Question

(a) Draw the Lewis structure for the amino acid valine, showing the amino group and the carboxylic acid group in their un-ionized forms. (b) Draw the Lewis structure for the zwitterionic form of valine. (c) Which of these structures will be the predominant form at physiological pH?

Short answer

Valine predominantly exists in its zwitterionic form at physiological pH.

Step-by-step solution

Valine Structure

Valine is an amino acid with the formula C5H11NO2. It consists of an amino group (-NH2), a carboxylic acid group (-COOH), and a side chain which is isopropyl group (-CH(CH3)2).

Un-ionized Form of Valine - Amino Group

The amino group (-NH2) is attached to the central carbon atom (alpha carbon) in its un-ionized form: each nitrogen atom shares a pair of electrons with two hydrogen atoms, forming single bonds. The nitrogen atom in the amino group also has a lone pair of electrons.

Un-ionized Form of Valine - Carboxylic Acid Group

The carboxylic acid group (-COOH) is also attached to the central carbon atom. It has a single bond between carbon and the hydroxyl group (O-H) and a double bond with the other oxygen atom (C=O). The hydroxyl group oxygen atom also draws a lone pair.

Lewis Structure for Un-ionized Valine

Draw the central carbon atom and connect it to: 1) one amino group (-NH2), 2) one side chain (-CH(CH3)2), and 3) one carboxylic acid group (-COOH). Ensure proper double, single bonds, and lone pairs are placed accordingly.

Zwitterionic Form of Valine

In its zwitterionic form, the amino group gains an extra hydrogen ion (proton), becoming -NH3⁺, and the carboxylic acid loses a proton, becoming -COO⁻. This results in the molecule having no net charge.

Lewis Structure for Zwitterionic Valine

Draw the central alpha carbon attached to: 1) the amino group now as -NH3⁺, 2) the side chain as -CH(CH3)2, and 3) the carboxylate group as -COO⁻. Show the positive charge on the nitrogen and the negative charge on the oxygen.

Dominant Form at Physiological pH

Under physiological pH (around 7.4), valine predominantly exists in its zwitterionic form. This is because the pH is between the pKa values of the amino group and the carboxylic group, allowing both to ionize to form the zwitterion.

Key concepts

Lewis Structure

The Lewis structure is a visual representation of a molecule that highlights its valence electrons and the arrangement of atoms. When depicting valine, an amino acid, using Lewis structures, we start by recognizing its key components: an amino group (-NH2), a carboxylic acid group (-COOH), and the distinctive isopropyl side chain (-CH(CH3)2). The un-ionized form of valine is illustrated by showing:

• The central alpha carbon (C) atom.
• The amino group (-NH2) attached to this carbon, where nitrogen (N) shares electrons with two hydrogen (H) atoms, forming single covalent bonds, while also possessing a lone pair of electrons.
• The carboxylic acid group with a C-O single bond and a C=O double bond, alongside the hydroxyl group (-OH) that holds a lone pair of electrons.

Lewis structures help us comprehensively evaluate the bonding and molecular layout, providing a groundwork for understanding molecular reactivity and properties.

Zwitterion

A zwitterion is a unique molecular form that possesses both positive and negative charges but remains overall neutral regarding its net charge. For amino acids like valine, the zwitterionic form is reached due to proton transfer: the amino group (\(-NH2\)) picks up an extra hydrogen ion to become \(-NH3^+\), while the carboxylic acid group (\(-COOH\)) donates a proton to transform into \(-COO^-\). This intriguing dual-charge setup stabilizes the molecule without a net electrical charge.

• Importance: Zwitterions are crucial in biological systems where they enable solubility and transport of amino acids in aqueous environments within the body.
• Representation: In the zwitterion form, nitrogen will carry a positive charge, signified by \(-NH3^+\), and one of the oxygens in the carboxylate will have a negative charge (-COO⁻).

This balancing of charges is fundamental in maintaining the structure and function of proteins.

Physiological pH

Physiological pH is the typical pH maintained within living organisms, often around 7.4 in humans. At this pH, many biochemical substances, such as amino acids, exist in their most stable forms. For valine, at a physiological pH of 7.4, it is predominantly in the zwitterionic form.

• The pH is critical because it aligns closely with the pKa values of the amino and carboxyl groups.
• Such alignment ensures both groups can ionize properly, allowing valine to adopt its predominant zwitterionic configuration.

Understanding physiological pH is essential not only for predicting amino acid behavior but also for diagnosing and treating various health conditions where pH balance is disrupted.

Valine

Valine is one of the 20 standard amino acids that play a critical role in protein synthesis and function. Its chemical formula is \(C_5H_{11}NO_2\), and it notably features a branched isopropyl side chain.

• Structural Characteristics: The diversity provided by valine contributes to the hydrophobic environment within protein structures, affecting protein folding and stability.
• Metabolic Role: It is particularly important in muscle metabolism, tissue repair, and the balance of nitrogen in the body.
• Dietary Importance: As an essential amino acid, valine cannot be synthesized by the body and must be obtained through diet, usually from protein-rich sources like meat, dairy, and legumes.

Recognizing valine's multifaceted roles in biochemistry and nutrition helps appreciate the complexity of protein functionality and the nutritional demands of living organisms.