Solvation, preferential, hydration

Extensive literature has developed related to the preferential interaction of different solvents with proteins or peptides in bulk solution.156-5X1 Similar concepts can be incorporated into descriptions of the RPC behavior of peptides and employed as part of the selection criteria for optimizing the separation of a particular peptide mixture. As noted previously, the dependency of the equilibrium association constant, /CassoCji, of a peptide and the concentration of the solvent required for desorption in RPC can be empirically described1441 in terms of nonmechanistic, stoichiometric solvent displacement or preferential hydration models, whereby the mass distribution of a peptide P, with n nonpolar ligands, each of which is solvated with solvent molecules Da is given by the following ... 

In the preferential interaction model, the equilibrium association constant, K.ISVIC, for a peptide docked to a RPC sorbent can be related via eq 6 to the changes in the preferential interaction of the solvent and water with the peptide P- the ligands L, and the peptide-ligand complex PtL as a function of solvent concentration, [3]m. The effect of preferential hydration/solvation of a peptide on k can be represented in terms of the following expression t85l... 

Preferential Solvation. Examination of the data of Figure 2 in terms of Equation 3 shows that below 65% chloroethanol, (dmi/dm2)r, io.ni is positive, while above this point it becomes negative. Negative values of this interaction parameter indicate a deficiency of component 1 in the immediate vicinity of molecules of component 2—i.e., preferential hydration of component 2. The extent of hydration is given by Equation 5. 

The solvation of metal ions in mixtures of solvating solvents, for example aqueous alcohols or aqueous dimethyl sulfoxide, provides examples of a special case of ternary complexes and, in preferential solvation, an indication of the relative affinities of the metal ion for water and for the organic cosolvent in question. The preferential hydration of Co2+ in aqueous methanol is shown by... 

An outstanding exception is the solvation of ions in dimethylsulphoxide (DMSO)-water mixtures. While in dimethylsulphoxide AG (Ag ) related to water is negative, in highly aqueous DMSO mixtures Ag is preferentially hydrated The change of preferential solvation of an ion with solvent composition has also been noticed for other ions This behavior can be attributed to the stabilization ... 

The present paper is devoted to the derivation of a relation between the preferential solvation of a protein in a binary aqueous solution and its solubility. The preferential binding parameter, which is a measure of the preferential solvation (or preferential hydration) is expressed in terms of the derivative of the protein activity coefficient with respect to the water mole fraction, the partial molar volume of protein at infinite dilution and some characteristics of the protein-free mixed solvent. This expression is used as the starting point in the derivation of a relationship between the preferential binding parameter and the solubility of a protein in a binary aqueous solution. 

The solvation behavior of a macromolecule such as a protein in a binary aqueous solvent is important in the understanding of such solutions [1-5]. A macromolecule can be preferentially hydrated when the concentration of water in the vicinity of the macromolecule (local concentration of water) is higher than the bulk concentration. The macromolecule can be preferentially solvated when the concentration of the cosolvent in the vicinity of the macromolecule is higher than the bulk cosolvent concentration. A measure of the solvation (or hydration) is the preferential binding parameter [2-6], which can be... 

Many characteristics of a protein in aqueous solvents are connected to its preferential solvation (or preferential hydration). The protein stability is a well-known example. Indeed, the addition of certain compounds (such as urea) can cause protein denaturation, whereas the addition of other cosolvents, such as glycerol, sucrose, etc. can stahihze at high concentrations the protein stiucture and preserve its en2ymatic activity [4-7]. The analysis of literature data shows that as a rule Ffor the former and r23 " <0 for the latter compounds. Recently, the authors of the present paper showed how the excess (or deficit) number of water (or cosolvent) molecules in the vicinity of a protein molecule can be calculated in terms of F2 the molar volume of the protein at infinite dilution and the properties of the protein-free mixed solvent [8]. The protein solubility in an aqueous mixed solvent is another important quantity which can be connected to the preferential solvation (or hydration) [9-13] and can help to understand the protein behavior [9-17]. 

The aim of the present paper is to establish a relation between (1) the preferential solvation (or hydration) of a protein and (2) the protein solubility in an aqueous mixed solvent. The obtained relation will be used to predict the protein solubility in an aqueous solvent in terms of the preferential binding parameter. 

Relation between the Preferential Binding Parameter and the Excesses (Deficits) Number of Water and Cosolvent Molecules around a Protein Molecule. It is well-known from numerous experimental measurements that F < 0 means preferential hydration of a protein in the presence of a cosolvent and F2T 0 means that the protein is preferentially solvated by the cosolvent. , 32,38-39,41-45 relation... 

Let us emphasize the physical significance of the obtained results regarding the excesses (deficits). A n > 0 means preferential hydration (or solvation in the case of toluene + polystyrene mixture) of the solvent molecules, and A i2 > 0 means that the polymer (protein) molecules are preferentially hydrated (or solvated for the toluene + polystyrene mixture). 

In contrast to Ann, Ann 0 for all mixtures investigated, and hence, the polymers or proteins are preferentially hydrated (solvated in the case of toluene + polystyrene mixture). Let us examine separately the contributions to Ann provided by the entropic and enthalpic factors. The contribution to Ann provided by the different sizes of the solvent and polymer (protein) molecules will be evaluated from the excess in an ideal... 

Figure 11.8 Schematic illustration of preferential binding and preferential hydration by solvent additives. In preferential binding the additive occurs in the solvation shell of the protein at a greater local concentration than in the bulk solvent, while preferential hydration results from the exclusion of the additive from the surface of the protein.

As a consequence of the preferential solvation of the dissolved ions in a solvent mixture, one or other solvent may become enriched in the solvate sphere of a given ion, and this changes the relaxation conditions reflecting the solvent-solvent interaction. Equations describing these have been elaborated by Capparelli et al [Ca 78a]. Their method was used to draw conclusions on the changes in the compositions of the solvate sheaths in solutions of magnesium perchlorate, potassium iodide and rubidium iodide in water-methanol solvent mixtures, purely from the influence of the interaction of the water molecules alone on the rate of intermolecular relaxation. It was found that all three cations coordinate water in a higher proportion than corresponds to the composition of the solvent mixture, i.e., they are preferentially hydrated. 

Other studies of the preferential solvation for which information can be derived from KBIs in ternary systems have also been made. The system -heptane -i- ethanol + 1-propanol at 313 K (Zielkiewicz 1995a) showed that ethanol and 1-propanol mix in a random manner in the presence of -heptane with no preferential solvation between these two solvents. The same author studied the solvation of N,N-dimethylformamide (C) in mixtures of water (A) and each of methanol, ethanol, and 1-propanol (B) at 313 K (Zielkiewicz 1995b). At Xc > 0.8 this component was solvated equally by A and B, but at Xc < 0.15 it was preferentially hydrated, that is, solvated by A, except when x > 0.8, where the solvation of C by A and B was random. A,A-dimethylformamide (C) featured also in the studies (Ruckenstein and Shulgin 2001a) of it in aqueous (A) methanol (B). The KBIs in the system n-hexane + 1-hexanol + methyl benzoate were studied at 298 K (Aparicio et al. 2005). They calculated the excess (or deficit) number of molecules of, say. A, around molecules of B in pseudobinary systems at constant mole fraction of C from... 

A word of caution is in order with regard to all such predictions, since in fact the reactions in an APCI source almost all involve solvated (usually hydrated) species, exemplified by the characteristic APCI protonation reaction described in Equation [5.5]. For example, benzene has a higher PA (750 kJ.moP ) than water (691 kJ.moP ), but is not protonated in positive ion APCI conditions because neither the neutral nor ionized form likes to be solvated (after all benzene is essentially immiscible with water) thus benzene is ionized preferentially by charge exchange under positive APCI conditions. 

A special NMR technique, H NOES Y, using the nuclear Overhauser effect, was applied by Bagno et al. [57] to Me NCl and Bu NI in A=W and B = MeCN mixtures, the ions of the former salt being preferentially hydrated and those of the latter one preferentially solvated by the organic component. Both ions of NaGHjCO are preferentially solvated by DMSO in its aqueous mixtures according to this technique. 

Kinetics of aquation, base hydrolysis, cyanide attack, and peroxodisul-fate oxidation have been investigated for the 4-methyl-1,10-phenanthroline complex [Fe(4Me phen)3]. The main interest here is that another demonstration is provided of the dominant role of solvation changes in determining activation volumes when heavily solvated ions, such as cyanide here, are involved. The minimal change in AF on going from water as solvent to 33% methanol is also ascribable to heavy and strongly preferential hydration of the cyanide. " In a similar vein, the zero activation volume for peroxodisulfate oxidation of the [Fe(bipy)(CN)4] anion, surely simple bimolecular outer-sphere electron transfer, can also be explained by balanc-... 

Remarkable data on primary hydration shells are obtained in non-aqueous solvents containing a definite amount of water. Thus, nitrobenzene saturated with water contains about 0.2 m H20. Because of much higher dipole moment of water than of nitrobenzene, the ions will be preferentially solvated by water. Under these conditions the following values of hydration numbers were obtained Li+ 6.5, H+ 5.5, Ag+ 4.4, Na+ 3.9, K+ 1.5, Tl+ 1.0, Rb+ 0.8, Cs+0.5, tetraethylammonium ion 0.0, CIO4 0.4, NO3 1.4 and tetraphenylborate anion 0.0 (assumption). 

Other preparative snags also occur in the addition of HHal to alkenes. Thus in solution in H20, or in other hydroxylic solvents, acid-catalysed hydration (p. 187) or solvation may constitute a competing reaction while in less polar solvents radical formation may be encouraged, resulting in anti-Markownikov addition to give 1-bromopropane (MeCH2CH2Br), via the preferentially formed radical intermediate, MeCHCH2Br. This is discussed in detail below (p. 316). 

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