Question

Draw the condensed structural formula for each of the following amino acids at pH below \(1.0\) : a. glycine b. cysteine c. serine d. threonine

Short answer

a. H3N+-CH2-COOH, b. H3N+-CH2-CH(SH)-COOH, c. H3N+-CH2-CH(OH)-COOH, d. H3N+-CH2-CH(CH3)(OH)-COOH

Step-by-step solution

Understanding the Concept

At pH below 1.0, amino acids are fully protonated. This means that the carboxyl group (-COOH) remains as -COOH and the amino group (-NH2) is protonated to form -NH3+.

Draw Condensed Structural Formula for Glycine

Glycine has the simplest structure among amino acids with the formula H2N-CH2-COOH. Since we are at pH below 1.0, the structure becomes H3N+-CH2-COOH.

Draw Condensed Structural Formula for Cysteine

Cysteine has a side chain containing a thiol group with the formula HS-CH2-CH(NH2)-COOH. At pH below 1.0, the structure becomes H3N+-CH2-CH(SH)-COOH.

Draw Condensed Structural Formula for Serine

Serine contains a hydroxyl side chain with the formula HO-CH2-CH(NH2)-COOH. At pH below 1.0, the structure becomes H3N+-CH2-CH(OH)-COOH.

Draw Condensed Structural Formula for Threonine

Threonine has an additional methyl group on the side chain with the formula CH3-CH(OH)-CH(NH2)-COOH. At pH below 1.0, the structure becomes H3N+-CH2-CH(CH3)(OH)-COOH.

Key concepts

amino acids

Amino acids are the basic building blocks of proteins. They contain a central carbon atom (known as the alpha carbon) that is attached to four different groups:

• An amino group (-NH2)
• A carboxyl group (-COOH)
• A hydrogen atom
• A side chain (R group) that varies between different amino acids

The side chain is what distinguishes one amino acid from another. For example, glycine has just a hydrogen atom as its side chain, whereas cysteine has a thiol group (-SH). Each amino acid can exist in different protonation states depending on the pH of its environment.

protonation

Protonation refers to the addition of a proton (H⁺) to an atom, molecule, or ion. In the case of amino acids, protonation affects both the amino and carboxyl groups. At very low pH levels (below 1.0), both groups are fully protonated:

• The amino group (-NH2) becomes -NH3+
• The carboxyl group (-COOH) remains as -COOH

These changes in protonation affect the overall charge and structure of the amino acid, impacting how it interacts with other molecules and environments. Understanding protonation is crucial for predicting the behavior of amino acids in different pH conditions.

pH levels

pH measures the acidity or basicity of a solution. The pH scale ranges from 0 to 14:

• pH below 7 is acidic
• pH of 7 is neutral
• pH above 7 is basic (or alkaline)

At different pH levels, amino acids take on different structures due to changes in protonation. For instance, at pH levels below 1.0, amino acids are fully protonated. This means their amino groups are in the -NH3+ form, and their carboxyl groups remain as -COOH. Understanding pH levels helps predict how amino acids and proteins will behave in various environments.

chemical structures

Chemical structures represent the specific arrangement of atoms within a molecule. For amino acids, the condensed structural formula provides a simplified way to visualize these structures. For example:

• Glycine at pH below 1.0: H3N+-CH2-COOH
• Cysteine at pH below 1.0: H3N+-CH2-CH(SH)-COOH
• Serine at pH below 1.0: H3N+-CH2-CH(OH)-COOH
• Threonine at pH below 1.0: H3N+-CH2-CH(CH3)(OH)-COOH

Each amino acid has a unique chemical structure that influences its properties and roles in proteins. Understanding these structures helps in visualizing how amino acids build up proteins and how they function within biological systems.