Small Molecule Solutions Including Aqueous Systems

2 Small Molecule Solutions Including Aqueous Systems 

At temperatures well below UCST, solubilities of hydrocarbons in water or water in hydrocarbons drop to very low values. The solutions are very nearly ideal in the Henry s law sense, and the isotope effects on solubility can be directly interpreted as the isotope effect on the standard state partial molar free energy of transfer from the Raoult s law standard state to the Henry s law standard state. Good examples include the aqueous solutions of benzene, cyclohexane, toluene, 

5 Condensed Phase Isotope Effects Isotope Effects in Non-ideal Gases 

Very large isotope effects like those shown in Fig. 5.12 seem to be limited to the hypercritical regions of phase diagrams, i.e. not too far from thermodynamic divergences of the type (dP/dT)c = 00 or (dT/dP)c = 00 (i.e. pressure-double critical points (p-DCP) or temperature-double critical points (T-DCP), respectively). 

There is a good deal of data which compares the solubilities of H2 and D2 in various solvents, but by far the most extensive information on gas solubility IE s compares gas solubilities in H20 and D20 (Table 5.11). The solubility of D2 is considerably higher than H2 in all solvents investigated (Table 5.12). The isotope effect increases sharply as temperature falls (although not shown in Table 5.12). The isotope effect increases sharply as temperature falls (although not shown in Ta- 

---

Additional modes of HPTC include normal phase, where the stationary phase is relatively polar and the mobile phase is relatively nonpolar. Silica, diol, cyano, or amino bonded phases are typically used as the stationary phase and hexane (weak solvent) in combination with ethyl acetate, propanol, or butanol (strong solvent) as the mobile phase. The retention and separation of solutes are achieved through adsorp-tion/desorption. Normal phase systems usually show better selectivity for positional isomers and can provide orthogonal selectivity compared with classical RPLC. Hydrophilic interaction chromatography (HILIC), first reported by Alpert in 1990, is potentially another viable approach for developing separations that are orthogonal to RPLC. In the HILIC mode, an aqueous-organic mobile phase is used with a polar stationary phase to provide normal phase retention behavior. Typical stationary phases include silica, diol, or amino phases. Diluted acid or a buffer usually is needed in the mobile phase to control the pH and ensure the reproducibility of retention times. The use of HILIC is currently limited to the separation of very polar small molecules. Examples of applications... 

In the course of preparing the SEC calibration curve. It is necessary to establish the F, F, and Fr for a series of compounds. It is important to choose solutes that are totally excluded for the F and ones that are totally included and have no interaction with the support for the total volume (F ). DNA is a good molecule for determination of exclusion limits in aqueous systems near neutral pH. Blue dextran may be suitable, but it should be used with caution because it may bind irreversibly to certain columns. A hydrophilic small molecule such as... 

The traditional qualitative uses of Ln + complexes to simplify complex NMR spectra in organic solvents, reviewed extensively, have been replaced by the use of high magnetic fields and two- and three-dimensional NMR methods. Applications of chiral SRs have also been extensively reviewed. This chapter describes LIS methodologies for those who wish to apply them to their particular systems, and their recent quantitative applications as NMR structural probes in aqueous solution of small Ln + complexes including some of their supramolecular constructs, as well as of small biological molecules and proteins, and as SRs for in vivo NMR spectroscopy. 

In this review isentropic compressibility data have been compiled for aqueous solutions of the amino acids, including all those found in proteins, of various peptides of low molar mass, and of many proteins. For both the small molecule and protein systems, it is clear that this thermodynamic property is a particularly sensitive measure of hydration effects in aqueous solution. For the small solutes attempts have been made to rationalize the compressibility data in terms of the interactions that occur between the various functional groups and solvent water. For proteins it has been shown that the compressibilities are not correlated with any one structural characteristic. Various characteristics such as amino acid composition, hydrophobicity and the degree of secondary structure all influence, to some degree, the compressibility of a protein. Compressibility measurements on protein solutions also provide an important means to determine the volume fluctuation of a protein. We believe that compressibility measurements on aqueous solutions of these biologically important molecules provide a very powerful means of probing and characterizing solute -water interactions in these systems. 

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). 

---