Let's assume that an arginine side chain has a pKa of exactly 12. Which one of the following best represents the deprotonated:protonated ratio at a pH of 11?

Question

BlackTom AI · Accepted Answer

Interpreting the question requires applying the Henderson–Hasselbalch relationship for a buffer system where the pKa of the conjugate acid is 12 and the pH is 11.
Option 1: 1:100. This would correspond to [A-]/[HA] = 0.01, which would give pH = pKa + log(0.01) = 12 + (-2) = 10. This is not consistent with the given pH value of 11, so this option is incorrect.
Option 2: 1:10. Here [A-]/[HA] = 0.1. Using Henderson–Hasselbalch for a conjugate acid/base pair, pH = pKa + log([A-]/[HA]) = 12 + log(0.1) = 12 + (-1) = 11, which matches the stated pH. This makes it a correct representation of the deprotonated to protonated ratio at pH 11.
Option 3: 1:1. This implies [A-]/[HA] = 1, giving pH = pKa + log(1) = 12 + 0 = 12. That would correspond to pH 12, not 11, so this option is wrong.
Option 4: 100:1. Here [A-]/[HA] = 100, which would yield pH = 12 + log(100) = 12 + 2 = 14 in this simplified framework, far from 11, so this option is incorrect.
Option 5: 10:1. This ratio gives [A-]/[HA] = 10, leading to pH = 12 + log(10) = 12 + 1 = 13, again not aligning with the given pH of 11, so this option is wrong.
In summary, among the options, the one that aligns with the Henderson–Hasselbalch calculation for pH 11 when pKa = 12 is the 1:10 ratio, since log(0.1) equals −1 and shifts the pH down from 12 to 11.

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Let's assume that an arginine side chain has a pKa of exactly 12. Which one of the following best represents the deprotonated:protonated ratio at a pH of 11?

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