Question 2

Tyrosine is a triprotic amino acid with pKa of 2.17, 9.19, and10.47 The first proton is removed

from the carboxylic acid ( –COOH), the second from theprotonated amine group (–NH3

+) . The

third, with a pKa of 10.47, is the phenolic proton (–OH) on thearomatic ring.

Tyrosine

a) What is the structure of the most protonated form of tyrosinein aqueous solution?

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b) Write the three acid dissociation equilibria with the correctcharge (use Tyr as shorthand

for the part of the molecule excluding the acidic protons, i.e.Tyr(charge), HTyr(charge),

H2Tyr(charge), H3Tyr(charge)).

c) Suppose we titrate a 0.100 M solution of tyrosinehydrochloride salt (often written

Tyr!HCl) with a 0.100 M solution of a strong base (NaOH).Calculate pH at the

following points:

i. initially,

ii. half way to the first,

iii. at the first,

iv. half way to the second,

v. at the second, and

vi. half way to the third equivalence points

d) Using your results calculated in (c) sketch an accuratetitration curve.

e) Based on your sketch, identify which equivalence point(s) canbe used to identify end

point with an indicator dye.

f) For part (e), indicate what the pKa (or range) of indicatordye should be to minimize

determinate titration error.

g) Write the general expressions for the mass balance and thecharge balance for part (c).