Question

Write the formulas of the species expected if the amino acid phenylalanine is maintained in (a) \(1.0 \mathrm{M}\) HCl; (b) \(1.0 \mathrm{M} \mathrm{NaOH} ;\) (c) a buffer solution with \(\mathrm{pH}=5.7\).

Short answer

In 1.0 M HCl, phenylalanine will be \(NH3^+-CH(C6H5)-COOH\). In 1.0 M NaOH, it will be \(-NH2-CH(C6H5)-COO-\). In a buffer solution at pH 5.7, it will be \(NH3^+-CH(C6H5)-COO-\).

Step-by-step solution

Understanding the pH values

The pKa values for the carboxyl group (\(-COOH\)) and the amino group (\(-NH3+\)) of an amino acid like phenylalanine are approximately 2 and 9, respectively. These pKa values are scriptural pH values at which the respective groups get ionized.

Analyzing Phenylalanine in 1.0 M HCl

HCl, a strong acid, will ionize completely, resulting in a very low pH. Both the carboxyl and the amino groups will be protonated and the species will have a net positive charge. Thus, in 1.0 M HCl, phenylalanine will exist as \(NH3^+-CH(C6H5)-COOH\).

Analyzing Phenylalanine in 1.0 M NaOH

In an alkaline medium like 1.0 M NaOH, the medium is highly basic (pH well over 9). Here, both the carboxyl and the amino groups will be deprotonated, and the species will have a net negative charge. So, phenylalanine in 1.0 M NaOH, will exist as \(-NH2-CH(C6H5)-COO-\).

Analyzing Phenylalanine in pH 5.7 Buffer Solution

In a buffer solution with pH 5.7, only the carboxyl group will be deprotonated since the pH is above its pKa but below the pKa of the amino group. The species will therefore have no net charge and exist as \(NH3^+-CH(C6H5)-COO-\).

Key concepts

Phenylalanine Chemical Structure

Phenylalanine is one of the essential amino acids, which are building blocks for proteins in the body. Its chemical structure is characterized by the presence of a benzyl side chain attached to the alpha carbon atom, which is next to the carboxyl (-COOH) and amino (-NH2) groups that all standard amino acids possess. The benzyl side chain is hydrophobic, influencing how phenylalanine interacts with other molecules. The formula for phenylalanine is C9H11NO2, and it has a three-letter abbreviation, Phe, and a one-letter abbreviation, F.
Phenylalanine can exist in different ionization states depending on the pH of the environment. At highly acidic or basic pH values, the molecule can gain or lose protons, changing its overall charge.

Acid-Base Chemistry

Acid-base chemistry is a fundamental concept in chemistry, involving the transfer of protons (H+) between molecules. Acids are proton donors, and bases are proton acceptors. The strength of an acid or base is often represented by its dissociation constant, Ka, or its base 10 logarithm, pKa.
When an acid donates a proton, it forms its conjugate base, while a base accepting a proton forms its conjugate acid. The reaction of an acid and a base can neutralize each other, producing water and a salt. The behavior of amino acids, such as phenylalanine, can be understood through acid-base chemistry as they contain both acidic (carboxyl group) and basic (amino group) sites that can undergo protonation and deprotonation.

pKa Values of Amino Acids

The pKa value is a measure of the acidity or basicity of a compound. It is the pH at which half of the species is deprotonated. Amino acids have more than one pKa value corresponding to their carboxyl and amino groups, and sometimes an additional one if there is an ionizable side chain.
For phenylalanine, the pKa of the carboxyl group is approximately 2, indicating that at a pH lower than 2, the carboxyl group is likely to be fully protonated (COOH). The pKa of the amino group is around 9, meaning that at a pH higher than 9, the amino group is likely to be fully deprotonated (NH2). In environments with pH between these values, the groups will be in their ionized or non-ionized states depending on the exact pH.

Buffer Solutions

Buffer solutions are mixes that can resist changes in pH when small amounts of acids or bases are added. This resistance is due to the presence of weak acids and their conjugate bases in the solution. Buffers are essential for maintaining pH levels in a range of applications, including biological systems where enzymes and other proteins require a specific pH range for optimal activity.
A buffer's capacity is defined by its components' pKa values and the concentration of the acid and conjugate base. For instance, if the buffer is made with acetic acid and sodium acetate, its effective buffering range is around the pKa of acetic acid, which is 4.76. For phenylalanine in a buffer solution with a pH near 5.7, the buffering system must maintain the pH to keep the amino acid in a zwitterionic form without a net charge.