Amino Acids, Peptides and Proteins | Organic Chemistry 3

Amino acids:

α-amino acids: 

Amino acids in which the amino group is attached to the carbon atom that is α to the carboxylic acid moiety. Many different amino acids are observed in nature, but only 20 α-amino acids are abundant in proteins. They differ from each other by the R group attached to the α-carbon.
 

With the exception of glycine (R = H), all α-amino acids have a chirality center on the α-carbon. Common, naturally occurring amino acids have an S configuration (except for cysteine) and are called L-amino acids because their Fischer projections are similar to those of L-sugars.
 

Acid-base properties of amino acids:

Amino acids are both basic and acidic due to the presence of amino (pKa ~ 9-10) and carboxylic acid (pKa ~ 2-3) groups. The form of an amino acid is pH dependent:
 

At physiological pH, amino acids exist as salts: the amino group is protonated while the carboxylic acid group is deprotonated ⇒ this salt is called a zwitterion (a net neutral compound that exhibits charge separation). Amino acids are therefore water soluble and have high melting points.

Isoelectric point (pI): 

pH at which an amino acid exists primarily in its neutral form. At the isoelectric point, the number of positive charges is equal to the number of negative charges in the molecule, resulting in an overall neutral charge. Each amino acid has a unique pI.

• For neutral amino acids, the pI is calculated from the average of the pKa values of the amino and carboxylic acid groups:
   

$\mathrm{pI} = \frac{\mathrm{pKa}{ }_{\mathrm{COOH}} + \mathrm{pKa}{ }_{\mathrm{NH}2}}{2}$

• For amino acids with acidic or basic side chains, the pI is calculated from the average of the 2 pKa values of the 2 acidic groups (acidic side chain) or the 2 basic groups (basic side chain).

Hell-Volhard-Zelinsky reaction:
 

Conversion of a carboxylic acid into a racemic mixture of α-amino acids via an α-haloacid.

Mechanism:

1. Halogenation of the α-carbon to form a racemic mixture of α-haloacids.
2. Nucleophilic attack of NH₃ (SN2 reaction) to form a racemic mixture of α-amino acids.
   
   
    

Polyalkylation of the primary amine is not observed because the amino group in the product is less basic and more sterically hindered than other primary amines.

Amidomalonate synthesis:
 

Conversion of diethyl acetamidomalonate into a racemic mixture of α-amino acids.

Mechanism: similar to the malonic ester synthesis.

1. Deprotonation of diethylacetamidomalonate to form a doubly stabilized enolate.
2. Nucleophilic attack of the enolate on the alkyl halide resulting in alkylation of the enolate.
3. Acid-catalyzed hydrolysis of the esters and the amide to form an amino-1,3-dicarboxylic acid.
4. Decarboxylation of the amino-1,3-dicarboxylic acid at high temperature to form the desired α-amino acid.
   
   
    

Strecker synthesis:
 

Conversion of an aldehyde into a racemic mixture of α-amino acids via an α-amino nitrile.

Mechanism:

First part: Conversion of the aldehyde into an α-amino nitrile.

1. Protonation of the aldehyde, making it even more electrophilic.
2. Nucleophilic attack of NH₃ on the carbonyl group to form a tetrahedral intermediate.
3. Deprotonation of the ammonium group to form a neutral intermediate.
   
   
    
4. Protonation of the hydroxyl group to form a good leaving group.
5. Loss of the leaving group, a molecule of water to form an iminium.
6. Nucleophilic attack of a cyanide ion to form a racemic mixture of α-amino nitriles.
   
   
    

Second part: Hydrolysis of the α-amino nitrile into an α-amino acid.

Resolution of a racemic mixture:

Optically pure amino acids can be obtained by separating a racemic mixture, a process called resolution. However, this technique is expensive, time-consuming, and half of the reaction product is useless.

Enantioselective synthesis of amino acids:

This approach provides a more efficient and selective method for obtaining optically pure amino acids compared to traditional resolution techniques.

Peptides:

A peptide is a short chain of amino acids linked together by amide bonds, called peptide bonds, resulting in the reaction between COOH and NH₂:
 

• The amino acid with the free amino group is called the N-terminal amino acid, while the one with the free carboxy group is called the C-terminal amino acid. By convention, peptides are always drawn with the N-terminus on the left.
• The sequence of amino acid residues in a peptide can be abbreviated with three-letter abbreviations starting with the N-terminus.
• Polypeptides with more than 40 amino acids are called proteins.
   

Geometry of peptide bonds:

As a result, there are 2 possible conformations for a peptide bond: s-trans and s-cis. The s-trans conformation is typically more stable because it lacks steric hindrance: the two bulkier R groups are oriented on opposite sides of the C-N bond.
 

A second consequence of resonance stabilization is that all atoms involved in the peptide bond are in the same plane.

Disulfide Bridges:

Cysteine is the only amino acid found in proteins that contains a thiol group. 2 cysteines can be joined via an oxidation process of their thiol moiety to form a disulfide.
 

Disulfide bridges between the same strand or different strands greatly affect the 3D structure and properties of peptides and proteins.

The Edman degradation:

In the Edman degradation, amino acids are cleaved one at a time from the N-terminal amino acid, the identity of the amino acid is determined, and the process is repeated to sequence the entire peptide.

In this process, phenyl isothiocyanate is used to convert the N-terminal amino acid into a phenylthiohydantoin (PTH) derivative resulting in a new peptide with one fewer amino acid.
 

Mechanism:

1. Nucleophilic attack of the N-terminal amino group on the carbon of phenyl isothiocyanate to form a thiolate ion. 
2. Intramolecular nucleophilic attack of the thiolate on the carbonyl group of the first peptide bond.
3. Proton transfers to form a neutral intermediate.
4. Protonation of the amine of the former first peptide bond followed by the loss of a leaving group, the peptide, to form a 5-membered thiazolinone ring.
5. Rearrangement of the thiazolinone ring to form a more stable phenylthiohydantoin derivative.
   
   

Sequencing of large peptides:

Automated peptide sequencers are capable of sequencing a peptide chain of up to 50 amino acid residues. To sequence larger peptides, they are first cleaved into smaller fragments by:

• Partial hydrolysis under acidic conditions to randomly form smaller fragments
• Enzymatic cleavage using peptidases, enzymes that can selectively hydrolyze peptide bonds.

There are a variety of peptidases with different cleavage sites:

• Carboxypeptidase catalyzes the hydrolysis of the amide bond closest to the C-terminal end ⇒ used to identify the C-terminal amino acid.
• Chymotrypsin catalyzes the hydrolysis of the amide bond with a carbonyl group from Phe, Tyr, or Trp.
• Trypsin catalyzes the hydrolysis of the amide bond with a carbonyl group from Arg or Lys.

Peptide bond formation:

The synthesis of a specific dipeptide consists of coupling an amino-protected amino acid and a carboxyl-protected amino acid. An additional reagent, a dehydrating agent such as dicyclohexylcarbodiimide (DCC), is used to promote amide formation by converting the carboxyl OH group into a better leaving group.

Mechanism:

1. Nucleophilic attack of the OH of the carboxylic acid on the carbodiimide.
2. Proton transfer to form a neutral activated carboxy group.
   
   
    
3. Nucleophilic attack of the amine on the carbonyl to form a tetrahedral intermediate.
4. Loss of the leaving group, the dicyclohexylurea, to re-form the carbonyl group.
5. Proton transfer to form an amide and a urea.
   
   

Protection of other reactive amino and carboxyl groups is necessary to prevent formation of undesired polypetides ⇒ The protected amino acids can be coupled regioselectively followed by removal of the protecting groups.

Protection and deprotection of the amino group:

• Protection:

Mechanism:

1. Nucleophilic attack of the amino group on the carbonyl group.
2. Loss of the leaving group to re-form the carbonyl group.
3. Deprotonation of the ammonium ion to form the desired protected amino acid.
   
   

• Deprotection of the Boc group:

Mechanism:

1. Protonation of carbonyl group with trifluoroacetic acid.
2. Loss of the leaving group, a carbamic acid. The by-product, a carbocation, undergoes an elimination reaction to form isobutylene.
3. Deprotonation of carbamic acid with trifluoroacetic acid.
4. Protonation of the amine followed by loss of CO₂ to form the deprotected amino acid.
5. Under acidic conditions, the amino group of the amino acid is protonated.
   
   
    

This process produces isobutylene and CO₂, both of which are gases ⇒ this drives the reaction to completion.

• Deprotection of the Fmoc group:

Mechanism:

1. Deprotonation of the benzylic proton to form a resonance-stabilized anion.
2. Protonation of the amino group followed by loss of CO₂ and dibenzofulvene.
   
   

Protection and deprotection of the carboxyl group:

• Protection:

A carboxylic acid moiety can be protected by converting it into an ester, commonly methyl esters or benzyl esters.

• Deprotection:

An ester protecting group can be removed with an aqueous base.

Benzyl esters can also be removed by hydrogenolysis with H₂ in the presence of Pt, or with HBr in acetic acid:
 

The Merrifield synthesis:

Large peptides can be prepared using the Merrifield synthesis, a solid-phase synthesis. In this method, an amino acid is attached to an insoluble polymer, usually a polystyrene derivative, and Fmoc-protected amino acids are added sequentially one at a time. In the final step, the polypeptide chain is cleaved from the polymer.

Mechanism:

1. A Fmoc-protected amino acid is attached to the polymer.
   
   
    
2. The Fmoc group is removed and an amide bond is formed with another Fmoc-protected amino acid using DCC.
   
   
    
3. Step 2 is repeated as many times as necessary to build the desired peptide.
4. At the end, the protecting group is removed and the peptide is cleaved from the polymer using HF.

The Merrifield synthesis can be fully automated.

Protein structure:

Polypeptides are characterized by their amino acid sequence. They are organized into 4 levels of structure:

• Primary structure: the linear sequence of amino acids. The specific arrangement of amino acids determines the unique identity of the protein and sets the foundation for higher order structures. A small change in the primary structure can lead to a large change in function.
• Secondary structure: the 3D conformations of localized regions of the protein. 2 particularly stable arrangements are the α-helix, a portion of the protein that twists into a clockwise spiral, and the β-pleated sheet, a portion of the protein in which 2 or more protein chains lie side by side.
• Tertiary structure: the 3D arrangement of the entire polypeptide chain. Interactions such as hydrophobic interactions, hydrogen bonding, disulfide bridges, and van der Waals forces contribute to the stability of the tertiary structure. The unique folding of a protein's tertiary structure determines its specific function and activity. A protein can unfold under conditions of mild heating in a process called denaturation.
• Quaternary structure: a functional, multi-subunit protein complex resulting from the interaction of multiple polypeptide chains (subunits).

Based on their shape, proteins are often classified as fibrous (linear chains that are bundled together) or globular (chains that are coiled into compact shapes).

Protein function:

Proteins play a variety of roles that are critical to cellular function:

• Structural proteins: Fiibrous proteins that provide support and maintain the structural integrity of cells and tissues.
• Enzymes: Proteins that serve as biological catalysts, accelerating all cellular processes without being consumed. Each enzyme is highly specific, recognizing and interacting with a specific substrate.
• Transport proteins: Proteins that facilitate the movement of molecules across biological membranes. For example, hemoglobin transports oxygen in the blood, while ion channels and carriers regulate the passage of ions and molecules across cell membranes.