Amino acids in different pH

Question

Hi!

I've got a question that I'm having trouble with solving:

How does the dipeptide asp-ser (aspartic acid and serine) look like at

a) pH = 1

b) pH = 7

c) pH = 13

Motivate your answers

I would appreciate it if someone could guide me through the answers!

Thanks!

Answer 1

So, you need to look up the pKa values for the different side groups of the amino acids. As the pH increases, the COOH gets deprotonated and as the pH decreases it get protonated and the amino group is deprotonated.

So, I looked up the pKa values and found the following:

Aspartic acid:

α carboxyl = 2.09

α amino = 9.82

side change = 3.86

Serine:

α carboxyl = 2.21

α amino = 9.15

So, at pH1, all carboxyl and amino groups would be protonated and the species would have a net + charge

At pH 7, the carboxyl groups would be deprotonated (- charge) and the amino groups would still be protonated so the species would have a net - charge.

At pH 13, all the carboxyl groups are still deprotonated (- charge) and the amino groups would also now be deprotonated (no charge) so the species would have a net - charge.