# Applying Henderson–Hasselbalch equation to amino acids: which pKa to use to calculate Z-/HZ ratio?

I was wondering which $$\mathrm{p}K_\mathrm{a}$$ to use when calculating the ratio of $$\ce{HZ}$$ to $$\ce{Z-}$$ of amino acids, the Henderson–Hasselbalch formula used:

$$\mathrm{pH} = \mathrm{p}K_\text{a} +\log\frac{[\ce{Z-}]}{[\ce{ZH}]}$$

Because most amino acids have two $$\mathrm{p}K_\mathrm{a}$$'s, one for the amino group and one for the carboxyl group (and sometimes a third $$\mathrm{p}K_\mathrm{a}$$ for the side chain). For example, leucine:

$$\begin{array}{cc} \hline \text{Residue} & \mathrm{p}K_\mathrm{a} \\ \hline \ce{NH3} & 9.74 \\ \ce{COOH} & 2.33 \\ \hline \end{array}$$

Let's say I want to know the ratio of $$\ce{HZ}$$ to $$\ce{Z-}$$ for a $$\mathrm{pH}$$ of $$1.00$$ and $$11.00$$, which $$\mathrm{p}K_\mathrm{a}$$ should be used in the Henderson–Hasselbalch equation and why?

• Both? I don't see why you can't use both at the same time. When $\mathrm{pH} = \mathrm{p}K_{\mathrm{a}}$, the corresponding dissociation is at 50%. When they're not that close, then one form will greatly predominate over the other... – Zhe Oct 26 '16 at 17:14
• how would you calculate the Hz/Z- ratio based on the two Pka's given at Ph = 1 e.g.? Offcourse it's possible to use both the Pka's in two different calculations but how would you combine these to get the Hz/Z- ratio? @Zhe – KingBoomie Oct 26 '16 at 17:37
• Check out this page – Karsten Theis Mar 17 at 20:41

2. The two pKa values are obtained experimentally from a titration.$$\ce{AH2 <=> AH- + H+ <=> A^2- + 2 H+}$$ Here, the $$\ce{AH-}$$ species could be ambiguous if the pKa values are close to each other (group 1 deprotonated and group 2 protonated or vice versa). Or, for a case like hydronium, water and hydroxide where the same atom can accept two protons, $$\ce{AH-}$$ would represent a single defined species. You can get the ratios of the concentrations from the pH and the respective pKa ($$\ce{AH2}$$ vs $$\ce{AH-}$$ via $$K_1$$, and $$\ce{AH2}$$ vs. $$\ce{AH-}$$ via $$K_2$$), and turn that into fractions of the total for each species. You can use that for a weighted average to calculate the average charge, for example.
This depends what HZ and Z- refer to. With two ionizable groups, there are really four species, $$\ce{HZYH, HZY^-, ^-ZYH and ^-ZY-}$$. If the pKa values are far apart, one of the mixed species would be very unlikely in aqueous solution (in your example, protonated carboxylate with deprotonated amino group). At pH 1.00, the amine would be 100% protonated, and the carboxylic acid would be mostly protonated (comparing its pKa of 2.33 to a pH of 1.00). At pH 11, the carboxylic acid would be 100% deprotonated, and the amine would be mostly deprotonated (comparing its pKa of 9.74 to a pH of 11.00).