5) (a) Look up the structure of the amino acid tyrosine. A tyrosine amino acid h

Question

5) (a) Look up the structure of the amino acid tyrosine. A tyrosine amino acid has a side chain with a pKa of about 10.5. Draw the chemical structure of the major form of tyrosine at pH 2, 7 and 12, with appropriate charges shown (b) Draw the structure of a complete molecule of tyrosine and a molecule of water, arranged so that the water molecule forms a hydrogen bond with the tyrosine side chain (R-group). Indicate all charges as they would be in aqueous solution at neutral pH. (c) What percentage of the tyrosine side chains are deprotonated at pH 10.5? (d) What percentage of the tyrosine side chains are deprotonated at pH 8.5? (e) What percentage of the tyrosine side chains are deprotonated at pH 8.0?

Explanation / Answer

C.Assuming the pKa = 10.5 for the Tyr side chain. When pH = pKa, there are equal amounts of protonated & deprotonated forms. So, 50%is deprotonated (and 50% is protonated).

(d)About 1%deprotonated (use Henderson-Hasselbalch equation).

(e)pH = pKa + log ( [A] / [HA] ) 8.0 = 10.5 + log ( [A] / [HA] ) -2.5 = log ( [A] / [HA] ) [A] / [HA] = 10^-2.5 = 0.0032 So about 0.32 %of the Tyr is deprotonated at pH 8.0.

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