Problem 19 - Chemistry: An Atoms-Focused Approach

Question

Why do most amino acids exist in the zwitterionic form at physiological \(\mathrm{pH}(\mathrm{pH} \approx 7.4) ?\)

Step-by-Step Solution

Answer

Answer: At physiological pH (approximately 7.4), amino acids favor the zwitterionic form, which has both positively charged amino groups (NH3+) and negatively charged carboxyl groups (COO-). This occurs because physiological pH is above the pKa of the carboxyl group, resulting in deprotonation and a negative charge, and below the pKa of the amino group, resulting in protonation and a positive charge. The zwitterionic form is important for maintaining the structural stability and function of proteins in living organisms.

Step 1: 1. Amino Acid Structure

First, let's understand the structure of amino acids. Amino acids are the building blocks of proteins and have a general structure that consists of a central carbon atom (known as the alpha carbon) bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a side chain (R group) that varies for each amino acid.

Step 2: 2. Ionizable Groups in Amino Acids

Amino acids have two ionizable groups: the amino group (NH2) and the carboxyl group (COOH). These groups can gain or lose protons (H+ ions) depending on the pH of the surrounding environment, resulting in charged forms of the amino acid.

Step 3: 3. Zwitterionic Form

The zwitterionic form of an amino acid occurs when both the amino group and the carboxyl group are charged, with the amino group carrying a positive charge (NH3+) and the carboxyl group carrying a negative charge (COO-). This gives the amino acid an overall neutral charge, as the positive and negative charges cancel each other out.

Step 4: 4. pH and Ionization of Amino Acids

The ionization state of the amino and carboxyl groups in an amino acid depends on the pH of the environment. At low pH (acidic conditions), the amino group (NH2) will be protonated and carry a positive charge (NH3+). At high pH (basic conditions), the carboxyl group (COOH) will lose a proton and carry a negative charge (COO-).

Step 5: 5. Zwitterionic Form at Physiological pH

At physiological pH (approximately 7.4), the ionization state of amino acids favors the zwitterionic form. This is because physiological pH is above the pKa of the carboxyl group (around 2), causing it to be deprotonated and carry a negative charge (COO-). Physiological pH is below the pKa of the amino group (around 9), causing it to be protonated and carry a positive charge (NH3+). With both groups carrying opposing charges, the amino acid exists as a zwitterion, which is important for maintaining the structural stability and function of proteins in living organisms.