Non-aqueous environments

Many substrates currently produced in the chemical industry are immiscible with water, but are readily miscible with organic solvents. Most enzymes, however, will not operate efficiently, or not operate at all, in non-aqueous media. Some exceptions do exist, such as lipases and esterases, which can operate in non-aqueous environments. Currently, there is considerable interest in extending the range of enzymes that do work in organic solvents. 

In a chemical model for mitrochondrial oxidative phosphorylation/ it has been proposed that the mitochondrial membrane, to which ATP and inorganic phosphate are attached, is held in an extended inactive form (50) by coulombic repulsion of positive charges. On reduction of the membrane by NADH one positive centre is removed, and folding of the membrane can occur with extrusion of water. This creates a non-aqueous environment around the ADP (51) and a metal ion can now catalyse the formation... 

Usually, activities of enzymes (hydrogenases included) are investigated in solutions with water as the solvent. However, enhancement of enzyme activity is sometimes described for non-aqueous or water-limiting surroundings, particular for hydrophobic (or oily) substrates. Ternary phase systems such as water-in-oil microemulsions are useful tools for investigations in this field. Microemulsions are prepared by dispersion of small amounts of water and surfactant in organic solvents. In these systems, small droplets of water (l-50nm in diameter) are surrounded by a monolayer of surfactant molecules (Fig. 9.15). The water pool inside the so-called reverse micelle represents a combination of properties of aqueous and non-aqueous environments. Enzymes entrapped inside reverse micelles depend in their catalytic activity on the size of the micelle, i.e. the water content of the system (at constant surfactant concentrations). 

It is well documented that amines oxidise differently in non-aqueous environments to those pathways seen in aqueous systems. In the former systems, hydrogen abstraction of the a-carbon predominates. The reactivity is in the decreasing order tertiary > secondary > primary amines. Oxidation in non-aqueous systems results in amides, aldehydes and carbon-nitrogen cleavage products [67]. 

Amylose consists of unbranched al 4-linked chains of 200-300 glucose residues. Due the a configuration at C-1, these chains form a helix with 6-8 residues per turn (1). The blue coloring that soluble starch takes on when iodine is added (the iodine-starch reaction ) is caused by the presence of these helices—the iodine atoms form chains inside the amylose helix, and in this largely non-aqueous environment take on a deep blue color. Highly branched polysaccharides turn brown or reddishbrown in the presence of iodine. 

In principle, placing a hydrophilic residue in a non-aqueous environment is energetically unfavorable. In integral proteins with multiple a helices that span the membrane, hydrophilic side chains from different helical segments may interact and in some cases form a channel through which ions may diffuse. Portions of the helical segments exposed to the lipid will contain primarily hydrophobic amino acid residues. 

Despite the advantages that the total non-aqueous environment offers the electroreductive studies on halocompounds, most of the degradation processes have to be applied to part of or total aqueous streams. 

Our studies have shown that both hydration and the placement of "essential" water molecules affect the RMS deviation, but not RMS fluctuation, of the protein when placed in a non-aqueous environment. As hydration increases, the structural similarities of the protein to the crystal structure increases. Although the deviation of the protein from the X-ray structure is higher in organic solvent than in water, the flexibility of the protein is higher in water. The protein remained spherical and the major movement is due to the folding back of the hydrophilic side chains on the protein surface exposed to hexane. The placement of bound water molecules affects the "local" mobility of the protein, mainly the surface loops. The total... 

The secondary structure of proteins may also be assessed using vibrational spectroscopy, fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy both provide information on the secondary structure of proteins. The bulk of the literature using vibrational spectroscopy to study protein structure has involved the use of FTIR. Water produces vibrational bands that interfere with the bands associated with proteins. For this reason, most of the FTIR literature focuses on the use of this technique to assess structure in the solid state or in the presence of non-aqueous environments. Recently, differential FTIR has been used in which a water background is subtracted from the FTIR spectrum. This workaround is limited to solutions containing relatively high protein concentrations. 

In practice, trade-offs between optimization of different membrane functions have to be accepted. For instance, the immobilization of the proton solvent will impede the leaking out of solvent and, thus, help to avoid membrane dehydration and cathode flooding. On the other hand this may only be achievable at the cost of lower proton conductivity. A good theoretical understanding of mechanisms of proton mobility in various aqueous and non-aqueous environments is thus of vital importance. 

One important feature of this tertiary structure is that the centre of the proteins is hydrophobic and non-polar. This has important consequences as far as the action of enzymes is concerned and helps to explain why reactions which should be impossible in an aqueous environment can take place in the presence of enzymes. The enzyme can provide a non-aqueous environment for the reaction to take place. 

Values of A(i.G° for the alkali metal cations together with the value for the tetraphenylarsonium ion are given in table 4.6 for the solvents considered in this chapter. For the alkali metal ions, A(rG° is both positive and negative, the latter values indicating that the ion is more stable in the non-aqueous environment. In the case of the Na ion, varies from —17 kJ moP in hexamethylpho-... 

The sodium azide pathway (Pathway A), Figure 4, begins with a thionylchloride treatment of the free acid to form the acyl chloride. Subsequent treatment with sodium azide may involve a non-aqueous environment (1,2-dimethoxy ethane, "dry method"), or an aqueous medium ("wet method"). The organic azide is recovered from the reaction mixture and converted into the isocyanate by the Curtius rearrangement. This may be accomplished in solid form ("dry method"), or in a non-aqueous solvent like dioxane or DMF ("solution method"). 

It has been a widespread assumption that enzymes are fragile molecules that only work in aqueous environments. However, over the last 20 years numerous reports have appeared that feature a wide range of enzymes used for organic synthesis in non-aqueous environments. The discovery that many enzymes retain their catalytic activity in non-aqueous media is often attributed to Klibanov, despite a few much earlier reports [1]. A non-aqueous system may be required for a given transformation due to solubility properties or to drive a reaction equilibrium (such as lipases working in the synthesis/acylation direction) and can even lead to advantages such as enhanced thermostability or altered substrate selectivity. 

The post-synthesis incorporation of aluminium into the lattice of pure siliceous zeolite-p was attempted using aluminium isopropoxide as aluminating agent in a non-aqueous environment. The XRD structural analysis of the Al-grafted materials showed an increase in the unit cell parameters which was associated with the insertion of aluminium into the framework. Quantitative multinuclear NMR investigation showed that the amount of framework aluminium incorporated into the zeolite lattice was related to the concentration of defect sites in the parent Si-p zeolite. This indicated that the alumination proceeds through a mechanism which involves the reaction between Al(OPr)3 and silanol groups at defect sites. Calcination after alumination led to the completion of the process, whereby octahedral-coordinated aluminium, (partially) attached to the framework, was transformed into tetrahedral-coordinated framework aluminium. 

In a non-aqueous environment a substantial steric repulsion term should also occur. 

Many industrial composite materials contain both fibres (often of different type or form) and particulates, so that their interaction with an environment can be complex. In order to discuss these issues, a simplistic approach will be used to indicate how a durable material can be achieved. One of the most important aspects is the chemical inertness of the polymer matrix. For most purposes, aqueous environments are of greatest significance and they will form the main part of the discussion in this chapter. In the case of non-aqueous environments, thermodynamic considerations can be used to assess the resistance of the matrix to solvent attack. 

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