ested that the acceleration and the increased selectivity of the reaction are ascribable to hydrophobic packing of diene and dienophile. 

Engberts reported evidence that enforced hydrophobic interactions and hydrogen-bonding interactions are responsible for the rate enhancement of aqueous Diels-Alder reaction. The term enforced has been used to emphasize that the rate enhancement is not the result of the hydrophobic packing of reactants but rather to stress that hydrophobic interactions occur simply because they are an integral part of the activation process . 

Hydrogen bonding has an important as shown in the relationship between the 

In contrast with previous hypotheses, it has been observed that the internal pressure of water cannot facilitate the hydrophobic packing of diene and dienophile and therefore it cannot be invoked to explain the strong rate enhancement of Diels-Alder cycloaddition when carried out in water with respect to organic solvent. 

The first mechanistic study on the benefits of a combination of Lewis acid catalysis and water as reaction medium for a Diels-Alder reaction appeared in 1995. Cu(DS)2 micelles (DS dodecyl sulfate) catalyze the cycloaddition of 3-(para-substituted 

Other workers have confirmed the importance of anion hydration in phase transfer catalysis. Increasing the base concentration in the aqueous phase from 15% to 50% was found to decrease the extractability of the hydroxide anion by up to five times, but the consequent decrease in the hydration of the anion in the organic phase caused an increase in its reactivity of up to ten thousand times [52]. 

Of course, there are many phase transfer reactions which must be conducted in the absence of water (solid-liquid PTC), and these include those where the product is water sensitive, or where the attacking anion is too hydrated to be transferred efficiently. For example, aromatic fluorinations suffer from phenol formation in the presence of water. 

It is clear that the amount of water used in these reactions is critical, and that it is not necessary for all of the inorganic substrate to be dissolved. The use of high salt concentrations in the aqueous phase can reduce catalyst poisoning by precipitation of attacking nucleophiles produced in the reaction [53], although the presence of solids in the reaction may increase the abrasiveness of the reaction, which would be undesirable on an industrial scale. 

Chulalaksananukul et al. measured the residual activity of a lipase from Mucor miehei after one day in SCCO2 at 40-100 °C at various water concentrations. As the temperature rises, the enzyme molecule at first unfolds reversibly and then undergoes one or more of the following reactions formation of incorrect or scrambled structures, cleavage of disulfide bonds, deamination of tryp-sine residues, and hydrolysis of peptide bonds. Each process requires water and is therefore accelerated with increasing water concentration [19]. The role of water on the performance of enzymes in SCFs is described in more detail in Section 4.9.4.3 

Enzyme stability generally decreases with increasing water concentration, whereas activity requires some water to be present. Therefore, the water content has to be optimized to find the best balance between enzyme life and activity. 

Enzymes are not catalytically active if water is completely absent. The often cited explanation is that at least a monolayer of water per enzyme molecule is necessary to keep the enzyme active [40]. Apparently, the essential noncova-lently bound water maintains the enzyme s native protein structure. In an enzymatic reaction under supercritical conditions, the water partitions between the enzyme, the enzyme support and the reaction mixture. In an essentially non-aqueous system, the existing water partitions preferrably to the solvent with increasing hydrophilicity. If there is little water in the system and if the solvent is relatively hydrophilic, the solvent may strip the essential water from the enzyme, making it inactive. When Zaks and Klibanov first noted that enzymes were more active in hydrophobic solvents than in hydrophilic organic solvents. 

Experiments with enzymes in SCFs revealed very early that scCOa could strip the essential water from the enzyme. Randolph et al. found that damp immobilized enzyme lost its activity when exposed to bone-dry carbon dioxide. The activity was quickly regained when they injected a small amount of water into the system [12]. Dumont also reported results from an immobilized lipase/C02 system where the conversion rate decreased when the enzyme was in contact with a dry COa/substrate flow. Reaction rates were restored completely when they directed the CO2 flow through a water saturator [13]. 

Kamat et al. dlso support this hypothesis [18]. They compared lipase-catalyzed transesterification rates in supercritical carbon dioxide, fluoroform, ethylene, ethane, propane, and sulfur hexafluoride as well as in several conventional liquid solvents of different polarities. The reaction rates increased with increasing hydrophobicity of solvent within the SCFs and also within the liquid solvent group. Because the solvent s immiscibility with water and its apolarity, by themselves, are irrelevant to enzymatic activity [8], it appears that the activity loss is the result of the enzyme losing essential water. Although SCCO2 is generally considered to be a hydrophobic solvent, it is more hydrophilic than fluoroform or hexane and capable of stripping essential water from the enzyme in an essentially nonaqueous environment. 

---

Surfaces can be active in inducing blood clotting, and there is much current searching for thromboresistant synthetic materials for use in surgical repair of blood vessels (see Ref. 111). It may be important that a protective protein film be strongly adsorbed [112]. The role of water structure in cell-wall interactions may be quite important as well [113]. 

The role of water in the life of plants is well known. In terms of its major effects this role consists in transporting the mineral nutrition, maintenance of intracellular pressure responsible for the vertical growth of plants and, finally, participation in photosynthesis which provide the biomass growth, or plainly speaking, the crop production. 

At 951 K, the reaction rate is directly proportional to Ph2o a catalytic effect that is attributed [808] to the role of water as an oxygen carrier. The reaction rate was also influenced by the method of salt preparation but for a given sample the effect of particle size was small. 

Reynolds and Lumry have discussed the role of water in this exchange and have suggested, for both steps, a mechanism involving water bridges. 

A series of AB cements can be prepared from aqueous solutions of oxides and halides (or sulphates) of magnesium or zinc. These cements are described in detail in Chapter 7. For the moment we will confine our discussion to a consideration of the role of water in these cements. 

The precise structural role played by the water molecules in these cements is not clear. In the zinc oxychloride cement, water is known to be thermally labile. The 1 1 2 phase will lose half of its constituent water at about 230 °C, and the 4 1 5 phase will lose water at approximately 160 C to yield a mixture of zinc oxide and the 1 1 2 phase. Water clearly occurs in these cements as discrete molecules, which presumably coordinate to the metal ions in the cements in the way described previously. However, the possible complexities of structure for these systems, which may include chlorine atoms in bridging positions between pairs of metal atoms, make it impossible to suggest with any degree of confidence which chemical species or what structural units are likely to be present in such cements. One is left with the rather inadequate chemical descriptions of the phases used in even the relatively recent original literature on these materials, from which no clear information on the role of water can be deduced. 

The role of water is important, for it acts as a reaction medium and... 

Hydropolymer gel has been considered as a possible candidate for an artificial articular cartilage in artificial joints because it exhibits very low friction when it is in contact with a solid. The origin of such low friction is considered to be associated with the water absorbed in the gel [83-86], some of which is squeezed out from the gel under the load and serves as a lubricant layer between the gel and solid surface, resulting in hydrodynamic lubrication [87, 88]. Although the structural information about the interfacial water is important to understand the role of water for the low frictional properties of hydrogel in contact with a solid and the molecular structure of lubricants other than water at solid/solid interfaces have been investigated theoretically [89-91] and experimentally [92-98], no experimental investigations on water structure at gel/solid interfaces have been carried out due to the lack of an effective experimental technique. 

Amadasi, A., Spyrakis, E., Cozzini, P., Abraham, D.)., Kellogg, G. E Mozzarelli, A. Mapping the energetics of water-protein and water-ligand interactions with the natural HINT force field predictive tools for characterizing the roles of water in biomolecules. J. Mol. Biol. 2006, 358, 289-309. 

Hartnig C, SpohrE. 2005. The role of water in the initial steps of methanol oxidation on Pt(lll). Chem Phys 319 185-191. 

Alkylating agents, clarifying the role of water and more generally the role of the general acid catalysis on QM reactivity.13-15... 

The role of water in S02 oxidation over activated carbon is to react with the S03 formed to yield sulfuric acid. This removes S03 from the catalyst... 

By using a mixture of ethyl acetate and D2O as solvent for hydrogenation, up to 75% deuterium is incorporated in the reduced product.13 This result indicates that the role of water here is not only as a solvent. Research on asymmetric hydrogenation in an aqueous medium is still actively being pursued. The method has been applied extensively in the synthesis of various amino acid derivatives.14... 

Recently, catalytic asymmetric Diels-Alder reactions have been investigated. Yamamoto reported a Bronsted-acid-assistcd chiral (BLA) Lewis acid, prepared from (R)-3-(2-hydroxy-3-phcnylphenyl)-2,2 -dihydroxy-1,1 -binaphthyl and 3,5A(trifluoromethy I) - be nzeneboronic acid, that is effective in catalyzing the enantioselective Diels-Alder reaction between a,(3-enals and various dienes.62 The interesting aspect is the role of water, THF, and MS 4A in the preparation of the catalyst (Eq. 12.19). To prevent the trimerization of the boronic acid during the preparation of the catalyst, the chiral triol and the boronic acid were mixed under aqueous conditions and then dried. Using the catalyst prepared in this manner, a 99% ee was obtained in the Diels-Alder reaction... 

Both of the above chemical studies point towards the increased importance of the burning process at 285°C in determining the initial rate of heat production. The role of water as yet remains undefined other than at the higher temperature of 285°C it appears to have the opposite effect on the bitumen sample compared to the process at 225°C i.e., it appears that water vapor encourages pathways by which the various components of bitumen react with oxygen. Preliminary calculations of the total heats evolved during the wet oxidation of bitumen sands indicate that they are independent of the partial pressure of oxygen in the system at... 

We present a molecular theory of hydration that now makes possible a unification of these diverse views of the role of water in protein stabilization. The central element in our development is the potential distribution theorem. We discuss both its physical basis and statistical thermodynamic framework with applications to protein solution thermodynamics and protein folding in mind. To this end, we also derive an extension of the potential distribution theorem, the quasi-chemical theory, and propose its implementation to the hydration of folded and unfolded proteins. Our perspective and current optimism are justified by the understanding we have gained from successful applications of the potential distribution theorem to the hydration of simple solutes. A few examples are given to illustrate this point. 

To investigate the role of water photolysis in ozone production in a prebiotic atmosphere, the following mechanism can be explored ... 

The results of Figure 2 (and other results (16,19,23) require rethinking as to the role of water in the double layer. It is still not known how much water is present in metal double layers emersed into gas ambient. But we have found in previous work that the amount of water in the emersed double layer on Au does not change with potential (23), and Koetz and Neff found that water was essentially absent in UHV (16) at all measured potentials. Results presented here do not decide the question of how much water is in the double layer in UHV, but they may severely restrict its role. Again, if water dipoles play an important role in double layer formation it is hard to see how their removal could cause a shift in work function which is constant with emersion potential. 

---