Hydrophobic bonds, intramolecular

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

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... 

Flavin mononucleotide (FMN)-adenosine and flavin adenine dinucleotide (FAD)-adenosine complexes show quenched triplet lifetimes compared to FMN alone, which is cited as evidence of intramolecular com-plexation between the flavins and adenosine by Shiga and Piette [142]. Adenosine phosphates also form complexes with FAD [143]. The com-plexation between a flavin and adenosine is identical to the intermolecular complexing of adenosine and flavin moieties, in the latter case enforced by hydrophobic bonding [144-146]. Rath and McCormick [147] have examined the riboflavin complexes of a series of purine ribose derivatives... 

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.

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

The Effect on Physical Properties. The most effective reaction of MTG is the crosslinking through 6-(y-Glu)Lys bonds (Fig. 1-b). When e-amino groups of lysine residues act as acyl acceptors, e-(y-Glu)Lys crosslinks are formed. The crosslinking reaction may be both intermolecular and intramolecular and causes significant physical property changes in protein-rich foods. The e-(y-Glu)Lys bonds are covalent bonds which are stable unlike ionic bonds and hydrophobic bonds. Therefore, even the few e-(y-Glu)Lys bonds in foods have a profound effect on the physical properties. 

In Chapter 1 we briefly described an interface as a layer with uncompensated intermolecular forces. The thermodynamics of a liquid interfaces covered with a soluble or insoluble monolayer layer has been describe in detail by many other competent authors and we want to present only the thermodynamic basis needed for the subsequent chapters of this book. Let us consider the interface between water and air. The specific properties of the bulk water, e.g. the freezing point, boiling point, vapour pressure, viscosity, cluster formation and hydrophobic bonds, are well described by long and short-range intermolecular forces and strong and weak intramolecular forces. Israelachvili recently (1992) remarked in a short note on the usefulness of this classification, although it is not clear whether the same interaction is counted twice or two normally distinct interactions are strongly coupled. 

In this equation ctj is Charton s electronic parameter for inductive effects of the substituents. The positive coefficient indicates that electron-attracting substituents are favourable. It is demonstrated in the paper that tt values are not position independent. Besides an intrinsic hydrophobic factor, intramolecular steric and hydrogen-bonding components are also included. The final equation including all compounds is equation (6). 

Some of the interactions that determine the three-dimensional structure of a protein molecule support a compact conformation, whereas others tend to expand the molecule. In aqueous solution hydrophobic parts of the protein are buried as much as possible in the interior of the molecule but in the adsorbed state the hydrophobic residues may be exposed to the sorbent surface, still shielded from water. Therefore, an expanded structure will be promoted upon adsorption if the compact structure in solution is stabilized by intramolecular hydrophobic bonding. More precisely, whether or not adsorbing protein molecules change their structure depends on the contribution from intramolecular hydrophobic bonding, relative to those from other interactions, to the overall stabilization of the structure in solution. In reference ( ) such an analysis of the structure determining factors has been made for HPA and RNase. It leads to the conclusion that HPA, more than RNase, is able to adapt its structure at sorbent surfaces. 

Interaction between hydrophobic amino acid residues stabilizes secondary structures such as a-helices and p-sheets. A reduction of the intramolecular hydrophobic bonding would cause a decrease of such secondary structures, which has indeed been found for, e.g.,... 

The collective effects of the double-bond character of the peptide bond, the intramolecular hydrogen bonding, the hydrophobic bonding, and the L configuration of the peptide residues induce most poly(L-a-amino acids) and protein sequences to adopt the shape of a right-handed a-helix. But there are also exceptions. Despite the l configuration of the peptide residues, poly(L-j8-benzyl aspartate) forms a left-handed a helix. In addition to a helices and the pleated-sheet structures 03 structures), other secondary structures are also known for poly(a-amino acids) (Table 30-2). 

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