What is the fraction of total tyrosine that will be in the complete deprotonated form at...

What is the fraction of total tyrosine that will be in the complete deprotonated form at pH 9.6?

Given pKa -COO = 2.2, pKa -NH3 = 9.11, pKa -OH = 10.07

Expert Solution

We want to completely deprotonate tyrosine; hence, we must remove the proton from the –OH group to form an anion.

The pH is 9.6 which is close to pK (-OH) = 10.07; hence, we shall use the value of pK as 10.07.

Use the Henderson-Hasslebach equation.

pH = pK + log [A^-]/[HA] where HA is the protonated form and A^- is the fully deprotonated form. Plug in values.

9.6 = 10.07 + log [A^-]/[HA]

===> -0.47 = log [A^-]/[HA]

===> [A^-]/[HA] = antilog (-0.47) = 0.3388

Thus, the ratio of the fully deprotonated and protonated forms of tyrosine at pH 9.6 is 0.3388:1. The fraction of the fully deprotonated form is given by

Fraction A^- = [A^-]/([A^-] + [HA]) = 0.3388/(0.3388 + 1) = 0.3388/1.3388 = 0.2531 ≈ 0.25 (ans).

queen_honey_blossom answered 2 years ago

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