Question

Polymers are not very soluble in water, but their solubility increases if they have charged groups. (a) Casein, a milk protein, contains many \(-\mathrm{COO}^{-}\) groups on its side chains. How does the solubility of casein vary with pH? (b) Histones are proteins essential to the function of DNA. They are weakly basic due to the presence of side chains with \(-\mathrm{NH}_{2}\) and \(=\mathrm{NH}\) groups. How does the solubility of a histone vary with pH?

Short answer

(a) Higher solubility at higher pH, (b) Higher solubility at lower pH.

Step-by-step solution

Understand Casein's Ionizable Group

Casein contains many \(-\mathrm{COO}^{-}\) (carboxylate) groups on its side chains. These groups are deprotonated and negatively charged in a basic environment (higher pH) and become protonated and neutral in an acidic environment (lower pH).

Casein's Solubility with pH

At higher pH, casein's \(-\mathrm{COO}^{-}\) groups remain ionized (negatively charged), increasing the solubility of casein in water due to repulsion between the like charges. At lower pH, these groups are protonated (neutral), reducing the solubility as the molecule becomes less charged and more hydrophobic.

Understand Histone’s Ionizable Group

Histones contain \(-\mathrm{NH}_{2}\) and \(=\mathrm{NH}\) (amino) groups. These groups are protonated and positively charged in an acidic environment (lower pH) and become deprotonated and neutral in a basic environment (higher pH).

Histone’s Solubility with pH

At lower pH, histone’s \(-\mathrm{NH}_{2}\) and \(=\mathrm{NH}\) groups are protonated (positively charged), increasing the solubility due to attractions with water molecules. At higher pH, these groups deprotonate (neutral), reducing the solubility as the molecule becomes more hydrophobic.

Key concepts

Protein Solubility

Proteins are complex molecules made of amino acids. They often have many ionizable groups, which can gain or lose electrons. The solubility of a protein in water is influenced by these ionizable groups. When proteins are soluble, they can easily mix with water. This is beneficial in biological and industrial processes where protein functionality is essential.

Examples of proteins with varying solubility include casein in milk and histones in cells. The presence of charged groups on these proteins helps them to dissolve better in water. The extent of this solubility is highly influenced by the surrounding pH.

pH Effects on Solubility

The solubility of proteins in water is greatly influenced by pH, which measures how acidic or basic a solution is. Each protein has specific ionizable groups that react differently to changes in pH. These reactions can affect the charge and, consequently, the solubility of the protein.

In a basic environment, casein's \(-\mathrm{COO}^{-}\) groups are deprotonated, making the protein more negatively charged and more soluble. Conversely, in an acidic environment, these groups become protonated, reducing the protein's solubility. For histones, the opposite is true. Their \(-\mathrm{NH}_{2}\) and \(=\mathrm{NH}\) groups become more positively charged in acidic conditions, increasing solubility. In a basic environment, they lose protons and become less soluble.

Ionizable Groups in Proteins

Proteins like casein and histones contain ionizable groups such as \(-\mathrm{COO}^{-}\), \(-\mathrm{NH}_{2}\), and \(=\mathrm{NH}\). These groups can donate or accept protons depending on the pH of the environment. This can change the overall charge of the protein, influencing its interactions with water molecules.

When ionizable groups are charged, they tend to attract water molecules, enhancing solubility. When these groups are neutral, the protein becomes more hydrophobic and less soluble in water. Understanding the behavior of these ionizable groups is critical in manipulating protein solubility for various applications.

Casein

Casein, a protein found in milk, contains numerous \(-\mathrm{COO}^{-}\) (carboxylate) groups. These groups are negatively charged at high pH levels, making casein more soluble in basic environments. In acidic environments, these groups become protonated and neutral, which decreases the protein's solubility.

This change in solubility with pH is leveraged in processes like cheese-making, where the acidification of milk causes casein to precipitate, forming curds.

Histones

Histones are proteins that interact closely with DNA in cells. They contain \(-\mathrm{NH}_{2}\) and \(=\mathrm{NH}\) groups which become positively charged in acidic environments. This positive charge enhances their solubility in water because they can form hydrogen bonds with water molecules.

At higher pH levels, these groups lose their protons and become neutral, decreasing solubility. The pH-dependent solubility of histones plays a significant role in their ability to bind and regulate DNA.