Solvent-induced interactions

The hydrophobic interaction term is used to describe the tendency of non-polar groups or molecules to aggregate in water solution. Hydrophobic interactions are believed to play a very important role in a variety of processes, specially in the behavior of proteins in aqueous media. The origin of this solvent-induced interactions is still unclear. In 1945 Frank and Evans proposed the so-called iceberg model where emphasis is made on the enhanced local structure of water around the non-polar solute. However, computational studies and ex-... 

We discuss here some aspects of solvent-induced interactions between H0O and Hsimplest example is a simple non-polar atom, say, neon or argon, and a water molecule. These are pure H(j)0 and Htf>I molecules. The more common cases are molecules that contain both HtpO and H0/ groups, such as alcohols, fatty acids, and proteins. 

If a solute A approaches a water molecule w in the gaseous phase, it has the same probability of attaching to any region on the surface w. (This is not precisely the case for real water molecules. However, we make this assumption to highlight the new effect of the solvent-induced interaction in producing the orientational preference discussed below.)... 

We now turn to the case of the intramolecular H(f>0—H I interaction. We present here one example of the solvent-induced interaction between a H(f)0 group, say, methyl, and an Htpl group, say, hydroxyl, on a benzene ring. Consider the formation of 2-methylphenol from toluene and phenol (Fig. 4.25a). The relevant reaction is... 

Solvent Induced Interactions Between Two Hydrophilic H0/Groups... 

In the previous sections of this chapter we have considered the solvent induced interactions between HcpO groups. We now discuss the solvent induced interactions between two hydrophilic H(pl) groups or molecules. 

Any interaction between two polar molecules can be described in terms of the superposition of charge-charge interactions. In a solvent, these interactions are modified by introducing the dielectric constant of the medium s. Thus, the solvent-induced interaction can be defined as the difference between the modified interaction and the vacuum interaction, i.e. 

We now turn to examining the question of the potential of mean force PMF), or the solvent-induced interaction between two H0J molecules or groups that interact either between themselves or with the solvent by a pair potential of the form (4.8.3). The simplest and the most important H(f>I molecules are water molecules themselves. We next consider the H I interactions between two water molecules at some specific configurations. 

Here, we have split the Gibbs energy change into two parts the direct AU and the indirect SG. The indirect part is further split into two contributions the first due to the process of bringing two LJ particles to the final configuration (i.e. the solvent-induced interaction when the HB part of the potential is switched off ), and the second due to turning... 

Repeating the same arguments for two functional groups attached to a polymer, we can reach the same conclusion namely, the solvent-induced interaction between two functional groups at R, is large and positive, of the order of... 

Here r is the center-to-center distance, and 0)i and (O2 are the orientations of two involved molecules in solvent.. ss(t, <01,0)2) h the total correlation function of the solute species (two solutes could be in different species but for simphcity we assume they are identical molecules). In the second fine in Eq. (70), the first term is the direct interaction of the solute molecules, and the second term is the solvent-induced interaction with yss(t,[Pg.58]

Appendix C. Estimate of the Solvent-Induced Interactions between Two, Three,... 

This function will appear in the study of solvent-induced interaction in Chapter 7. The quantity defined in (5.6.3) is the average local density of particles at R3 (with orientation H3) of a system subjected to an external field of force produced by fixing two particles at Xi and X2. The arguments for this interpretation are exactly the same as those given in section 5.2. 

From now on we shall treat the extreme dilute solution of s in vv. We shall drop the subscripts s whenever we discuss the potential of average force between two solute particles. Suppose we have two simple solutes s and we are interested in the solvent-induced interaction at some distance R = a, where a is of the order of the molecular diameter of the solute. Relation (7.14.6) can be written as... 

When we say that the solvent-induced interaction between charged particles is well understood, we mean that the phenomenon can be well described by the macroscopic dielectric constant . This, of course, is not a molecular theory of the dielectric constant. In principle one should be able to express the dielectric constant in terms of molecular parameters, e.g., the dipole moments of the molecules. 

This is quite an unimpressive minimum well. However, in considering the contribution of HBing between two H0I groups in biomolecules, we have to deal with two groups that have fixed orientation relative to each other and that are attached to some carrier or backbone. The situation is the same as in Fig. 7.21 but with H0I groups, say hydroxyl or carbonyl, replacing the methyl group. Therefore, we should examine the conditional contribution of the solvent-induced interaction between two H0I at some fixed relative orientation. 

We are interested here in the solvent-induced contribution 5G. Following similar reasoning as in section 7.14, we can write the total solvent-induced interaction as... 

We discuss here some aspects of solvent-induced interactions between H0O and Hcpl molecules or groups. The simplest example is a simple nonpolar atom, say neon or argon. 

In the previous four sections, we discussed the solvent-induced interactions between a pair of solvatons. These could be H0O-H0O, H0I-H( I, or a mixed H0O-H I pair. 

The extension to any number of solvatons is quite straightforward. In this section we discuss some aspects of the solvent-induced interactions among many simple H0O solvatons in water. An extension to more complex molecules is discussed in section 8.9. 

As a prototype of our new measure we consider the neopentane molecule. Using the same arguments as in section 7.15, the solvent-induced interaction among five methane molecules brought to the final configuration of a neopentane molecule (see Fig. 7.32) is written as follows ... 

Thus for a small conformational change the difference between the solvation Gibbs energies of A and B is simply the conditional indirect or solvent-induced interaction between the two FGs, given a.f The ratio of the solubility of A and B is therefore... 

Next, we estimate the solvent-induced interaction among three groups forming an equilateral triangle with an edge of 4.5 A and oriented so that they can simultaneously form three HBs with one water molecule, as in Fig. C.lb. If we neglect quantities of order one, we have approximately, as in (C.3),... 

The principle of transferability is commonly used in the construction of the intramolecular potential function of a macromolecule. It has been recently used to construct intermolecu-lar interactions or solute-solvent interaction. The main idea is to transfer the parameters describing the interaction between small molecules, e.g., methane and water, on to larger molecules, say methane-ethanol, or ethane-water. In this book we used a similar idea to extract information from small model compounds and apply it to biopolymers. The information we are interested in is the conditional solvation Gibbs energies of various groups, e.g., methyl, ethyl, hydroxyl, and so on, and intramolecular solvent-induced interactions between such groups. In this appendix we describe the methodology of this transferability principle and examine its adequacy and extent of its reliability. 

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