Question

Measurements on several variants of a DNA double helix are performed. Although the free energy of double helix formation does not vary significantly, the enthalpy and entropy measurements show a large range. A scientist observes that mutations that are favorable with respect to binding enthalpy are entropically disfavored. What effect has the scientist observed and why is this phenomenon common in biological molecules?

Short answer

The scientist has observed enthalpy-entropy compensation, common in biology for balancing stability and flexibility.

Step-by-step solution

Understanding Key Concepts

First, let's understand the terms involved. Enthalpy (\(\Delta H\)) is a measure of the total heat content, and entropy (\(\Delta S\)) measures disorder in a system. In biological systems, changes in these values affect Gibbs free energy (\(\Delta G\)), given by the equation \(\Delta G = \Delta H - T \Delta S\).

Analyzing Enthalpy and Entropy Changes

In this case, mutations are favorable concerning binding enthalpy, meaning the process releases more heat (more negative \(\Delta H\)). However, they are entropically disfavored, indicating a decrease in entropy (\(\Delta S\) is less positive or more negative), suggesting less disorder.

Observing the Compensation Effect

The scientist has observed an enthalpy-entropy compensation effect. This occurs when a favorable change in enthalpy is balanced by an equally unfavorable change in entropy, keeping the free energy change (\(\Delta G\)) relatively constant.

Explaining the Phenomenon's Commonality

This effect is common because biological systems often require a delicate balance between structural stability (enthalpy) and flexibility or disorder (entropy). This balance allows proteins and DNA to maintain their structure while being able to perform dynamic functions in the cell.

Key concepts

Gibbs free energy

Gibbs free energy, often denoted as \( \Delta G \), is a measure that helps us understand whether a process or reaction can occur spontaneously. In simpler terms, it's like a judge that tells us if a chemical reaction will happen on its own or if it needs some help. It combines two important factors: enthalpy (\( \Delta H \)) and entropy (\( \Delta S \)). The equation that links them is:\[ \Delta G = \Delta H - T \Delta S \]

• \( \Delta G \) represents Gibbs free energy change.
• \( \Delta H \) is the change in enthalpy, or heat content.
• \( T \) is the temperature, usually measured in Kelvin.
• \( \Delta S \) signifies the change in entropy, or disorder.

A negative \( \Delta G \) implies that the reaction can happen on its own, meaning it's spontaneous. Conversely, a positive \( \Delta G \) indicates the reaction needs external energy to proceed.
In the context of a DNA double helix, the Gibbs free energy remains fairly constant even though enthalpy and entropy might vary widely. This is due to the enthalpy-entropy compensation, where changes in one component are often offset by the other.
This intricate balance is crucial for many biological processes, ensuring they proceed smoothly and efficiently.

DNA double helix

The DNA double helix is a remarkable structure that resembles a twisted ladder. It's composed of two long strands that wind around each other, with each strand made up of nucleotides. Each nucleotide is in turn made of a sugar group, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

• The bases pair specifically, A with T and C with G, facilitating the helix's structure.
• The hydrogen bonds between these base pairs keep the two strands together.
• The sequence of bases along the DNA strand carries genetic information crucial for biological development and function.

Enthalpy and entropy play significant roles in the formation and stability of the DNA double helix.
Enthalpy relates to the hydrogen bonds that hold the double helix together, while entropy involves the freedom and randomness of the nucleotide chains.
When mutations occur, they can alter these properties, affecting how the helix forms and functions. Understanding these changes is essential for insights into genetic variations and their impacts.

Biological molecules

Biological molecules, the building blocks of life, include proteins, nucleic acids, carbohydrates, and lipids. These macromolecules have unique structures and functions, pivotal in sustaining life processes. The interplay of enthalpy and entropy within these molecules often triggers the enthalpy-entropy compensation effect.

• Proteins are made of amino acids and fold into intricate structures essential for their function.
• Nucleic acids like DNA and RNA store and transfer genetic information.
• Carbohydrates provide energy and structural support to cells.
• Lipids form cellular membranes and serve as signaling molecules.

In biological molecules, a perfect balance between structure (enthalpy) and flexibility (entropy) is crucial.
For instance, protein stability often requires a careful trade-off between strong chemical bonds (high enthalpy) and the molecule’s ability to move and interact with others (high entropy).
This balance allows molecules to perform complex tasks, supporting the dynamic nature of cells and organisms. The enthalpy-entropy compensation observed in DNA can also be extended to other biological molecules, illustrating the universal need for stability and adaptability in life's chemistry.