Apolar solvation energies

Unlike the ion-ion PMF case, substantial apolar surface area is often buried during biomolecular conformational changes and/or complex formation. Therefore, we must also consider the apolar solvation energies discussed in a previous section. For the current example, a constant apolar coefficient method would provide an additional term for the conformational change ... 

Surprisingly, the low solubility of small-sized particles does not stem from a weak interaction of particles with their surrounding water environment (77). For example, the heat of solvation of methane in water at ambient temperature is of similar magnitude as the heat of vaporization of pure liquid methane (80). The positive solvation free energy of small apolar particles at low temperatures is the consequence of negative solvation entropy, which overcompensates for the negative solvation enthalpy. It is widely believed that this entropy penalty is caused by the orientation order introduced to the hydration-shell water molecules as they try to maintain an intact hydrogen bond network (77). Parallel to the entropy decrease observed for low... 

Point 2 above is understandable (Eq. I) since in apolar solvents the energy for charge separation is large and makes electron transfer slow. In the case of aromatic nitriles as acceptors, with which we are particularly concerned, there is obviously also a strong n-interaction that can occur between donor and acceptor, and irradiation in aprotic solvents usually causes no reaction but only quenching of the nitrile monomolecular fluorescence and appearance of a new red-shifted emission attributable to the exciplex (see Section 2.2). Independent solvation of the radical ions is also important. For example, the bichro-mophoric molecules 5 and 6 show strong exciplex emission and... 

Excited states of ketones (compound I) and positive solvatochromic dyes, e.g. diethylamino p-nitrobenzene (compound II of table VII) or related compounds are more strongly solvated by polar solvents when comparki with apolar ones. The shift of the K a transition of substituted p-nitro enoles as well as the n a transition of ketones are used as empirical polarity scales, a and % of a solvent. The energy shifts of the a a transition of substituted pyridinium-N-phenoIate betaines (compound III) and the charge-transfer absorption... 

Smithrud, D.B. Diederich. F. Strength of molecular complexation of apolar solutes in water and in organic solvents is predictable by linear free energy relationships A general model for solvation effects on apolar binding. J. Am. Chem. Soc. 1990. 772. 339-343. 

For few very apolar solutes, the steric term is the main contribution to solvation, but for most solutes of interest the electrostatic term (AGeie) is the main component of the free energy of hydration (Table 2). AGeie can be defined as the work necessary to built up the solute charge distribution in... 

Free energies of solvation in water and chloroform indicate that in general the nucleic acid bases are better solvated in water than in chloroform, but the results are not extremely different. This suggests that the nucleic acid bases have a dual character they bear a large number of polar groups which favor their hydration, but the aromatic cores are probably better solvated in apolar solvents like chloroform. The theoretical results in Tables 6 and 7 agree well... 

Smithrud, D. B. and Diederich, F. (1990) Strength of Molecular Complexation of Apolar Solutes in Water and in Organic Solvents Is Predictable by Linear Free Energy Relationships A General Model for Solvation Effects on Apolar Binding , J. Am. Chem. Soc. 112, 339-343. 

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