Water mixture, conformation

The difference in replacement of the lEP for 19a (pHiEp = 3.6) and 19b (pHiEP = 2.9) is probably connected with the different hydrophobicity of polycarboxybetaine chains, e.g., the cationic group of 19b is more effectively screened by ethyl substituents, as they are bulkier than the methyl groups of 19a. The effect of organic solvent on the solution behavior of 19b was studied [193]. In saline water, the conformation of 19b is compact due to the screening of the electrostatic repulsion by the neutral salt KCl. Addition of ethanol improves the thermodynamic quality of the solvent with respect to the hydrophobic parts of the macromolecules, and the reduced viscosity increases and has the maximal value at 60-80 vol % of ethanol in a water/ethanol mixture. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

In most cases, the complex array of interactions at work within an actual surfactant-water mixture leads to variations of the surfactant parameter with surfactant dilution and temperature. In general then, the phase behaviour of a binary surfactant-water mixture follows a curved trajectory through the local/global domain plotted in Fig. 4.11. If these variations in molecular conformation are small, the phase progression with water dilution is expected to follow a nearly-vertical line in the plot if the molecular architecture is sensitive to these external parameters, die succession of phases with water dilution is more nearly horizontal. 

Intermolecular NMR-spectroscopic Nuclear Overhauser Enhancements of [C4mim] [BFJ-water mixtures led to the suggestion that, in addition to anion-water interactions, solvation (hydration) of the cation also occurs [96], Water replaces the cation-anion hydrogen bonds and also reduces cation-cation associations. Conformational changes in the structure of the butyl chain with respect to the plane of the imidazolium ring were suggested, which were evidenced in various studies in 2003 [114-117],... 

Wang JM, Takeda A, Yang JT, and Wu CSC. Conformation of Concanavalin A and its Fragments in Aqueous Solution and Organic Solvent Water Mixtures. J Protein Cheml992 11 157-164. 

D. Sprous, M. A. Young, and D. L. Beveridge, Molecular dynamics studies of the conformational preferences of a DNA double helix in water and an ethanol/water mixture theoretical considerations of the A B transition, J. Phys. Chem. B 102, 4658-4667 (1998). 

Preliminary X-ray analytical data of [Phe4, Val6] antamanide, crystallized from organic solvent-water mixtures, indicate that in polar solvents the peptide assumes a conformation different from either the one in non-polar solvents or in the com-... 

The trichlorothiatriazine 7 can be converted with secondary amines bearing bulky substituents into the corresponding iV rV-dialkylthiatriazin-l -amines 8.42,71 These compounds are relatively stable and can be recrystallized from alkanol/water mixtures. It is recommended recrystallizing amounts of maximum 3-5 g. In some cases decomposition of this type of thiatriazines is observed. The conformation and molecular structure of 8a are known.29... 

NMR (self-diffusion and H-NMR) and ATR-FTIR measurements were used to evaluate conformational modifications at the molecular level and demonstrated that the GMO/TAG/12.5 wt% water mixture undergoes interrelated structural changes as a function of temperature that led to the formation of a fluid Hu phase. 

As shown in Figure 5, in 40 and 50% methanol-water mixtures, the behaviour of PMA solutions becomes similar to that of PAA, i.e. there is no longer a detectable discontinuity in both potentiometric and viscosity titrations. It can thus be admitted that hydrophobic interactions are virtually destroyed and therefore the conformational behaviour of PMA follows directly the electrostatic repulsions between the carboxylic groups [10],... 

The different titrations curves are extrapolated to pK=3.6. From Figure 7, it appears that for both acetone-water mixtures containing respectively 42% and 50% of acetone, the curves superimpose in the whole range of ionization this already justifies their use as correct reference curves. Moreover, viscosity measurements show also that conformational transition no more exists in such mixtures. For high a, the curves also merge into a single one except for water and acetone-water (12%) where it can be admitted that the transition is not completed at a=1. 

As AO is not soluble in acetone-water mixtures, the effect of AO on the conformation of PLL has been investigated in 50% methanol-water mixtures. 

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