Problem 35 - General Chemistry: Principles and Modern Applications

Question

Draw condensed structural formulas showing what form you would expect for the essential amino acid threonine (a) in strongly acidic solutions; (b) at the isoelectric point; and (c) in strongly basic solutions.

Step-by-Step Solution

Answer

Threonine will exist in different forms under different pH conditions: (a) positively charged in acidic solutions (\(\text{NH}_3^{+}\text{-CHC}(\text{OH})\text{H}_3\text{-COOH}\)), (b) neutrally charged at the isoelectric point (\(\text{NH}_3^{+}\text{-CHC}(\text{OH})\text{H}_3\text{-COO}^{-}\)), and (c) negatively charged in basic solutions (\(\text{NH}_2\text{-CHC}(\text{OH})\text{H}_3\text{-COO}^{-}\)).

Step 1: Threonine structure

Firstly, it is essential to know the structure of threonine. It is an amino acid with the chemical formula \(\text{C}_4\text{H}_9\text{NO}_3\). The structure consists of a carboxylic group (-COOH), an amino group (-NH2), and a side chain (-CH(OH)CH3), attached to a central carbon atom. Amino acids exist as zwitterions in solutions due to the presence of the acidic and basic functional groups.

Step 2: Threonine in Acidic Solution

In a strongly acidic solution (low pH), the -COOH group remains protonated and the -NH2 group gets protonated due to the high concentration of \(\text{H}^+\) ions. In this case, threonine will exist in the form of a positively charged species (\(\text{NH}_3^{+}\text{-CHC}(\text{OH})\text{H}_3\text{-COOH}\)).

Step 3: Threonine at Isoelectric Point

At the isoelectric point, the pH of the solution equals the pI of the amino acid. The threonine will have a net charge of zero as the -COOH group is deprotonated (-COO-) and the -NH2 group is protonated (NH3+), resulting in the zwitterionic form (\(\text{NH}_3^{+}\text{-CHC}(\text{OH})\text{H}_3\text{-COO}^{-}\)).

Step 4: Threonine in Basic Solution

In a strongly basic solution (high pH), both the -COOH and the -NH2 groups get deprotonated due to the high concentration of \(\text{OH}^-\) ions. Therefore, threonine will exist in the form of a negatively charged species (\(\text{NH}_2\text{-CHC}(\text{OH})\text{H}_3\text{-COO}^{-}\)).

Key Concepts

Zwitterions in Amino Acids

Amino acids are fascinating molecules that serve as the building blocks of proteins. One of their unique properties is their ability to exist as zwitterions. A zwitterion is a molecule that has both a positive and a negative charge, but the overall charge of the molecule is neutral. This occurs because amino acids contain both an amine group, which can accept a proton to become positively charged, and a carboxyl group, which can lose a proton to become negatively charged.

In water and biological fluids, amino acids typically exist in this zwitterionic form, which is crucial for their solubility and reactivity. The zwitterionic nature of amino acids also affects their behavior in electric fields, a property used in techniques such as electrophoresis for separating proteins.

Threonine Chemical Structure

The amino acid threonine has a distinct structure that makes it an essential component of many proteins. It is characterized by its aliphatic side chain containing a hydroxyl group, making it a polar amino acid. The basic chemical structure of threonine, as described in the exercise, includes a central carbon (the \(\alpha\)-carbon) to which four different groups are attached: a carboxylic acid group (-COOH), an amine group (-NH2), a hydrogen atom, and a variable side chain represented as -CH(OH)CH3, where the hydroxyl group is part of a methyl group. This functional group is responsible for the specific reactivity and interactions of threonine within peptides and proteins.

Understanding the structure of threonine is essential for predicting its behavior in various environments, a subject of great importance in biochemistry and nutrition.

Threonine in Different pH Environments

The behavior of threonine can dramatically change depending on the pH of its surroundings. In acidic environments, the amino group accepts an extra proton, resulting in a positively charged amine. Conversely, in basic conditions, the carboxylic acid group loses its proton, leading to a negatively charged carboxylate ion.

At the isoelectric point, threonine carries no net electric charge. The isoelectric point (pI) is the specific pH at which the molecule becomes a zwitterion, and it is different for each amino acid. For threonine, this pH value is where the carboxyl group is deprotonated while the amine group is protonated, balancing the charges within the molecule. These pH-dependent characteristics underscore the importance of understanding how amino acids like threonine can be affected by and, in turn, affect their biochemical environment.