JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Biomolecules

The total number of negative charge in the tetrapeptide, Gly-Glu-Asp-Tyr, at pH 12.5 will be ______. (Integer answer)

Answer: 4

Solution

At pH

12.5

every acidic group of the peptide lies well above its

p K_{a}

and is deprotonated, while the terminal ammonium group has lost its proton and is left neutral. The groups that then carry a negative charge are the C-terminal

- CO O^{-}

of tyrosine, the side chain

- CO O^{-}

of glutamic acid, the side chain

- CO O^{-}

of aspartic acid, and the phenolate

- O^{-}

of the tyrosine side chain, whose

p K_{a}

of about

10

is below the given pH. Glycine has no ionisable side chain and the peptide linkages carry none. The total number of negative charges is

4

JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Integer answer

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