Question

1. The carboxylic acid group of an amino acid has a pKa of approximately 2. However, carboxylic acids like benzoic acid and acetic acid have pKas in the range of 4-5 pKa units. Explain why the carboxylic acid of an amino acid is more acidic.

2. Consider the amino acid arginine: a. At physiological pH (pH = 7.4), what is the predominant form in solution? b. What percent of the carboxylic acid group is ionized at this pH? c. What percent of the amino group is ionized at this pH?

Answer 1

1)Basically, the acidicity of -COOH group depends on how much hydroxyl bond is polarized. The more the common electron pair of OH is withdrawed towards oxygen, the more the acidicity is. Hence, groups with electron withdrawing effect increase the acidicity of caroxylic acid.

For example In acetic acid, -COOH is bound to methyl group which is electron donating group while in glycin, a hydrogen in methyl group was replaced by amino group which is electron withdrawing group. So the aciditicy of -COOH in glycine is larger than in acetic acid.

2)There are three amino acids that have basic   side chains at neutral pH. These are arginine (Arg), lysine (Lys), and histidine (His). Their side chains contain nitrogen and resemble ammonia, which is a base. Their pKa's are high enough that they tend to bind protons, gaining a positive charge in the process.