We discuss the detailed facts of peptide bonds such as definition, properties , functions and some frequently asked questions are discussed briefly.
Proteins are basically cellular macromolecules that are comprised by various type of amino acids residue which are joined with each other by peptide bonds which are called amide(-CONH2) bond also. Proteins are present in all living organisms and perform a vast array of functions.
In this article the properties of peptide bond and other detailed facts are discussed clearly.
What is Amino Acid?
Amino acid is an organic molecule that is known as building blocks of life. Amino acid has both the carboxyl acid and amino group in its structure. Though there are more than 500 naturally occurring amino acids are familiar but only 20 amino acids determine the genetic code of all living organisms.
The structure of a general amino acid is drawn below-
They can be classified as alpha(α), beta(β) and gamma(γ) or delta (δ) amino acids. Proteins are formed when –CPPH group of one amino acid reacts with the –NH2 group of another amino acid and one H2O molecule is removed.
What is Peptide Bond
Peptide bond is mainly a covalent chemical bond which is called amide (-CONH2) type bond present in protein. It is formed by linking two consequitive alpha amino acids. Peptide bonds are kinetically stable. The peptide bonds can be intact even after thousand years in absence of any catalyst.
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Peptides are classified as –
- Dipeptide- It is derived from two amino acids .
- Oligopeptide- it consists of two to twenty amino acids and include dipeptides, tripeptides, tetrapeptides and so on.
- Polypeptide- It is a single linear chain made by many amino acids ( more than 50 amino acids).
Formation of Peptide Bond
Peptide bond formation is mainly a condensation process ( release of H2O) . Carboxyl (-COOH) group of one amino acid reacts with the amino (-NH2) group of another amino acid and one water molecule is eliminated after that. This reaction is kinetically feasible. Thus the equilibrium of the peptide bond formation reaction is shifted towards the right hand side that is product side. Hence biosynthesis of peptide bond requires an input of free energy. The formation process of peptide bond is described below-
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Properties of Peptide Bond
The characteristics and properties of the chemical covalent peptide bond is discussed below-
The structure of peptide bond shows a planar geometry which leads the peptide bond to achieve towards most stable conformation. The peptide bonds formed are mostly in trans configuration. In peptide bond, both –C=O and –N-H bonds are polar (due to electronegativity difference between two consisting atoms) and take part in the hydrogen bond formation. Each residue contains a carbonyl bond (-C=O), which is a good hydrogen bond acceptor.
All proteins are made of α amino acid having L configuration. This determines the steric arrangement at α- carbon atom. The peptide bond ( substituted amide bond) has a planar structure. The 6 atoms that reside in the same plane are related with each other by bond lengths and bond angles that vary minorly from one amino acid residue to another. Only three of these six bond lengths are part of the peptide chain. Those bonds are-
The α- carbon to carbonyl bond, the C-N bond and the imide nitrogen to α carbon bond. Since the C-N bond possesses some double bond character (due to delocalization of unshared electron pair), the rotation about this bond is not possible. Only the first and last bond allows rotation about it.
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The presence of a peptide linkage can be proved by a chemical experiment. Alkaline solution of a peptide is prepared and then a drop of dilute copper sulfate solution will be added into the alkaline solution. Color change occurs from blue violet to pink color. The intensity of the color, and the absorption at 540nm is (λmax =540 nm) directly proportional to the concentration of protein. This experiment is known for “Biuret Test”.
Polypeptides show IR (Infra red) frequency near 3300 and 3100 cm-1 which are the characteristic of nu bar (wave number) for N-H (hydrogen bonded) stretching in their IR spectra. Another group of bands are observed near 1650 cm-1 and 1550 cm-1. Those are responsible for C=O stretching frequency. All of these two frequencies are also hydrogen bonded. The peptide bond also absorbs UV ( ultra violet) radiation in the region 180-220 nm.
Hydrolysis of Peptide Bond
Peptide linkage can be broken by hydrolysis reaction ( reaction with water) of amide. The peptide bonds of proteins are very much stable. Thus this bond will break spontaneously in a extremely slow process. In this hydrolysis process the value of Gibbs free energy is approximately 8-16 KJ/mol. The half life of the reaction is between 350-400 years per bond at 298K.
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