Question

Show the structure of the product you would expect to obtain by \(S_{N} 2\) reaction of a cysteine residue with iodoacetic acid.

Short answer

S-carboxymethyl-L-cysteine is formed.

Step-by-step solution

Understand the Reactants and Reaction Type

We have a cysteine residue that contains a thiol group (-SH), and iodoacetic acid with an iodine atom available for nucleophilic substitution through an \(S_N2\) mechanism. The \(S_N2\) reaction involves a nucleophile attacking an electrophilic center, resulting in the displacement of a leaving group in a single concerted step.

Identify the Nucleophile and Electrophile

In this reaction, the thiol group (-SH) of cysteine is the nucleophile because it has a lone pair of electrons on the sulfur atom. Iodoacetic acid acts as the electrophile, where iodine serves as the leaving group and the carbon atom bonded to iodine is the electrophilic center.

Examine the Stereochemistry and Mechanism

The \(S_N2\) reaction proceeds with a backside attack by the nucleophile, resulting in inversion of the configuration at the electrophilic center. However, since the electrophilic center here is not chiral, we focus only on the bond-forming and bond-breaking process.

Predict the Product Structure

The sulfur atom of the thiol group will attack the electrophilic carbon of iodoacetic acid, displacing the iodine atom in this \(S_N2\) reaction. The product will therefore be a thioether linkage (a sulfur bridge) between the cysteine residue and acetic acid, forming S-carboxymethyl-L-cysteine.

Key concepts

Cysteine

Cysteine is an amino acid that plays a vital role in various biological functions. It is known for its reactive thiol (or sulfhydryl) group, which consists of a sulfur atom bonded to a hydrogen atom ( - SH ). This thiol group is crucial in enzymatic activities and protein structure stabilization. Cysteine serves as a building block for proteins, and its presence is essential for forming disulfide bonds, which contribute to the tertiary structure of proteins. These bonds aid in maintaining protein integrity and functionality in various physiological processes.

Moreover, cysteine is involved in detoxification within the body, as it can bind to and neutralize harmful substances. It also plays a role in producing other essential molecules like glutathione, an antioxidant that helps protect cells from damage.

Understanding cysteine's role and reactivity is crucial when exploring biochemical reactions, such as those involving thiol groups and nucleophilic substitution mechanisms.

Thiol Group

The thiol group, often represented as -SH, is a functional group consisting of a sulfur atom bonded to a hydrogen atom. This group is highly reactive, primarily due to the presence of sulfur, which can easily engage in a variety of chemical reactions. In organic chemistry, thiol groups are prominent for their ability to undergo oxidation to form disulfide bonds or participate in nucleophilic substitution reactions, such as the $S_N2$ reaction.

In the context of cysteine, the thiol group serves as an important nucleophile. Because it contains a lone pair of electrons, it can readily attack electrophilic centers, initiating reactions that lead to the formation of new bonds. This characteristic makes thiol groups central to many biochemical processes, including post-translational modifications of proteins. This is why understanding the thiol group is essential for fields like biochemistry and molecular biology.

Iodoacetic Acid

Iodoacetic acid is an organic compound known for being a strong alkylating agent. It contains an iodine atom bonded to a methyl group on an acetic acid molecule, making it highly reactive. In $S_N2$ reactions, the iodine atom serves as the leaving group due to its ability to stabilize the extra electrons when detached from the molecule.

Because of its reactivity, iodoacetic acid is commonly used in chemical biology to modify proteins, particularly by targeting thiol groups. The presence of the carboxylic acid group ( -COOH ) in iodoacetic acid contributes to its solubility in the aqueous biological systems, which is crucial for facilitating reactions with biological molecules such as proteins and peptides.

Understanding iodoacetic acid is important when studying chemical modifications in proteins, as it illustrates how specific chemical reactions can alter protein function and structure.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental concept in organic chemistry, involving the replacement of an atom or group of atoms (the leaving group) by a nucleophile. The type of nucleophilic substitution most commonly discussed is $S_N2$ , where a nucleophile attacks an electrophilic carbon atom and displaces the leaving group in one step.

In an $S_N2$ reaction, the nucleophile approaches the electrophilic center from the side opposite the leaving group (backside attack). This leads to an inversion of stereochemistry at the electrophilic site, known as the Walden inversion. Such reactions are concerted, meaning they occur in a single step without intermediates.

In the context of thiol groups like those in cysteine, the thiol acts as a nucleophile due to its electron-rich sulfur atom. In combination with an electrophile like iodoacetic acid, the thiol group attacks the carbon atom bonded to iodine, resulting in the displacement of iodine and formation of a new sulfur-carbon bond. Understanding nucleophilic substitution reactions is essential for mastering organic synthesis and reactivity.