Problem 81 - General Chemistry: Principles and Modern Applications

Question

The \(R\) group \(-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) is that of what amino acid? (a) alanine; (b) serine; (c) threonine; (d) arginine; (e) none of these.

Step-by-Step Solution

Answer

The correct answer is (e) none of these.

Step 1: Identify the acid-base problem

Determine strong/weak acid/base, buffer, or titration.

Step 2: Write relevant equations

Use pH = -log[H+], Ka expression, or Henderson-Hasselbalch.

Step 3: Calculate

Substitute values and solve.

Step 4: State the answer

The answer is: The correct answer is (e) none of these.

Key Concepts

Alanine Structure

Alanine is a simple yet significant amino acid within the family of proteinogenic amino acids, which are those incorporated into proteins during the translation process. The structure of alanine is distinguished by its aliphatic R group consisting of a single methyl group, denoted as \( -\mathrm{CH}_3 \). This compact R group contributes to alanine's nonpolar, hydrophobic properties, making it characteristically inert in interactions with other amino acids within protein structures.

The alanine side chain does not engage in hydrogen bonding, which can influence the folding and stability of proteins. Its simplistic structure is often used as a reference when comparing the complexities of other amino acid side chains.

Serine Structure

Serine stands out among amino acids with its polar side chain that contains a hydroxymethyl group represented by the formula \( -\mathrm{CH}_{2}\mathrm{OH} \). This hydrophilic R group enables serine to participate in hydrogen bonding, significantly affecting protein structure and function.

Serine is often found in active sites of enzymes where its hydroxyl group can be involved in catalytic mechanisms. Additionally, serine is a target for various post-translational modifications, which can alter protein function or localization. The presence of the hydroxyl group in serine's side chain is an essential feature for distinguishing it from other similar amino acids.

Threonine Structure

Threonine is another amino acid with a polar side chain. It is structurally similar to serine but with an additional methyl group, giving it the side chain formula \( -\mathrm{CH}(_{\mathrm{OH}})\mathrm{CH}_3 \). Threonine's R group, due to the presence of both a hydroxyl and a methyl group, makes it more structurally complex than serine and is also involved in hydrogen bonding.

The duality of the hydrophobic methyl group and the hydrophilic hydroxyl group allow threonine to be embedded within various regions of a protein. Threonine is particularly crucial in signaling pathways as it is a common site for phosphorylation, a reversible modification that regulates protein activity.

Arginine Structure

Arginine is well-known for its complex R group that harbors a guanidinium group at the end of a three-carbon aliphatic straight chain. This can be represented by the chemical structure \( -\mathrm{H}_{2}\mathrm{N}\mathrm{C}(\mathrm{NH}_{2})_{2} \). The guanidinium group is positively charged at physiological pH, making arginine a highly polar, hydrophilic amino acid.

This charged group is essential for arginine's role in protein-protein interactions, signal transduction, and as a precursor for the synthesis of nitric oxide, a vital cellular signaling molecule. The distinctive guanidinium group also allows arginine to form multiple hydrogen bonds, contributing to the stabilization of protein structures and binding specificity.

Amino Acid Identification

Identifying amino acids by their R groups is a fundamental skill in biochemistry and molecular biology. The R group, or side chain, essentially determines the chemical properties and reactivity of the amino acid within proteins. Knowing the specific R group structures, such as those of alanine, serine, threonine, and arginine, helps in recognizing the amino acid's role in the protein's function and structure.

For instance, the exercise in question highlighted the importance of recognizing side chains. The given R group, \( -\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2} \) does not correspond to any of the provided options, showcasing the need for precise knowledge in amino acid identification to accurately predict protein chemistry and interactions.