Question

Valinomycin is an antibiotic. It functions by binding \(\mathrm{K}^{+}\) ions and transporting them across the membrane into cells to offset the ionic balance. The molecule is represented here by its skeletal structure in which the end of each straight line corresponds to a carbon atom (unless a different atom is shown at the end of the line). There are as many \(\mathrm{H}\) atoms attached to each \(\mathrm{C}\) atom as necessary to give each \(\mathrm{C}\) atom a total of four bonds. Using the "like dissolves like" principle, explain how the molecule functions. (Hint: The \(-\mathrm{CH}_{3}\) groups at the two ends of each Y shape are nonpolar.)

Short answer

Valinomycin dissolves in cell membranes due to nonpolar groups and transports K+ ions via its polar binding sites.

Step-by-step solution

Understand the Chemical Principle

The 'like dissolves like' principle states that substances with similar polarity tend to dissolve in each other. Nonpolar substances dissolve well in nonpolar solvents, while polar or ionic substances dissolve well in polar solvents or water.

Analyze Valinomycin's Structure

Valinomycin is a large, complex cyclic compound with both polar and nonpolar regions. The skeletal structure shows -CH3 groups, which are nonpolar, and polar functional groups that can interact with K+ ions.

Interaction with K+ Ions

Valinomycin binds to K+ ions through its polar regions. The ion fits into the interior of the molecule where it can be surrounded by polar groups, stabilizing the complex due to complementary ionic and dipole interactions.

Membrane Transport Mechanism

The nonpolar exterior of valinomycin allows it to be soluble in the lipid bilayer. Thus, carrying the K+ ion, it can transport across the hydrophobic portion of the membrane, effectively shuttling K+ from one side to the other.

Conclusion

Valinomycin's mixed polar and nonpolar characteristics allow it to dissolve in cell membranes while binding and stabilizing K+ ions, facilitating their transport across the membrane.

Key concepts

like dissolves like principle

The "like dissolves like" principle is a fundamental concept in chemistry that helps to understand solubility and interactions between molecules. This principle states that substances with similar types of polarity will dissolve in one another.

For example, nonpolar substances like oils are soluble in other nonpolar liquids like petrol. Similarly, polar substances, such as water, tend to dissolve other polar substances or ionic compounds. This principle is crucial when considering how chemicals interact in different environments, particularly in biological systems.

ion transport

Ion transport refers to the movement of ions across a cell membrane, a vital process for many cellular functions. In the case of valinomycin, it acts as a carrier for the potassium ion (\( \mathrm{K}^{+} \)) across the membrane.

The molecule surrounds the ion in a way that makes it possible for the ion to move through the hydrophobic environment of the cell membrane. This ion transport is essential for maintaining the ionic balance within cells and is involved in processes like nerve impulse transmission and muscle contraction.

cell membrane permeability

Cell membrane permeability is a measure of how easily substances can pass through the cell membrane. Membranes are selectively permeable, meaning they allow some things to pass through while preventing others.

Valinomycin affects this permeability by acting as an ionophore, facilitating the transport of \( \mathrm{K}^{+} \) ions across the hydrophobic lipid bilayer of cell membranes. By doing so, it alters the ionic balance that is critical for various cellular mechanisms, including signal transduction and homeostasis.

polar and nonpolar regions

The structure of molecules like valinomycin includes both polar and nonpolar regions, contributing to its unique functions. Polar regions typically have functional groups that can form hydrogen bonds or ionic interactions, enabling them to engage with polar molecules or ions.

Nonpolar regions are composed primarily of hydrocarbon chains or similar groups that avoid interacting with water. This bifunctional nature allows valinomycin to bind ions like \( \mathrm{K}^{+} \) through its polar regions while its nonpolar exterior makes the whole complex soluble in the lipid-rich cell membrane, illustrating the duality required for its role as an ion carrier.