Question

Discuss the role of the hydrophobic interactions in stabilizing the following. (a) Double-stranded DNA (b) Lipid bilayers (c) Soap micelles

Short answer

In summary, hydrophobic interactions play a crucial role in stabilizing various biological structures, including double-stranded DNA, lipid bilayers, and soap micelles. This is mainly due to their tendency to avoid water, which leads to the formation of nonpolar regions that contribute to the stability and function of these structures. In double-stranded DNA, the hydrophobic interactions occur between stacked base pairs, while in lipid bilayers, they occur between the lipid tails. In soap micelles, hydrophobic interactions contribute to the formation of a nonpolar core that can dissolve nonpolar substances for effective cleaning.

Step-by-step solution

Part (a) - Double-stranded DNA

Hydrophobic interactions play a significant role in the stabilization of double-stranded DNA. The DNA molecule consists of two polynucleotide chains that twist around each other, forming the double helix. Each nucleotide unit in a chain contains a phosphate group, a sugar molecule, and a nitrogenous base. There are four types of bases: adenine (A), cytosine (C), guanine (G), and thymine (T). The hydrophobic interactions in double-stranded DNA occur between the stacked base pairs inside the helix. The nitrogenous bases are hydrophobic, and they tend to avoid water by stacking on one another in a parallel fashion. These hydrophobic interactions contribute to the stability of the double helix, as the bases avoid contact with the polar, aqueous environment, which is present outside the double helix.

Part (b) - Lipid bilayers

Lipid bilayers are a fundamental component of cell membranes. They consist of two layers of lipid molecules, where their hydrophilic (water-loving) polar heads face the water on either side of the bilayer, while the hydrophobic (water-fearing) nonpolar tails face each other, forming the interior of the bilayer. The hydrophobic interactions between the lipid tails are responsible for the stability of lipid bilayers. These interactions cause the lipid tails to avoid water and to interact with each other, leading to the formation of a continuous, nonpolar region in the interior of the bilayer. This nonpolar region acts as a barrier, preventing the passage of ions and polar molecules through the membrane while allowing the passage of nonpolar molecules.

Part (c) - Soap micelles

Soap micelles are formed when soap molecules aggregate in water. Soap molecules have a polar, hydrophilic head, and a nonpolar, hydrophobic tail. When soap is added to water, these molecules arrange themselves into spherical structures, where the hydrophilic heads face the water and the hydrophobic tails are within the interior of the structure, forming a micelle. The hydrophobic interactions between the soap tails are essential for the formation and stability of micelles. These interactions cause the hydrophobic tails to avoid water and to aggregate together, forming the nonpolar core of the micelle. Consequently, the micelle can dissolve nonpolar substances like dirt or grease, which become trapped in the hydrophobic core and can then be washed away by water.