Tyrosine has a pKa of 10.0 for the ionization of the phenolic hydrogen. The appr
ID: 1071481 • Letter: T
Question
Tyrosine has a pKa of 10.0 for the ionization of the phenolic hydrogen. The approximate absorption spectra (on a scale of molar extinction coefficients) for protonated and deprotonated forms of tyrosine is given below: The absorption spectrum (on the absorbance scale) of a peptide that contains three tyrosine residues and no tryptophan or phenylalanine in a 1 cm cell is shown below: Answer the following questions, explaining or demonstrating your method clearly. What is the concentration of the peptide? What are the concentrations of the protonated and unprotonated species? What is the pH of the solution?Explanation / Answer
From the given data for tyrosine
a. concentration of peptide = 6.67 x 10^-4 + 1.27 x 10^-4 = 7.94 x 10^-4 M
b. concentration of potonated species = 0.8/1200 = 6.67 x 10^-4 M
concentration of unprotonated species = 0.18/1420 = 1.27 x 10^-4 M
c. pH = pKa - log(base/acid)
= 10 - log(1.27 x 10^-4/6.67 x 10^-4)
= 10.72
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