Amino Acid Zwitterions and Predominant Structures at Different pH

Concept: Zwitterion

Definition and Properties

The zwitterion is the predominant form of most free amino acids at physiological pH (~7). A zwitterion is a molecule that contains both positive and negative charges, but is overall electrically neutral.

• Backbone Structure: Amino acids have a central carbon (α-carbon) bonded to an amino group (–NH3+), a carboxyl group (–COO−), a hydrogen atom, and a variable side chain (R group).

• Charge Distribution: At physiological pH, the amino group is protonated (–NH3+) and the carboxyl group is deprotonated (–COO−), resulting in a net charge of zero.

• Acid-base Reactions: The zwitterion form arises due to acid-base reactions involving the amino and carboxyl groups.

Example: At pH 7, the structure of glycine is:

• NH3+–CH2–COO−

Determining Predominant Amino Acid Structures

pKa and Protonation States

The pKa value is a measure of the strength of an acid. It indicates the pH at which half of the species is protonated and half is deprotonated. The predominant structure of an amino acid at a given pH depends on the pKa values of its functional groups.

• Hydrogen Ion Concentration: Lower pH means higher concentration of hydrogen ions (more acidic).

• Carboxyl Group: Typical pKa ~2. At pH above pKa, the carboxyl group is deprotonated (–COO−).

• Amino Group: Typical pKa ~9. At pH below pKa, the amino group is protonated (–NH3+).

• Side Chains: Some amino acids have ionizable side chains with their own pKa values (e.g., Lys, Arg, Asp, Glu, His, Cys, Tyr).

Example: For alanine at pH 7:

• Carboxyl group: deprotonated (–COO−)

• Amino group: protonated (–NH3+)

• Net charge: 0 (zwitterion)

Practice: Identifying Structures at Different pH

• At low pH (acidic): Both amino and carboxyl groups are protonated (–COOH and –NH3+), net charge +1.

• At high pH (basic): Both groups are deprotonated (–COO− and –NH2), net charge –1.

• At physiological pH: Zwitterion form predominates.

Table: Predominant Species of Amino Acid Backbone at Different pH

pH Range	Predominant Species	Net Charge
Low pH (<2)	–COOH, –NH3+	+1
Physiological pH (~7)	–COO−, –NH3+	0 (zwitterion)
High pH (>9)	–COO−, –NH2	–1

Example: Valine at pH 12

Given pKa values: pK1 (carboxyl) = 2.32, pK2 (amino) = 9.62.

• At pH 12, both carboxyl and amino groups are deprotonated:

• Structure: –COO−, –NH2

• Net charge: –1

Practice Problems

• Fill in the groups for the predominant structure of Ala at pH 13: –COO−, –NH2

• Fill in the appropriate groups for Arg at pH 4.3: –COOH, –NH3+, side chain (guanidinium) protonated

• Draw the predominant structure of Arg at pH 8.91 (pK1 = 1.82, pK2 = 9.04, pKR = 12.48): –COO−, –NH3+, side chain (guanidinium) protonated

Key Equations

• Henderson-Hasselbalch Equation:

$\text{pH} = \text{p}K_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right)$

• Used to calculate the ratio of protonated to deprotonated forms at a given pH.

Summary Table: Amino Acid Backbone Net Charge vs. pH

pH	Carboxyl Group	Amino Group	Net Charge
<2	–COOH	–NH3+	+1
~7	–COO−	–NH3+	0
>9	–COO−	–NH2	–1

Additional Info

• The concept of zwitterions is crucial for understanding amino acid behavior in solution, protein structure, and enzyme function.

• Knowing the predominant form of amino acids at different pH values is essential for predicting their reactivity and interactions in biological systems.