Question

Draw the zwitterion for each of the following amino acids: a. glycine b. threonine c. glutamic acid d. phenylalanine

Short answer

Glycine: NH3+-CH2-COO-, Threonine: NH3+-CH(CH3OH)-COO-, Glutamic acid: NH3+-CH(CH2CH2COOH)-COO-, Phenylalanine: NH3+-CH(C6H5)-COO-.

Step-by-step solution

Title - Understand Zwitterions

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. For amino acids, the amino group (NH3+) is protonated, and the carboxyl group (COO-) is deprotonated.

Title - Identify Glycine Structure

Glycine is the simplest amino acid with the structure NH2-CH2-COOH. To draw its zwitterion form, the NH2 becomes NH3+ and the COOH becomes COO-.

Title - Draw Zwitterion of Glycine

The zwitterion form of glycine is drawn as NH3+-CH2-COO-.

Title - Identify Threonine Structure

Threonine has the structure NH2-CH(CH3OH)-COOH. To draw its zwitterion form, convert NH2 to NH3+ and COOH to COO-.

Title - Draw Zwitterion of Threonine

The zwitterion form of threonine is NH3+-CH(CH3OH)-COO-.

Title - Identify Glutamic Acid Structure

Glutamic acid has the structure NH2-CH(CH2CH2COOH)-COOH. For the zwitterion form, the NH2 becomes NH3+ and the α-carboxyl group becomes COO- while the side chain carboxyl group remains COOH.

Title - Draw Zwitterion of Glutamic Acid

The zwitterion form of glutamic acid is NH3+-CH(CH2CH2COOH)-COO-.

Title - Identify Phenylalanine Structure

Phenylalanine has the structure NH2-CH(C6H5)-COOH. To draw its zwitterion, convert NH2 to NH3+ and COOH to COO-.

Title - Draw Zwitterion of Phenylalanine

The zwitterion form of phenylalanine is NH3+-CH(C6H5)-COO-.

Key concepts

amino acids

Amino acids are the building blocks of proteins. They consist of a central carbon atom (known as the alpha carbon) bonded to four different groups: an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable R group or side chain. The side chain determines the unique properties of each amino acid. These molecules play a crucial role in various biological processes, including the formation of enzymes, hormones, and neurotransmitters.

Each amino acid can exist in different forms depending on the pH of the environment, influencing their function and interactions.

zwitterions

A zwitterion is a special form of a molecule that contains both a positive and a negative charge but has an overall neutral charge. In amino acids, the amino group is protonated to NH3+ and the carboxyl group is deprotonated to COO-. This unique structure helps amino acids to act as buffers, maintaining stable pH levels in biological systems.

For instance, at physiological pH (around 7.4), amino acids predominantly exist as zwitterions. This dual-charged nature is essential for their solubility in water and their ability to interact with other molecules. Understanding zwitterions is fundamental to grasping how amino acids work in various chemical and biological contexts.

chemical structures

The chemical structure of an amino acid features a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and an R group or side chain. The specific arrangement and composition of these groups determine the chemical characteristics and behavior of the amino acid.

For example, glycine is the simplest amino acid with an R group that is just a hydrogen atom. Threonine has a more complex side chain, represented by CH(CH3OH). Glutamic acid contains an additional carboxyl group in its side chain (CH2CH2COOH), while phenylalanine has a benzyl side chain (C6H5). These diverse structures influence the amino acids' role and function in proteins and other biological molecules.

protonation

Protonation is the process where a molecule gains a proton (H+). For amino acids, this typically occurs at the amino group. When an amino acid is in an aqueous solution, the amino group (NH2) can accept a proton and become NH3+. This protonation results in the amino group holding a positive charge.

This change is pH-dependent; under low pH (acidic) conditions, protonation is more likely to occur. For instance, in the zwitterion form seen in amino acids like glycine or threonine, the amino group is protonated, contributing to the molecule's overall charge balance and functionality in biological systems.

deprotonation

Deprotonation is the removal of a proton (H+) from a molecule, typically resulting in a negatively charged species. In amino acids, this process occurs at the carboxyl group. The carboxyl group (-COOH) loses a proton, becoming -COO-. This deprotonation happens more readily in higher pH (basic) environments.

In the context of zwitterions, the carboxyl group is deprotonated (-COO-), contributing to the overall neutral charge of the molecule. This balance between protonation and deprotonation is vital for the stability and reactivity of amino acids in different biochemical environments.