Question

Amino acids are the building blocks for all proteins in our bodies. A structure for the amino acid alanine is All amino acids have at least two functional groups with acidic or basic properties. In alanine, the carboxylic acid group has $K_{\mathrm{a}}=4.5\times {10}^{-3}$ and the amino group has $K_{\mathrm{b}}=7.4\times {10}^{-5}.$ Because of the two groups with acidic or basic properties, three different charged ions of alanine are possible when alanine is dissolved in water. Which of these ions would predominate in a solution with $\left[{\mathrm{H}}^{+}\right]=1.0M?$ In a solution with $\left[{\mathrm{OH}}^{-}\right]=1.0\mathrm{M}?$

Short answer

In a solution with $\left[{\mathrm{H}}^{+}\right]=1.0M$, the alanine ion with a net positive charge (+1) will predominate. In a solution with $\left[{\mathrm{OH}}^{-}\right]=1.0M$, the alanine ion with a net negative charge (-1) will predominate.

Step-by-step solution

Calculate the pH and pOH of the given solutions

First, we will calculate the pH and pOH of the solutions with [H+]=1.0M and [OH-]=1.0M. For the solution with [H+]=1.0M: pH = -log([H+]) = -log(1.0) = 0 Now find the pOH of this solution using the relationship: pH + pOH = 14 0 + pOH = 14 pOH = 14 For the solution with [OH-]=1.0M: pOH = -log([OH-]) = -log(1.0) = 0 Now find the pH of this solution using the relationship: pH + pOH = 14 pH + 0 = 14 pH = 14

Determine the pKa and pKb of alanine

Given the $K_a=4.5\times {10}^{-3}$ and $K_b=7.4\times {10}^{-5}$, we can find the pKa and pKb values. pKa = -log($K_a$) = -log($4.5\times {10}^{-3}$) ≈ 2.35 pKb = -log($K_b$) = -log($7.4\times {10}^{-5}$) ≈ 4.13

Compare pH, pKa, and pKb for the solution with [H+]

In a solution with pH=0, we can make the following comparisons: - Since pH < pKa, the carboxylic acid group will be protonated and present in the cationic form. - Since pH < pKb, the amino group will be protonated and also present in the cationic form. Therefore, in the solution with [H+]=1.0 M, the predominant ion of alanine will have both the carboxylic acid and amino groups protonated, resulting in a net positive charge (+1).

Compare pH, pKa, and pKb for the solution with [OH-]

In a solution with pH=14, we can make the following comparisons: - Since pH > pKa, the carboxylic acid group will be deprotonated and present in the anionic form. - Since pH > pKb, the amino group will be deprotonated and present in the anionic form. Therefore, in the solution with [OH-]=1.0 M, the predominant ion of alanine will have both the carboxylic acid and amino groups deprotonated, resulting in a net negative charge (-1). Answer: In a solution with [H+]=1.0 M, the alanine ion with a net positive charge (+1) will predominate. In a solution with [OH-]=1.0 M, the alanine ion with a net negative charge (-1) will predominate.

Key concepts

Exploring Alanine

Alanine is an amino acid, which means it is one of the building blocks of proteins, essential for various biological functions. It belongs to a class of molecules known as alpha-amino acids. This particular amino acid is made up of the following components:

• A central carbon atom.
• An amino group (-NH${}_2$).
• A carboxylic group (-COOH).
• A side chain specific to alanine, which is a methyl group (-CH${}_3$).

Together, these groups allow alanine to engage in the formation of proteins by linking together through peptide bonds with other amino acids, making them integral to the structure and function of proteins.
Alanine's role extends beyond structure, as it is involved in various metabolic processes, like glucose metabolism. Understanding alanine is fundamental to grasping how proteins are structured and function in living organisms.

Understanding pH

The pH scale is a measure that indicates how acidic or basic a solution is. It is calculated as the negative logarithm of the hydrogen ion concentration, $\text{pH}=-\log [{\text{H}}^{+}]$. A pH of 7 is considered neutral, below 7 acidic, and above 7 basic or alkaline. The scale essentially helps determine the extent to which substances can donate or accept protons in water.
Correctly identifying pH conditions can indicate how alanine behaves in different environments such as acidic stomach or neutral cytoplasm, altering its charge and thus its role in biological systems.

Functional Groups in Alanine

In the world of amino acids, functional groups are specific groups of atoms within molecules that have characteristic properties and reactivity. Alanine contains two primary functional groups:

• The amino group (-NH${}_2$) that acts as a base, accepting protons.
• The carboxylic acid group (-COOH) that acts as an acid, donating protons.

These groups can undergo chemical reactions such as ionization, where their protonation states are changed.
The behavior of these functional groups under different pH conditions significantly affects the structure and function of proteins. When dissolved in water or bodily fluids, they can pick up or release protons, impacting alanine's net charge and thus its behavior in physiological processes.
Understanding these functional groups helps in modeling the reactivity and solubility of amino acids in biological and chemical contexts.

Protonation Explained

Protonation is the process by which a molecule, like an amino acid, gains a proton (H${}^{+}$), making it positively charged. This happens typically when the environmental pH is lower than the pKa (acid dissociation constant) of the functional group.
For alanine:

• The carboxyl group (-COOH) will remain protonated below its pKa.
• The amino group (-NH${}_2$) will also accept a proton and become -NH${}_3^{+}$.

Protonation affects the solubility, structure, and interaction of alanine with other molecules. This becomes particularly important when considering the protein-folding process, as the charge and interactions of amino acids dictate the overall shape and function of proteins.
Knowing when and how alanine protonates allows for predicting how it might interact in different systems, such as in acidic solutions found within certain parts of the body.

Deprotonation Dynamics

Deprotonation involves the loss of a proton from a molecule, typically occurring when the environmental pH is above its pKa. This process can change the charge on functional groups, which is a key factor in determining how molecules like amino acids behave in solutions.
In alanine:

• The carboxylic acid group -COOH can lose a proton resulting in -COO${}^{-}$.
• The amino group remains uncharged when deprotonated to -NH${}_2$.

Deprotonation plays a crucial role in the structural integrity and interaction of proteins, especially when considering environments such as basic solutions. These dynamics can determine the tertiary structure of proteins by affecting how amino acids interact with each other and with surrounding molecules.
Understanding deprotonation is vital for grasping how biochemical processes involving alanine function, making it critical for fields such as medicine and biochemistry.