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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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.

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-.

Memory hook

Peptide bond has carbonyl then nitrogen: CO-NH.

Examples and method

Worked example

Write the dipeptide linkage between glycine and alanine. Remove 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 to apply

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

Diagram support

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

How CBSE asks it

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

Avoid common mistakes

Common confusion

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

Common wrong answer

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

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.

Quick check

What small molecule is eliminated during peptide bond formation?

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

Answer writing and exam use

1-mark answer

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-mark answer

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

3-mark answer

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. 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-. Write the dipeptide linkage between glycine and alanine. Remove 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-. Asked as definition of peptide bond, zwitterion explanation, structure identification, or product of condensation between amino acids. Writing an ester linkage between amino acids is wrong; amino acids form amide-type peptide bonds, not ester bonds.
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