Apolar contributions

It is conceptually convenient to define the enthalpy changes with reference to the temperature at which the apolar contribution to the overall AH° is zero (Baldwin, 1986), so that the apolar enthalpy change AHap is simply... 

Fig. 1. Analysis of the apolar contribution to the dissolution thermodynamics of cyclic dipeptides into water. Each thermodynamic quantity is plotted against the number of apolar hydrogens (aH) (i.e., hydrogens bonded to carbon) (a) AC , (b) AH°, (c) AS0, and (d) AG°. Lines are the linear regression fit of the data. As described in the text, the slope gives the hydrophobic contribution. Data are from Murphy and Gill (1990).

Because the apolar contribution to AH° of dissolution into water at 25°C is negative for the solids rather than near zero as seen in the... 

The apolar contribution to AS0, ASap, is better characterized than AHap. The value of Tt has been shown to be a universal temperature for all processes involving the transfer of an apolar surface into water and has a value of 112°C (Murphy et al., 1990). At this temperature the AS0 of transfer, ASf, represents the mixing entropy of the process. The universal value of Tt was determined using mole fraction concentration units, so that the liquid transfer ASf takes on a value of zero. The value of Tt remains the same using the local standard state of Ben-Naim (i.e., molar concentration units) (Ben-Naim, 1978), but the value of Ais increased by R ln(55.5), where R is the gas constant and 55.5 is the molarity of water. 

Recently it has been shown that convergence of thermodynamic quantities at some temperature will occur when there are two predominant interactions that independently contribute to the thermodynamics (i.e., group additivity), provided that one of these interactions is constant for the set of compounds being investigated (Murphy and Gill, 1990, 1991). For example, the 1-alkanols have varying amounts of apolar surface, but each compound has only one —OH group. Under these conditions it was demonstrated that the apolar contribution to AH° is zero at (i.e., Th = Th) and that the... 

In contrast to the relatively constant number of hydrogen bonds per residue, a set of proteins must bury variable amounts of apolar surface area in order to show convergence (Murphy and Gill, 1991). At the temperature at which the apolar contribution to AH° is zero, no variation would be observed in AH° per residue and the constant polar contribution is all that should be observed. The breakdown into polar and apolar interactions can also be viewed in terms of buried surface area. Proteins bury an increasing amount of surface area per residue with increasing size, but the increase is due to increased burial of apolar surface, whereas the polar surface buried remains constant. This is illustrated in Fig. 2 for 12 globular proteins that show convergence of AH°. These proteins bury a constant 39 2 A2 of polar... 

The value of AH can also be compared to the helix unfolding AH0 of Scholtz et al. (1991). The buried surface area, relative to the extended chain, was calculated for a 50-residue alanine a helix. An average of 19.5 A2 of polar surface is buried per residue and an average of 3.2 A2 of apolar surface is overexposed (i.e., is less accessible in the extended chain than in the helix). Using the fundamental parameters for the polar and apolar ACP described above, a value of — 6.5 cal K-1 (mol res)-1 is estimated for ACp for the helix denatur-ation. At 100°C the extrapolated value of AH0 is about 1.0 kcal (mol res)-1, again in reasonable agreement with the value of AH of 1.35 kcal (mol res)-1. These results strongly support the assertion that the apolar contribution to AH0 is close to zero at 7h. 

As has been discussed recently (Murphy etal., 1992), the formalism developed by Lee (1991) predicts not only the convergence behavior discussed by the author (i.e., when the apolar and polar contributions to the enthalpy are identical). For the case in which the buried polar area per residue is constant it also predicts convergence at the point at which the apolar contribution to the enthalpy is zero. In Fig. 3 we have plotted AH0 versus ACp normalized either per residue (Fig. 3a) or per buried total surface area (Fig. 3b), in order to compare the results of the two approaches. It is clear that the linearity is better when the data are normalized to the number of residues than when they are normalized to the buried surface area. This is presumably due to variabilities in the surface area calculation. The slope of the line in Fig. 3a is —72.4, which corresponds to a convergence temperature, Th, of 97.4°C for this set of proteins. If the above analysis is correct, then this temperature corresponds to The value of AH is 1.32kcal (mol res)-1 or 33.6 cal (mol A2)-1 of polar surface area. 

Fig. 3. Linear correlation of AH0 versus ACp at 25°C for protein denaturation for the proteins listed in Table IV. (a) Normalized per number of residues (b) normalized per total buried area. The line in (a) is the linear regression fit with a slope of — 72.4 corresponding to a convergence temperature of 97.4°C. The line in (b) represents the line calculated from the parameters in Table II for convergence at the temperature at which the polar and apolar contributions to AH are equal per unit area. See text for details.

The decrease of the polarity of the polymer chain microenvironment results, as for one-component solvents, from the following factors (a) the apolar contribution of the polymer backbone and its substitutents (i.e., shielding against polar and mobile solvent molecule), (b) the structure of the solvent in the vicinity of the polymer, and (c) reduced "solvation" of dipole reporter SB molecules by groups of the polymer chains since they are less polar and are not quite free to orient their dipoles toward the embedded compound SB. 

With preferential sorption of one component of the binary solvent on the polymer coil, an increase or decrease of the polarity of the polymer microenvironment occurs depending on whether the more polar (water) or less polar (organic solvent) component is sorbed. Preferential sorption occurs for PHEMA in 1-propanol/water, dioxane/water, and acetone/water mixtures (Figures 4 and 5). When the more polar component (water) is preferentially sorbed from mixtures in which its concentration is low, then the apolar contribution of the polymer may be compensated to that extent, since the polarity of the polymer chain microenvironment is even higher than the bulk solvent polarity. As a result, the curves of the dependence of Ej for the polymer on the solvent composition intersect the same dependence for mixed solvents. This phenomenon was observed for PHEMA in 1-propanol/water (Figure 4), dioxane/water, and acetone/water (Figure 5). Preferential sorption is also indicated by the results for PMMA and PBMA in methanol/toluene mixtures. Preferential sorption was previously found in this system by dialysis equilibria. ... 

Based on these contributions (a-d), we may arrive at the predictive scheme presented in Table 1. Because of the relatively large contribution from dehydration, essentially all proteins adsorb from an aqueous environment on apolar surfaces, even under electrostatically adverse conditions. With respect to polar surfaces, distinction may be made between proteins having a strong internal coherence ( hard proteins) and those having a weak internal coherence ( soft proteins). The hard proteins adsorb at polar surfaces only if they are electrically attracted, whereas the structural rearrangements (i.e., reductions in ordered structure) in the soft proteins lead to a sufficiently large increase in conformational entropy to make them adsorb at a polar, electrostatically repelling surface. 

An ideal polarity probe based on photoinduced charge transfer and solvent relaxation should (i) undergo a large change in dipole moment upon excitation but without change in direction, (ii) bear no permanent charge in order to avoid contributions from ionic interactions, (iii) be soluble in solvents of various polarity, from the apolar solvents to the most polar ones. 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

Another important result that was obtained recently concerns the evaluation of the contribution to the reorganization energy arising from the polarization of the medium, protein and solvent from a microscopic model including the residual charges and induced dipoles of the protein as well as bound water molecules, a value of about 0.2 eV was calculated for different eleetron transfer processes [97], This weak value results from the apolar character of the medium, and is compatible with the kinetic data which indicate that reorganization energies are small in the reaction center (Sect. 3.2.2)... 

When the urea and thiol are removed by dialysis (see p. 78), secondary and tertiary structures develop again spontaneously. The cysteine residues thus return to a suf ciently close spatial vicinity that disulfide bonds can once again form under the oxidative effect of atmospheric oxygen. The active center also reestablishes itself In comparison with the denatured protein, the native form is astonishingly compact, at 4.5 2.5 nm. In this state, the apolar side chains (yellow) predominate in the interior of the protein, while the polar residues are mainly found on the surface. This distribution is due to the hydrophobic effect (see p. 28), and it makes a vital contribution to the stability of the native conformation (B). 

Attractive forces (sometimes also referred to as dispersion forces) between apolar molecules arising from the mutual polarizability of the interacting molecules. London forces also contribute to the interactive forces between polar molecules. See also van der Waals Forces Noncovalent Interactions... 

---