Hydrophilic attraction

Most proteins contain more than one polypeptide chain. The manner in which these chains associate determines quaternary structure. Binding involves the same types of noncovalent forces mentioned for tertiary structure van der Waals forces, hydrophobic and hydrophilic attractions, and hydrogen bonding. However, the interactions are now interchain rather than infrachain (tertiary structure determination). The quaternary structure of hemoglobin (four almost identical subunits) will be discussed in Chapter 4, that of superoxide dismutase (two identical subunits) will be discussed in Chapter 5, and that of nitrogenase (multiple dissimilar subunits) will be discussed in Chapter 6. 

Hydrophilic attractions between a protein and an aqueous medium, such as cytoplasm or blood, also help maintain tertiary structure. In a protein dissolved in an aqueous medium, the polypeptide chain is folded so that nonpolar side groups are on the inside of the molecule and polar side groups are on the outside, where they interact with the water. 

It appears that the interactions leading to fibril formation result from the monomeric form, and from change in monomer conformation and hydrophilic attraction of the parallel /J-sheet forms. Fibril formation is also encouraged by contact of the insulin solution with hydrophobic surfaces. Contact with gamma-irradiated PVC leads to instability, apparently induced by chemical changes in the insulin. 

How soaps and detergents work A soap molecule has two different parts one end is hydrophilic, attracted to water, and the other end is hydrophobic, repelled by water. The hydro-phobic part of the soap molecule consists of a long hydrocarbon chain that is structurally similar to oil and is therefore soluble in oil. 

The very polar, ionic end of the soap anion on the left is hydrophilic (attracted to water), and the nonpolar, hydrocarbon portion of the structure is hydrophobic. 

The charged end of the soap and detergent molecule is hydrophilic (attracted to water). The hydrocarbon end of the molecule is attracted to nonpolar solutes but not to water (hydrophobic). A layer of soap or detergent molecules forms around the greasy solute. This envelops the solute particles so the solute particles float away in the water. 

The bilayer structure shown in Fig. 1 maximizes both the hydrophobic interaction of the alkyl chains and the hydrophilic attractions of the polar groups with the aqueous medium and thereby provides a thermodynamically stable structure. However, because there are no strong attractive forces... 

Most dirt on clothing or skin adheres to a thin film of oil. If the oil film can be removed, the dirt particles can be washed away. A soap molecule consists of a long, hydrocarbon-like chain of carbon atoms with a highly polar or ionic group at one end (Figure 15.3). The carbon chain is lipophilic (attracted to or soluble in fats and oils), and the polar end is hydrophilic (attracted to or soluble in water). In a sense, soap molecules are schizophrenic, having two different personalities. Let us see what happens when we add soap to water. 

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