Explain peptide bond formation

1.4.3 · Biology › Biomolecules — Proteins & Nucleic Acids

Peptide Bond HAI kya?

Yeh kyun zaroori hai?

• Peptide bonds saare proteins ki backbone hain — yeh amino acids ko polypeptides mein chain karte hain.
• Yeh reaction dehydration synthesis (condensation) hai: tum paani hatake build karte ho.
• Is bond mein partial double-bond character hota hai (resonance ki wajah se), jo rotation restrict karta hai aur proteins ko structural rigidity deta hai.

Mechanism: Step-by-Step Derivation

Chalo do generic amino acids se first principles use karke peptide bond formation derive karte hain.

Starting Materials:

• Amino Acid 1: H₂N-CHR₁-COOH
• Amino Acid 2: H₂N-CHR₂-COOH

(R₁ aur R₂ side chains hain — yeh kuch bhi ho sakti hain: glycine ke liye H, alanine ke liye CH₃, etc.)

Step 1: Reactive Groups Identify Karo

Yeh step kyun? Amino acids ke alpha carbon par do functional groups hote hain: ek amino group (nucleophile, electron-rich) aur ek carboxyl group (electrophile, carbonyl carbon par electron-poor). Amino group carboxyl group ke carbonyl carbon par attack karta hai.

• AA1 ka Carboxyl group: -COOH → iska ek carbonyl (C=O) carbon hota hai jo electrophilic (δ+) hai.
• AA2 ka Amino group: -NH₂ → nitrogen ke paas ek lone pair hai, jo ise nucleophilic (δ-) banata hai.

Step 2: Nucleophilic Attack

Yeh step kyun? Nitrogen ka lone pair electron-deficient carbonyl carbon par attack karta hai. Yeh ek classic nucleophilic acyl substitution hai.

${\text{H}}_2{\text{N-CHR}}_1\text{-}\underset{\text{electrophilic}}{\text{C}\text{=O-OH}}+{\text{H}}_2{\text{N-CHR}}_2\text{-COOH}$

AA2 ke amino group ka nitrogen AA1 ke carboxyl group ke carbonyl carbon par attack karta hai:

$\text{AA1-C(=O)-OH}+:{\text{NH}}_2\text{-AA2}\to \text{AA1-C(=O)-}\stackrel{+}{N}{\text{H}}_2\text{-AA2 + }{\text{OH}}^{-}$

Tetrahedral intermediate banta hai (carbonyl oxygen negatively charged ho jaata hai, nitrogen positively charged ho jaata hai).

Step 3: Proton Transfer aur Water Elimination

Yeh step kyun? Tetrahedral intermediate unstable hota hai. Oxygen nitrogen se ek proton (H⁺) le leta hai, aur OH group paani ke roop mein nikal jaata hai.

Carboxyl se: -OH nikalti hai. Amino se: -H nikalta hai. Milake: H₂O release hota hai.

Step 4: Peptide Bond

Final structure:

$\boxed{{\text{H}}_2{\text{N-CHR}}_1\text{-}\underset{\text{peptide bond}}{\text{CO-NH}{\text{-CHR}}_2\text{-COOH}}}$

Yeh ek dipeptide hai. -CO-NH- linkage hi peptide bond hai.

Key insight:

• Yeh bond resonance ki wajah se planar hota hai — C=O aur C-N dono mein partial double-bond character hota hai:

Yeh C-N bond ke aas-paas rotation restrict karta hai, jisse proteins ko defined 3D shapes milti hain.

Worked Examples

Common Mistakes

Connections

• Amino Acid Structure — building blocks jinke paas -NH₂ aur -COOH hote hain
• Primary Structure of Proteins — peptide bonds sequence define karte hain
• Dehydration Synthesis — general mechanism (carbohydrates, lipids mein bhi)
• Protein Translation — ribosomes peptide bond formation catalyze karte hain
• Hydrolysis Reactions — peptide bonds todna (digestion, proteases)
• Resonance Structures — kyun C-N bond rigid hai
• Secondary Structure — α-helices aur β-sheets planar peptide bond geometry par depend karte hain

#flashcards/biology

Peptide bond kya hota hai? :: Ek covalent C-N bond jo ek amino acid ke carboxyl group aur doosre ke amino group ke beech banta hai, jisme H₂O eliminate hoti hai.

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