Question

Write the formulas of two amino acids with polar side chains and of two amino acids with nonpolar side chains. Explain why the side chains you have chosen have their respective polarities.

Short answer

The formulas of two amino acids with polar side chains are serine (\(\mathrm{NH_2 - CH(-CH_2 - OH) - COOH}\)) and glutamine (\(\mathrm{NH_2 - CH(-CH_2 - CH_2 - CONH_2) - COOH}\)). The side chains of these amino acids have electronegative atoms or charged functional groups, which leads to a separation of charges and a net dipole, making them polar. The formulas of two amino acids with nonpolar side chains are alanine (\(\mathrm{NH_2 - CH(-CH_3) - COOH}\)) and phenylalanine (\(\mathrm{NH_2 - CH(-CH_2 - C_6H_5) - COOH}\)). Their side chains do not have electronegative atoms or charged groups, so they have an even distribution of charges and no net dipole, making them nonpolar.

Step-by-step solution

Review the general structure of an amino acid

Amino acids have a central carbon (C) atom, called the alpha carbon, to which an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H) and a side chain (R) are attached. The side chain varies among different amino acids and determines their properties.

Understand polar and nonpolar side chains

A polar molecule has a net dipole, meaning there is a separation of charges where one part of the molecule is more negatively charged than the other. This is due to differences in electronegativity between the atoms involved. Nonpolar molecules, on the other hand, have no net dipole and an even distribution of charges. Side chains with electronegative atoms, such as oxygen or nitrogen, or with charged functional groups, such as -COOH or -NH2, will be polar. Side chains without such functional groups or atoms will be nonpolar.

Choose and write formulas for two amino acids with polar side chains

Two examples of amino acids with polar side chains are serine and glutamine. 1. Serine: Its side chain has a hydroxyl group (-OH) attached to a carbon atom. The formula for serine can be written as: \[ \mathrm{NH_2 - CH(\underline{R}) - COOH} \] where \(\underline{R}\) = \(\mathrm{-CH_2 - OH}\). 2. Glutamine: Its side chain has an amide functional group (-CONH2) attached to a carbon chain. The formula for glutamine can be written as: \[ \mathrm{NH_2 - CH(\underline{R}) - COOH} \] where \(\underline{R}\) = \(\mathrm{-CH_2 - CH_2 - CONH_2}\).

Choose and write formulas for two amino acids with nonpolar side chains

Two examples of amino acids with nonpolar side chains are alanine and phenylalanine. 1. Alanine: Its side chain has a methyl group (-CH3) attached to the central carbon atom. The formula for alanine can be written as: \[ \mathrm{NH_2 - CH(\underline{R}) - COOH} \] where \(\underline{R}\) = \(-CH_3\). 2. Phenylalanine: Its side chain has a phenyl group (a benzene ring) connected to a methyl group. The formula for phenylalanine can be written as: \[ \mathrm{NH_2 - CH(\underline{R}) - COOH} \] where \(\underline{R}\) = \(\mathrm{-CH_2 - C_6H_5}\) (with C6H5 representing the benzene ring).

Explain the polarities of the chosen side chains

The chosen polar side chains (serine and glutamine) exhibit a separation of charges due to the presence of electronegative atoms (oxygen or nitrogen) or charged functional groups (-OH or -CONH2). These properties give them a net dipole and make them polar. On the other hand, the side chains of the chosen nonpolar amino acids (alanine and phenylalanine) do not have electronegative atoms or charged groups that could lead to a separation of charges. Both the methyl group in alanine and the phenyl group in phenylalanine have an even distribution of charges, resulting in no net dipole, making them nonpolar.

Key concepts

Polar Side Chains

In the realm of amino acids, side chains play a crucial role in determining their polarity. A polar side chain implies that there is a charge separation within the molecule. This happens because certain atoms within the side chain structure have different electronegativities. Atoms like oxygen and nitrogen can attract electrons more strongly than others, leading to this charge separation.
For instance, serine and glutamine are amino acids with polar side chains. Serine has a hydroxyl group (-OH) in its side chain, while glutamine carries an amide group (-CONH2). Both of these groups contain electronegative atoms, either oxygen or nitrogen, which contribute to their polarity. As a result, these amino acids can form hydrogen bonds with water, making them hydrophilic.

Nonpolar Side Chains

Nonpolar side chains, unlike their polar counterparts, lack significant differences in electronegativity among their atoms, leading to an even distribution of charge. This results in no net dipole moment. Amino acids like alanine and phenylalanine exemplify nonpolar side chains.
Alanine has a simple methyl group (-CH3) attached as its side chain, which does not have an electronegative atom, ensuring a nonpolar characteristic. Similarly, phenylalanine features a phenyl group, essentially a benzene ring, which maintains an equal charge distribution. Such side chains are hydrophobic, meaning they tend to avoid water and interact more easily with other nonpolar substances.

Chemical Structure

Amino acids, the building blocks of proteins, share a common basic structure but vary significantly in their side chains. The general formula consists of:

• A central carbon atom (alpha carbon)
• An amino group (-NH2)
• A carboxyl group (-COOH)
• A single hydrogen atom
• A variable side chain (R group specific to each amino acid)

The side chain is what differentiates one amino acid from another, affecting its chemical behavior, polarity, and how it interacts with other molecules.
This variation in side chains can lead to amino acids being classified broadly as either polar or nonpolar, influencing how proteins fold and function in biological systems.

Functional Groups

Functional groups are specific groupings of atoms within molecules that have characteristic properties. They are key in defining the chemical behavior of organic molecules, including amino acids. In amino acids, side chains can contain different functional groups that determine whether the side chain is polar or nonpolar.
For example:

• -OH (hydroxyl group) in serine contributes to its polarity by forming hydrogen bonds.
• -CONH2 (amide group) in glutamine offers sites for hydrogen bonding due to electronegative nitrogen.
• -CH3 (methyl group) in alanine, being nonpolar, doesn't cause charge separation.
• Phenyl ring (benzene) in phenylalanine is a large nonpolar structure, leading to hydrophobic properties.

Understanding these functional groups helps in predicting how amino acids will interact with their surroundings.

Dipole Moments

Dipole moments occur when there is a separation of charge within a molecule, leading to positive and negative ends. It is primarily seen in molecules with polar covalent bonds, where electrons are not shared equally.
In the context of amino acids, polar side chains are responsible for dipole moments.
For instance:

• Serine, with its -OH group, can have points of positive and negative charge, leading to a dipole.
• Glutamine's amide group also leads to dipole moments, providing the molecule with a directional electric charge.

Nonpolar side chains, such as those in alanine and phenylalanine, lack dipole moments because they have uniform charge distributions. The presence or absence of dipole moments impacts how these amino acids behave in physiological environments, especially in protein folding and interactions with other molecules.