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# Isoelectric point and zwitterions

The isoelectric point of an amino acid is the pH at which the amino acid has a neutral charge. You will learn how to calculate the isoelectric point, and the effects of pH on the amino acid's overall charge. We will also discuss zwitterions, or the forms of amino acids that dominate at the isoelectric point. By Tracy Kovach. Created by Tracy Kim Kovach.

## Want to join the conversation?

• What are the consequences of an amino acid being charged electrically?
• Because the sides chains of amino acids can be positive and negative, amino acids from different parts of the polypeptide chain will attract or repel each other so will determine how the protein folds and so the overall shape of the protein.

The charges will also determine how the protein interacts with other proteins and molecules. For example, a protein that interacts with DNA will need some positive charges on or near its surface so it can bind the negative charges of the phosphates on DNA.

Charges will affect how a protein binds small molecules and so are very often crucial in the reactions an enzyme catalyses.

Finally, the charges are useful for researchers because different proteins will have different combinations of amino acids and therefore different isoelectric points, which allows proteins to be separated from one another and thus purified.
• can you please do a video showing how we would find the pi with an actual amino acid with a side chain, how we would go about figuring it out
• Calculating pI for different side chains
(So I just learned this by getting an answer wrong on an MCAT practice exam...)

Neutral side chain: pI = 0.5[(pKa of main carboxyl group] + [pKa of main chain amino group])

Acidic side chain: pI = 0.5([pKa of main carboxyl group] + [pKa of side chain])
the pKa of the main chain amino group is NOT included in the equation

Basic side chain: pI = 0.5([pKa of main chain amino group] + [pKa of side chain])
the pKa of the main chain carboxyl group is NOT included in the equation
• what's the difference between a zwitterion and a molecule that is amphoteric?
• A zwitterion is a molecule that possesses both a positive and negative charge on the same molecule. One stipluation is that these two opposing charges must exist on that one molecule at the same time. For example, at certain pH's, some amino acids will be zwitterionic. Note the diprotic amino acid, Alanine. Alanine has a non-protic side chain (a methyl) , and thus at pH = 7.4 (physiological pH), the carboxyl group has a negative charge (COO-), and the amino group has a positive charge (RNH3+). Therefore, at pH 7.4, L-Ala is zwitterionic. Amphoteric molcules are not necessarily zwitterionic. Take for example ammonia (NH3). Ammonia has a pKa of 38. Thus, any molecule with a pKa less than 38 will protonate ammonia, and if the pKa of the molecule is greater than ammonia, NH3 will deprotonate it. Ammonia will be protonated by acetic acid (pKa 4.76), but deprotonated by an alkane with a negative charge/lone pair (pKa 50). In this case ammonia can be acidic or basic, making it amphoteric (can act as an acid or a base). However, it can never have both a positive and negative charge at the same time, so ammonia cannot be zwitterionic. Really and truly, pretty much any molecule can be amphoteric, if put it in contact with molecules that have higher and lower pKa values then itself. This has limits, however, in the human body, as our pH homeostasis is tightly regulated. Hope that helps.
• Does pH stand for "power hydrogen"?
• Yes! Søren Peter Lauritz Sørensen, a Danish scientist coined the term pH in 1909 to mean "power of hydrogen".
• please elaborate the concept of pka
• You will need a separate video to understand it well. Take a generic dissociation of an acid
HA <-> H+ + A-
The Ka value (K being equilibrium constant and a being the denotation for acid) for this dissociation is equal to the concentration of products over reactants
[H+]*[A-]/[HA]. So if your Ka value is >1, you know more products formed than reactants remained. If it is <1, you know that the product didn't dissociate into completion very well. Just like pH is -log[H+], pKa is defined to be -log[Ka].
• You wrote NH3 in the video, isn't it suppose to be NH2
• -NH2 is the uncharged form, but at low pH (high concentration of H+), some of the H+ binds the nitrogen atom to form -NH3+.
• What kind of general reaction would zwitterions undergo if dissolved into water instead of strong base or strong acid?
• This is a great question! For most amino acids, zwitterions would be present at pH 7. Which is significant as, the physiological pH of the cells in our bodies is approximately 7.4. The amino acids in water would have the carboxy group unprotonated and the amino group protonated (zwitterion), and this is would be fluidly changing. However, it would reach equilibrium at the zwitterion state.
• If NH3 was a H+ acceptor then why did it lose a H+ in the basic solution?
• The way I like to think of it is that the basic solution was MORE basic than the basic NH3 group. Because both the solution and NH3 are basic, they both want to accept protons. However, the solutions was MORE basic than the NH3 groups, so it wants to accept protons more than the NH3 group. For that reason the NH3 lost it's proton. It didn't want the proton as much as the solution did.

Not necessarily an empirical way of thinking about it but makes sense to me and helps me wrap my mind around it! Hope it helps you too.