Question

Write the zwitterion form of alanine and show its reaction with the following. (a) \(1 \mathrm{~mol} \mathrm{NaOH}\) (b) \(1 \mathrm{~mol} \mathrm{HCl}\)

Short answer

Answer: When alanine in its zwitterion form reacts with 1 mol of sodium hydroxide, the products are deprotonated alanine ion and water. In contrast, when alanine reacts with 1 mol of hydrochloric acid, the products are the protonated alanine ion and chloride ion.

Step-by-step solution

1. Zwitterion form of alanine

First, let's write the structure of alanine. Alanine is an α-amino acid with the chemical formula \(CH_3CH(NH_2)COOH\). It contains an amino group (-NH2), a carboxyl group (-COOH), and a side chain methyl group (-CH3), making it a non-polar, aliphatic amino acid. The zwitterion form of alanine refers to the state when the amino group (-NH2) is protonated, and the carboxyl group (-COOH) is deprotonated. It occurs when alanine is in an aqueous solution near its isoelectric point (pI) of 6.01. The zwitterion form of alanine is: \(CH_3CH(NH_3^+)COO^-\).

2. Reaction with 1 mol NaOH

When alanine in its zwitterion form reacts with 1 mol of sodium hydroxide (NaOH), the base (hydroxide ion, OH⁻) removes a proton (H⁺) from the protonated amino group (-NH3⁺), resulting in a negatively charged alanine molecule. The reaction can be represented as follows: \(CH_3CH(NH_3^+)COO^-(aq) + NaOH(aq) \rightarrow CH_3CH(NH_2)COO^-(aq) + Na^+(aq) + H_2O(l)\) The product of this reaction is the deprotonated alanine ion and water.

3. Reaction with 1 mol HCl

When alanine in its zwitterion form reacts with 1 mol of hydrochloric acid (HCl), the acid (proton, H⁺) reacts with the deprotonated carboxylate group (-COO⁻) of alanine, resulting in a positively charged alanine molecule. The reaction can be represented as follows: \(CH_3CH(NH_3^+)COO^-(aq) + HCl(aq) \rightarrow CH_3CH(NH_3^+)COOH(aq) + Cl^-(aq)\) The product of this reaction is the protonated alanine ion and chloride ion.

Key concepts

Amino Acids

Amino acids are the building blocks of proteins, essential molecules for life. Every amino acid consists of a central carbon atom bonded to four different groups: a hydrogen atom, a carboxyl group (-COOH), an amino group (-NH2), and a unique side chain or R-group that varies between different amino acids. In the case of alanine, the side chain is a simple methyl group (-CH3).

Amino acids exist in different forms depending on the pH of their environment. One essential form is the zwitterion, which is electrically neutral but carries both a positive and a negative charge on different atoms within the molecule. In water, amino acids can act as both acids and bases, which is a property that stems from their ability to donate and accept protons. Understanding the behavior of amino acids in varying conditions is crucial for grasping how they interact within biological systems.

Isoelectric Point

The isoelectric point, or pI, is the specific pH at which an amino acid has no net electric charge. At this point, the number of positive charges on the amino acid is exactly equal to the number of negative charges. For alanine, the pI is approximately 6.01. At this pH, alanine exists predominantly in its zwitterionic form with a positively charged ammonium group (-NH3+) and a negatively charged carboxylate group (-COO-).

The isoelectric point is an essential concept because it impacts the solubility and mobility of amino acids in an electric field, which is leveraged in techniques such as isoelectric focusing for protein purification and identification. By knowing the isoelectric point of an amino acid, scientists can predict its behavior in biological systems and manipulate conditions for desired outcomes in biotechnological applications.

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes in which protons are exchanged between molecules. Amino acids, with their amphoteric nature, can participate in such reactions acting either as acids (proton donors) or bases (proton acceptors). The zwitterion form of an amino acid is particularly interesting as it can react differently with acids and bases.

When an amino acid in its zwitterionic form encounters a base such as NaOH, it acts as an acid, losing a proton from its ammonium group to form a negatively charged molecule. Conversely, when it meets an acid like HCl, it acts as a base, gaining a proton on its carboxylate group to become a positively charged molecule. These acid-base reactions are not only pivotal in organic chemistry but also play a significant role in enzymatic activity and protein structure within biochemistry.