Need help preparing for the Organic Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach you Part 4 about isoelectric point of amino acids. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Summary of Isoelectric Point Determination
Now that you’ve read about finding the isoelectric point under various circumstances, it is time to review and summarize. First, all polar and nonpolar amino acids have an isoelectric point around 6. Since they only have two acidic groups, the carboxylic acid group with a pKa of ~2, and the amino group with a pKa of ~9 to ~10, we will always observe a value around 6 for the isoelectric point of all polar and non-polar amino acids.
Acidic amino acids and basic amino acids have a third acidic group on the side chain. As we saw with our test case of aspartic acid, acidic amino acids have an isoelectric point less than 6. On the other hand, basic amino acids like lysine have an isoelectric point greater than 6.
The other thing that's important is the isoelectric point is the pH at which you either have exclusively your zwitterion in solution or the mean net charge of all species in solution is zero. However, if the pH is less than the pI or the pH is greater than the pI, then your molecule is not all in its neutral form or the mean net charge of species in solution is not zero. When the pH is less than the pI, there will be a net positive charge (as a mean) on the amino acids in solution and when the pH is greater than the pI there will be a net negative charge (again, as a mean) on the molecules in solution.
Isoelectric Points and Charge Prediction for Peptides
On the MCAT, you're not always going to be working with individual amino acids. Sometimes, you're going to get questions asking about the charge of peptides, which are simply polymeric structures of 2 or more amino acids linked together.
From our previous discussion about the protonation states of amino acids, we can make some simple predictions: At physiological pH, the deprotonated carboxylate group of acidic amino acids should contribute a -1 charge, while the protonated (conjugate acid) form of basic amino acids should contribute a +1 charge to any peptide it is a part of. Further, at physiological pH, the +1 from the N-terminal amine and the -1 from the C-terminal carboxylate should cancel out. We can also further predict that at non-physiological pHs we need only compare the pKa of the residue to the pH of the environment and determine whether any particular group is protonated or deprotonated - protonated carboxylic acids add +0 while deprotonated ones will count for -1, and protonated bases will account for +1 and deprotonated ones for 0.
N-ANDLIFPE-C
What is the charge of this peptide at pH eight? Notice first that we have only one N-terminus and one C-terminus, and all other amines and carboxylates of the backbone are currently participating in peptide bonds. This means, we only need to consider the net charge of these two termini and the charges on any acidic groups.
Recall from earlier in this chapter that the pKa of a carboxylic acid terminus is approximately 2, and that of a terminal amine is approximately 9. So, if we are in an environment of pH of eight, we can expect the carboxylic acid to be deprotonated and the amine to still be almost exclusively protonated. Consequently, we can expect a -1 charge on the carboxylate group and a +1 on the amine, for a total of +0 for the two termini. This also suggests that we are likely to only care about the contribution from our N and C termini in extreme environments, in cyclized peptides or in post-translationally modified ones.
So now that we've dealt with the N and C terminus, we need to consider the charges on our residues. Logically, any polar and nonpolar amino acids will not be carrying labile protons and therefore not be charged within our solution: Alanine (A), asparagine (N), leucine (L), isoleucine (I), phenylalanine (F), proline (P) - their side chains can't take on a charge so you can ignore them. You only need to take note of the basic and acidic amino acids: We have aspartic acid (D) and glutamic acid (E). Both have acidic side chains that will be deprotonated in a solution of pH 8, and therefore contribute a -1 charge each, for a total of -2.
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