Acetylsalicylic acid (active molecule in Aspirin - Asp) is a prototypical analge

Question

Acetylsalicylic acid (active molecule in Aspirin - Asp) is a prototypical analgesic used in the treatment of mild to moderate pain. Acetylsalicylic acid is a weak acid with a pKa=3.45. When taken orally acetylsalicylic acid will reach the stomach, where parietal cells control the concentration of HCl by secreting H+ and Cl-. While fasting, they maintain stomach [H+] = 10-3.

I HAVE WORKED A-C, I NEED THE ANSWER TO D.

a. What is the ratio of Asp to HAsp in the stomach of the patient while fasting?

Answer I got: ratio Asp:HAsp = 0.355 or 1:2.82

b. After a meal, the parietal cells decrease the stomach pH to 1.4. To which value would the ratio of Asp- to HAsp change after a meal?

Answer I got: Asp:HAsp ratio = 8.9x10^-3 or 1:112

c. Under normal condition the blood pH is maintained to 7.4 by the bicarbonate buffer system. Which will be the predominant form of acetylsalicylic acid once absorbed in the blood stream and how many fold its deprotonation state will change compared to the stomach after a meal? Explain the rationale behind your answer.

Answer I got:

Part A: when pH > pKa => deprotonated (Asp-)

Part B: 10^6 = one million fold

d. Phosphate is the most important buffer in cells where the pH is typically maintained between 6.9 and 7.4. For H3PO4, pKa1 = 2.12, pKa2 = 7.21, pKa3 = 12.32. Which will be the main phosphate species in solution for buffering the pH at a value of 7.2? Will the protonation state of acetylsalicylic acid change significantly once uptaken in the cytosol from the blood stream?

Explanation / Answer

D. HPO24   Pka2 = 7.2

At pKa - 3.5, aspirin is in its protonated (neutral) form at pH below 2.5. At higher pH, it is in deprotonated form (anionic). So if the aspirin in the blood at pH 7.4 it is deprotonated form (anionic). When it enter into the stomach or intestine the environment is in acidic condition it becomes protonated (neutral) form. So aspirin now absorbed in to the cells.

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