The substituents R1 and R2 for nine peptides are listed in the table given below. How many of these peptides are positively charged at pH = 7.0 ? Peptide R1 R2 I H H II H CH3 III CH2COOH H IV CH2CONH2 (CH2)4 NH2 V CH2CONH2 CH2CONH2 VI (CH2)4 NH2 (CH2)4 NH2 VII CH2COOH CH2CONH2 VIII CH2OH (CH2)4 NH2 IX (CH2)4 NH2 CH3

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Chemistry

pH Scale and Relative Strength of Acids

pH Calculation for Acids and Bases

Amino acids

Question

The substituents R₁ and R₂ for nine peptides are listed in the table given below. How many of these peptides are positively charged at pH = 7.0 ?

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Any amino acid will exist in a cationic form in solution of pH < pI (isoelectric point)

$∴$ pH = 7 is less than pI given which implies pI > 7 $∴$ basic amino acid.

Given peptide will have basic nature if number of basic sites are greater than acidic sites.

$∴$ Peptide IV R₁ = CH₂CONH₂ R₂ = (CH₂)₄NH₂

Peptide VI R₁ = (CH₂)₄NH₂ R₂ = (CH₂)₄NH₂

Peptide VIII R₁ = CH₂OH R₂ = (CH₂)₄NH₂

peptide IX R₁ = (CH₂)₄NH₂ R₂ = CH₃