Amino Acids

Question 1

When the pK of the amine group of an amino acid is 3, in a solution with a pH of 8, which of the following will be true?

a. The amine group will be charged.
b. The amine group will be deprotonated.
c. The amine group will be protonated.

Answer 1

The amine group will be deprotonated (neutral for NH2). This is because when pK < pH, there will be a small amount of protonated amino acid, and high number of deprotonated.

Question 2

When the pKa of an amino acid is greater than the pH of its solution:

Answer 2

The COOH and NH2 groups will be protonated (NH3+ and COOH).

Question 3

In which of the following solutions would you find a large concentration of zwitterions?

a. pK = PKa.
b. pK = pH.
c. pK > pH.
d. pK < pH.

Answer 3

pK = pH - this is where the ratio of protonated to deprotonated will be equal, forming a zwitterion where the amine group is protonated (NH3+) and the carbocyl group is deprotonated (COO-), giving it an overall neutral charge to the amino acid.