medium difficulty6 min

Amino Acids and Peptide Bond Formation

Proteins are polymers of alpha-amino acids joined through peptide bonds, and a peptide bond is the amide linkage -CO-NH- formed between the carboxyl group of one amino acid and the amino group of another.

In one line: Peptide bond has carbonyl then nitrogen: CO-NH.

Practice This Concept

Understand it

Amino acids contain both an amino group and a carboxyl group.
In alpha-amino acids, both groups are attached to the same alpha carbon, along with hydrogen and a side chain.
In neutral conditions, amino acids often exist as zwitterions, carrying both positive and negative charges within the same molecule.
Proteins form when amino acids join by condensation, eliminating water and forming peptide bonds.
The sequence and nature of amino acids determine the protein's properties.

Example

Glycine and alanine can combine by loss of water to form a dipeptide containing the -CO-NH- peptide linkage.

Rule to remember

Condensation pattern: R-CH(NH2)-COOH + H2N-CH(R')-COOH gives R-CH(NH2)-CO-NH-CH(R')-COOH + H₂O.

Zwitterion form contains -NH3+ and -COO-.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1Write the dipeptide linkage between glycine and alanine.
2Remove OH from the -COOH group of glycine and H from the -NH2 group of alanine, then join the remaining carbonyl carbon to nitrogen to form -CO-NH-.

Method you can reuse

1Identify the alpha carbon, amino group, carboxyl group, and side chain.
2For peptide formation, select -COOH from one amino acid and -NH2 from another.
3Remove H₂O and write the amide linkage as -CO-NH-.

Visual tip

A peptide bond formation diagram is required, showing two amino acids, loss of H₂O, and the -CO-NH- linkage.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What small molecule is eliminated during peptide bond formation?

Think first, then show answer

Water is eliminated when the carboxyl group of one amino acid reacts with the amino group of another.

Common wrong answer

Writing an ester linkage between amino acids is wrong; amino acids form amide-type peptide bonds, not ester bonds.

Don't mix this up

Students often write the peptide bond as -COOH-NH2-. The correct peptide linkage after water loss is -CO-NH-.

Exam tip

In peptide bond questions, circle the carboxyl group of one amino acid and the amino group of the next before writing the product.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as definition of peptide bond, zwitterion explanation, structure identification, or product of condensation between amino acids.

1
Enough for 1 mark
Proteins are polymers of alpha-amino acids joined through peptide bonds, and a peptide bond is the amide linkage -CO-NH- formed between the carboxyl group of one amino acid and the amino group of another.
2
Add for 2 marks
Example: Glycine and alanine can combine by loss of water to form a dipeptide containing the -CO-NH- peptide linkage.
3
Add for 3 marks
Amino acids contain both an amino group and a carboxyl group.

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