Valine is an amino acid with a nonpolar side chain and serine is one with a polar side chain. Draw the two amino acids.
a. Why is the side chain for valine nonpolar, whereas the side chain for serine is polar?

Verified step by step guidance

1. Step 1: Understand the structure of an amino acid. Each amino acid consists of a central carbon atom (called the alpha carbon) bonded to four groups: an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (R group) that determines the properties of the amino acid.
2. Step 2: Draw the structure of valine. The side chain (R group) of valine is an isopropyl group (-CH(CH₃)₂), which consists only of carbon and hydrogen atoms. These atoms share electrons almost equally, making the side chain nonpolar and hydrophobic.
3. Step 3: Draw the structure of serine. The side chain (R group) of serine is a hydroxymethyl group (-CH₂OH), which contains an -OH group. The oxygen atom in the -OH group is highly electronegative, creating a polar bond due to the unequal sharing of electrons between oxygen and hydrogen.
4. Step 4: Explain why valine's side chain is nonpolar. The isopropyl group in valine's side chain lacks electronegative atoms like oxygen or nitrogen, so it does not form significant dipoles. This makes the side chain nonpolar and hydrophobic, meaning it does not interact well with water.
5. Step 5: Explain why serine's side chain is polar. The hydroxyl (-OH) group in serine's side chain creates a dipole due to the electronegativity difference between oxygen and hydrogen. This makes the side chain polar and hydrophilic, meaning it can form hydrogen bonds and interact well with water.

Key Concepts

Amino Acid Structure

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid consists of a central carbon atom, an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group). The nature of the side chain determines the amino acid's properties, such as polarity and hydrophobicity.

Polarity of Side Chains

The polarity of an amino acid's side chain is determined by the presence of functional groups that can form hydrogen bonds with water. Nonpolar side chains, like that of valine, consist mainly of hydrocarbons, making them hydrophobic and less likely to interact with water. In contrast, polar side chains, such as that of serine, contain hydroxyl (-OH) or other polar groups, allowing them to interact favorably with water.

Hydrophobic and Hydrophilic Interactions

Hydrophobic interactions occur when nonpolar molecules or regions of molecules aggregate to avoid contact with water, while hydrophilic interactions involve polar molecules that readily interact with water. These interactions are crucial in determining the structure and function of proteins, influencing how amino acids with different side chains behave in aqueous environments.