Question

What would charge would you expect on alanine when placed in a solution with a pH of 1.00?

+1. Since alanine is nonpolar, we know that the only parts of the amino acid that can be charged are the N-terminus and the C-terminus.

In an acidic solution, there is an excessive amount of protons available to protonate the amino acid. As a result, the carboxylic acid end and the amine end will both be fully protonated. This will result in an overall charge of +1, due to the nitrogen having three hydrogens attached.

Let's say I am given a certain pH of 2.00 rather than 1.00 for the acidic solution and the pH in this example is of that of a non-polar acid (alanine) changes to +1.

Does the pH dictate how many protons are added to amino or carboxyl groups? What determines the number of protons being added to that amino acid on the carboxylic end? I know a carboxylic acid can take up to four protons being added on the molecule.

Answer 1

In a solution with a pH of 1.00, alanine would have a charge of +1. The number of protons added or removed is dictated by the solution's pH relative to the amino acid's pKa values. Alanine acts as an acid or base and has its own isoelectric point where it exists as a zwitterion with no net charge.

Step-by-step explanation:

When alanine is placed in a solution with a pH of 1.00, we would expect it to have a charge of +1. This is because in an acidic solution like pH 1, the carboxylic acid (-COOH) group of alanine will be protonated to form -COOH, while the amino (-NH₂) group will also be protonated to form -NH₃⁺. Since alanine does not have any side chain that can be charged, the only capable sites of gaining or losing protons are these two groups.

The pH of a solution dictates how many protons are added to or removed from the amino and carboxyl groups. This is governed by the pKa values of the respective functional groups and the current pH of the solution. Essentially, acids lose their proton when in a medium with a pH higher than the pKa, while bases gain protons in a medium with a pH lower than the pKa of their conjugate acid.

Alanine, like all amino acids, can act as both an acid and a base due to its amino and carboxyl functional groups, having an isoelectric point (pI) at which it exists in a zwitterionic form with both a protonated -NH₃⁺ group and a deprotonated -COO⁻ group, resulting in a net charge of zero. Below its pI, an amino acid like alanine will have a net positive charge due to the presence of excess H⁺ ions inducing the carboxylate group to capture a hydrogen ion and the amino group to remain protonated.