d. Which amino acid has a greater negative charge at pH = 6.20, glycine or methionine?

Verified step by step guidance

1. Determine the pKa values of the functional groups in glycine and methionine. Glycine has a carboxyl group (pKa ≈ 2.34) and an amino group (pKa ≈ 9.60). Methionine also has a carboxyl group (pKa ≈ 2.28) and an amino group (pKa ≈ 9.21). Neither amino acid has an ionizable side chain.
2. At pH = 6.20, compare the pH to the pKa values of the carboxyl and amino groups for each amino acid. If pH > pKa, the group will be deprotonated; if pH < pKa, the group will be protonated.
3. For both glycine and methionine, the carboxyl group (pKa ≈ 2.3) will be deprotonated at pH = 6.20, resulting in a negatively charged carboxylate ion (\( \text{COO}^- \)).
4. For both glycine and methionine, the amino group (pKa ≈ 9.5 for glycine and 9.2 for methionine) will remain protonated at pH = 6.20, resulting in a positively charged ammonium ion (\( \text{NH}_3^+ \)).
5. Since neither glycine nor methionine has an ionizable side chain, the net charge of each amino acid at pH = 6.20 will depend only on the charges of the carboxyl and amino groups. Both amino acids will have the same net charge at this pH, so neither has a greater negative charge.

Key Concepts

Amino Acid Structure

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid has a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R group (side chain). The nature of the R group determines the amino acid's properties, including its charge at different pH levels.

pH and Charge

pH is a measure of the acidity or basicity of a solution, influencing the ionization of functional groups in amino acids. At a given pH, the carboxyl group of an amino acid typically donates a proton, becoming negatively charged, while the amino group can accept a proton, becoming positively charged. The overall charge of the amino acid depends on the pH relative to its pKa values.

Isoelectric Point (pI)

The isoelectric point (pI) is the pH at which an amino acid has no net charge, meaning the positive and negative charges balance each other. For glycine and methionine, their pI values help determine their charge at pH 6.20. Amino acids with pI values lower than the solution's pH will carry a net negative charge, while those with higher pI values will be positively charged.