Intramolecular hydrophobic

As has been described in Chapter 4, random copolymers of styrene (St) and 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) form a micelle-like microphase structure in aqueous solution [29]. The intramolecular hydrophobic aggregation of the St residues occurs when the St content in the copolymer is higher than ca. 50 mol%. When a small mole fraction of the phenanthrene (Phen) residues is covalently incorporated into such an amphiphilic polyelectrolyte, the Phen residues are hydrophobically encapsulated in the aggregate of the St residues. This kind of polymer system (poly(A/St/Phen), 29) can be prepared by free radical ter-polymerization of AMPS, St, and a small mole fraction of 9-vinylphenanthrene [119]. 

Hansch C., Anderson, S. (1967) The effect of intramolecular hydrophobic bonding on partition coefficients. J. Org. Chem. 32, 2583-2586. 

Ben-Naim, A., Wiff, J. (1979) A direct measurement of intramolecular hydrophobic interactions. J. Chem. Phys. 70, 771-777. 

Funasaki, N., Hada, S., Neya, S., and Machida, K. Intramolecular hydrophobic association of two chains of oligoethyleneglycol diethers and diesters in water, J. Phys. Chem., 88(24) 5786-5790, 1984. 

Favored conformations of maltose arrived at by free-energy calculations were in good agreement with those for the solid state and in solution.17-18 From thermal-expansibility experiments, it has been suggested that, in aqueous solution, the maltose molecule folds, and undergoes extensive, intramolecular, hydrophobic bonding.19... 

Tc depends on the balance between the ability of the polymer to form hydrogen bonds with water and on the inter- and intramolecular hydrophobic forces. The former raises Tc, while the latter lowers Tc. By incorporating... 

The hydrophobicity and hence the radius of gyration of PVP derivatives can be changed. Despite their increased molecular masses, the alkylated PVP derivatives migrate faster than the starting PVP. This can be explained by intramolecular hydrophobic interactions in the aqueous buffer resulting in a smaller radius of gyration. Also, the effective ionization can be different. It was possible to separate the ethyl and the benzyl derivative from the underivatized PVP with identical contour length. 

Table II shows some literature examples of the breakdown in additivity which can occur in flexible molecules (Compounds 1-4) or in sterically crowded molecules such as the diphenylmethanes (Compounds 5 and 6). In the first four cases, a dipole is probably interacting with polarizable 7r-electrons of the aromatic ring. In Compound 5, the overall shape probably prevents water molecules from forming a solvate iceberg between the two rings—a situation which can be considered an intramolecular hydrophobic bond. In Compound 6 a combination of intramolecular hydrophobic effects and possibly interaction of the side chain dipole with one or both aromatic rings leads to a wide discrepancy between experimentally determined and calculated log F values.

Table V shows some sterically crowded salicylic acid derivatives. Compound 1 is bibenzyl and is included for comparison. The Air value indicates some degree of interaction between the aromatic rings, which can be considered as an intramolecular hydrophobic effect. Log F is known for both salicylic acid and for phenyl methyl sulfide, and these values can be added to give the expected value for Compound 2. Experimentally, log F is smaller than this, so some intramolecular interaction probably occurs.

Table VI shows a progression in the intramolecular hydrophobic interaction between two rings. In diphenyl, additivity holds. In di-phenylethane, which has some conformational freedom, there is a small but quite definite interaction. In the tricyclic Compound 3, the rings are held at a definite angle to one another, with no possibility for rotation. Adding log F values for N-methylaniline and toluene, and making a correction for cyclic methylene groups, we obtain an estimate far greater than the experimental log F value.

Ion interaction in the stationary phase [15] and intramolecular hydrophobic interactions [16] have been mentioned as other possible reasons for the occurrence of system peaks. Knox and Kaliszan [17] have discussed the origin of system peaks in their classical investigation of the methods of void volume determination. 

FIGURE 3. Dependence of molecular size on pH due to carboxylate repulsion (upper) and intramolecular hydrophobic bonding (lower). 

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