Dry Organic Solvents Research Articles

Enzymatic catalysis in nonaqueous solvents.

Subtilisin and alpha-chymotrypsin vigorously act as catalysts in a variety of dry organic solvents. Enzymatic transesterifications in organic solvents follow Michaelis-Menten kinetics, and the values of V/Km roughly correlate with solvent's hydrophobicity. The amount of water required by chymotrypsin and subtilisin for catalysis in organic solvents is much less than needed to form a monolayer on its surface. The vastly different catalytic activities of chymotrypsin in various organic solvents are partly due to stripping of the essential water from the enzyme by more hydrophilic solvents and partly due to the solvent directly affecting the enzymatic process. The rate enhancements afforded by chymotrypsin and subtilisin in the transesterification reaction in octane are of the order of 100 billion-fold; covalent modification of the active center of the enzymes by a site-specific reagent renders them catalytically inactive in organic solvents. Upon replacement of water with octane as the reaction medium, the specificity of chymotrypsin toward competitive inhibitors reverses. Both thermal and storage stabilities of chymotrypsin are greatly enhanced in nonaqueous solvents compared to water. The phenomenon of enzymatic catalysis in organic solvents appears to be due to the structural rigidity of proteins in organic solvents resulting in high kinetic barriers that prevent the native-like conformation from unfolding.

Nucleophilic ion pairs. Part II. Micellar catalysis of proton abstraction by hydroxamate anions

Hydroxamate anions in a cationic (cetyltrimethylammonium bromide, CTAB) micelle were found to serve as excellent catalysts for proton abstraction from α-ketols. Proton removal from benzoin by N-methylmyristohydroxamate anion (MMHA) in the CTAB micelle was 3 500 times faster than the corresponding reaction in a non-micellar system, and 56 times faster than that by the much more basic hydroxide ion. A similar, dramatic rate enhancement was attainable by addition of small amounts of CTAB or methyltri-n-octylammonium chloride (TMAC) to MMHA solubilized in aqueous non-ionic micelles. On the other hand, the MMHA, NEt4 ion pair in organic solvents was also effective for proton abstraction. MMHA, NEt4 in highly dehydrated acetonitrile gave a rate 103 higher than that in aqueous media. Minute amounts of H2O and Nal efficiently quenched the rearrinn in dry organic solvents, and it was inhibited in protic solvents. It was suggested from these experimental data that the rate enhancement of MMHA in the oresence of a cationic micelle largely stems from formation of a hydrophobic ion pair which is loose and desolvated in the micellar phase.

Heat of immersion studies on characterized silicas

A differential wetting calorimeter suitable for measuring heats of immersion of solids in either aqueous solutions or dry organic solvents is described. With this apparatus the heats of immersion of a number of well-characterized Aerosil silicas in both water and benzene have been measured. The heat of immersion of unannealed silicas in water is dependent on the temperature of the wetting process. For annealed silicas this temperature dependence is not always present. These results indicate that the heat of wetting contains terms which reflect disturbances of the liquid layers surrounding the solid. Also, for microporous adsorbents the heats of wetting cannot be reliably expressed in terms of heats per unit area of surface when the area is determined by the B.E.T. method.