Peptide Bonds

Peptide Bonds - Formation, Characteristics & Structure

What is A Peptide Bond?

According to organic chemistry, a peptide bond is a chemical bond made between two molecules, where one molecule's carboxyl group combines with the other molecule's amino group. In the process, it releases a molecule of water. This is referred to as a dehydration synthesis process or a condensation reaction, commonly occurring between amino acids. The resultant CO-NH bond is known as a peptide bond, and the molecule is known as an amide. An amide or peptide group in the protein's context is the four-atom functional group -C(=O)NH-. And proteins and polypeptides are chains of amino acids joined together by peptide bonds.


How Do Peptide Bonds Form?

A peptide bond is formed through the molecules of the two amino acids, resulting in the carboxylic acid group of one amino acid, which can react with the amine group of the other. In its simplest form, this is demonstrated by two lone amino acids joining via peptide bond formation to produce a dipeptide, the smallest peptide.

Furthermore, more amino acids can be chained together to generate new peptides: 

  • 50 or fewer amino acids forming a peptide are called peptides.
  • 50-100 amino acids are called polypeptide bonds.
  • Lastly, peptides with more than 100 amino acids are called proteins.

A peptide bond breaks down by hydrolysis, which is the chemical breakdown of a substance caused due to coming in contact with water (metastable bonds). The peptide bonds generated inside peptides, polypeptides, and proteins are prone to breakage if exposed to water; however, this reaction is relatively slow. The reaction of a peptide bond with water produces around 10 kJ/mol of free energy. A peptide bond has an absorption wavelength of 190-230 nm.

Enzymes found within living organisms may both build and decompose peptide bonds. Peptides are found in various hormones, antibiotics, anti-aging products, anticancer medicines, weight loss supplements, and neurotransmitters, most are known as proteins, because of the number of amino acids present.

Are Peptide Bonds Covalent?

Peptide bonds are generated due to electron sharing among carbon and nitrogen, which is one of the most fundamental characteristics of a covalent bond. A peptide bond is an amide sort of covalent chemical bond that connects two successive alpha-amino acids from C1 to N2 of the other in a peptide chain.

What Are the Characteristics of Peptide Bonds?

Peptide bonds are strong and have a partial double bond. A few of its characteristics are:

  • Peptide bonds don't see a breakage by high salt concentrations or heating. These bonds are broken down by introducing them to a strong acid or base for an extended period of time at a high temperature. Some particular enzymes can also break them, like digestive enzymes.
  • As peptide bonds are stiff and planar, they help to maintain protein structure.
  • Peptide bonds comprise both slightly positive charge groups, like the polar hydrogen atoms of amino groups, and partly negative charge groups comprising polar oxygen atoms of carboxyl groups.

Structure of Peptide Bonds

Linus Pauling and Robert Corey discovered peptide bonds' inflexible and planar nature. A peptide bond is a trans, planar, and rigid structure. It also depicts a figure with a partial double bond. The peptide bond's coplanarity refers to the semi-sharing of two pairs of electrons among the amide nitrogen and carboxyl oxygen. 

The peptide bond's atoms C, H, N, and O are in the same plane as the oxygen atom of the carboxyl group and the hydrogen atom of the amide group, which are trans to one another.

The Polarity of the Peptide Bond

Typically, free rotation should be possible around a single bond among a carbonyl carbon and an amide nitrogen, which is the structure of a peptide bond. But, in this scenario, nitrogen possesses a single pair of electrons. These electrons are quite close to a carbon-oxygen bond. As a consequence, a plausible resonance structure with a double bond between carbon and nitrogen may be sketched. As a result, nitrogen has a positive charge, while oxygen has a negative charge. The resonance structure for the peptide bond thus prevents movement around the peptide bond. 

Furthermore, the true structure is a weighted combination of these two structures. The resonance structure is important in displaying the real electron distribution since the peptide bond has a roughly 40% double bond. Hence, it is rigid.