Isoelectric Point of Amino Acids, Part 1 | MCAT Organic Chemistry Prep

Need help preparing for the Organic Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach you Part 1 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!

Useful Terms

In order to carry on a discussion about isoelectric points and the arithmetic methods to determine them, we need to define some terms. A zwitterion, sometimes also called a dipolar ion, is a molecule with an equal number of positive and negative charges. Meaning it is an ion, but its net charge is zero.

An isoelectric point (pI) is the pH at which the statistical mean of charges on a particular zwitterion is zero – for simple divalent zwitterions such as amino acids, virtually all molecules in solution at the pI will be in their zwitterion form. This can be a bit more complicated for more complex molecules, a situation that the MCAT may present.

It may seem counterintuitive to not simply say that all molecules are in their zwitterionic form at the isoelectric point. But recall however that acids and bases have dissociation constants (Ka and Kb, respectively) and logarithmic simplifications to make comparison with the chemical environment easy to perform (pKa and pKb). These values correspond to the pH value at which 50% of the functional group of interested will dissociate. In the case of an acid, this means that when the pH is less than pKa, the moiety of interest will be predominantly protonated. Conversely, when the pH is greater than the pKa, then the moiety of interest will be primarily deprotonated. And finally, if the pH of the solution is equal to the pKa of a particular functional group, that functional group will be protonated in 50% of instances and deprotonated in the other 50%.

Polar and Non-Polar Amino Acids

With these terms now well-defined, we can take a look at amino acids, starting with polar and non-polar amino acids. We are starting our discussion with both polar and non-polar amino acids as a matter of simplicity: Because they have only a single amine and carboxylic acid group each, they provide an instance where finding the isoelectric point is relatively straight-forward. This is in contrast to what may happen when determining the isoelectric points of acidic and basic amino acids, in which case we will have three reactive groups to consider.

With the molecule in its left-most state, we see that both the amine and carboxylic acid group are protonated – this is the form that predominates in low-pH solutions. Note that the pKa of the carboxylic acid group is approximately 2, and the pKa of the amine (note: pKa of the conjugate acid of the amine, also called the pKb) is approximately 9. This is a pattern that holds true for most α-amino acids and is very well worth being aware of for the MCAT – you may well be asked to predict the approximate properties of amino acids in solution.

As we go from left to right, the pH of the environment that our amino acid is in increases and consequently the protonation state of both its termini should change. At a pH of approximately 2 we observe that 50% of our carboxylate groups are deprotonated and 50% protonated. At a neutral pH (7), virtually all carboxylic acid groups are deprotonated, and the amino acid exists in solution predominantly in the zwitterionic form.

Continuing this trend, in an environment with a pH of approximately 9, we can see that half the amine will be in its non-protonated, basic form. And in a strongly basic environment, all amine functional groups will be deprotonated and the amino acid as a whole can be expected to exist almost exclusively in its negatively charged form.

Looking at these three molecules, we can see that polar and nonpolar amino acids can take on three different states of protonation or deprotonation in solution. Let’s take a look at what that means for the charge of these molecules. On the left, at the lowest pH, we see a +1 charge. In neutral solution, we see a net zero charge on a zwitterion: The amino group has a +1 charge, the carboxylate group has a -1 charge and the net charge ends up being zero. And on the last and right-most molecule, we simply see a net charge of -1.

It may be tempting to think that the isoelectric point should simply be the pH at which the zwitterion is present. However, consider the situation at a pH of 2. In this situation, the pH is equivalent to the pKa of the carboxylic acid group. That means half of our carboxyl group is protonated and half of it is deprotonated.

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