Zwitterions and the Isoelectric Point

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Glycine is an amino acid, a small organic molecule. An amine or an acid of similar molar mass melts at a low temperature. But glycine melts only above five hundred kelvin, and even then it decomposes. And it dissolves in water, like a salt. So how can a small organic molecule behave like a salt? And there is more: at one special p H value, it does not move at all in an electric field. Today we find out why. This is an important idea for JEE, so let's begin.

0:37The zwitterion: a salt inside one molecule

So, every amino acid carries two groups: the amino group, N H two, which is basic, and the acid group, COOH; the small two is a subscript. Now let me ask you this: an acid and a base in the same molecule, what will they do? Yes, exactly, they react with each other. The acid group hands its proton to the amino group, so the proton moves from COOH to N H two, okay? So the nitrogen end becomes N H three plus; the small raised plus is its positive charge. The acid end becomes COO minus. R is the side chain, and each dash is a bond. One molecule with a plus end and a minus end: this is a zwitterion, or dipolar ion, the middle box in the figure. Amino acids exist like this in the solid and in water. With me? Now back to our puzzle. Strong attractions between these plus and minus charges hold the crystal together, just like a salt. So amino acids are crystalline solids with a high melting point, and they dissolve in water. That explains glycine. Clear? And just like a salt, they are insoluble in non-polar solvents. Got it?

1:53Amphoteric: reacts with acids and with bases

Now, amino acids are amphoteric: they react with acids and with bases. So tell me, in the zwitterion, which group is the acid, the one that can give a proton? If you said COOH, then careful, because the zwitterion has no COOH any more! The acidic group is N H three plus; it can give back its proton, H plus. Right? And the basic group is COO minus, because it can take a proton back. Remember this pair. Okay? So add an acid, H plus, and the COO minus takes a proton. We get the cation, the left box in the figure. With me? Add a base, O H minus, and it pulls the proton off the N H three plus. We get the anion, the right box. The arrows point both ways, so these changes are reversible. Clear?

2:48The isoelectric point (pI)

Now look at the p H arrow under the figure: as it rises, we go from cation, to zwitterion, to anion. In between, at one p H value, the charges cancel and the net charge is zero. This is the isoelectric point, written small p, capital I, on screen. Okay? Now put the amino acid in an electric field at exactly this p H value. Which way will it move? It does not move at all, yes, exactly. With no net charge, the pulls on its plus end and its minus end cancel, so there is no net force. That was our second surprise. Right? And at the isoelectric point the solubility is least: with no net charge, the molecules do not repel one another, so they pack together most easily. Got it? Now, suppose the p H is below the isoelectric point, with extra H plus around. Will it be a cation, or an anion? It is a cation, that is correct. The extra H plus turns the COO minus back into COOH, so only the plus charge is left, and a cation moves to the cathode. Clear? And above the isoelectric point, the N H three plus loses its proton, so we get an anion, moving to the anode. An easy way to remember it: low p H, lots of H plus, so a plus charge. With me?

4:12Finding the isoelectric point

So how do we find it? Take a neutral amino acid, with equal numbers of amino and acid groups. It has two values, p K a one and p K a two; the a one and a two are subscripts. The isoelectric point is their average, one half times, p K a one plus p K a two, about six; the wavy equals sign means approximately. Okay? Acidic amino acids, with an extra acid group, have it near three, and basic amino acids, with an extra basic group, near ten. Right? These have three p K a values, so here is the rule: average the two values on either side of the neutral form, the form with zero net charge. Example two needs this rule. Clear?

4:57Example 1: pI of glycine from pKa 2.34 and 9.60

Alright, example one. For glycine, p K a one is two point three four, and p K a two is nine point six zero. Find its isoelectric point. Pause the video and try it first. Done? Okay, let's check. Glycine has one amino group and one acid group, so it is neutral, and we average its two p K a values. With me? Two point three four plus nine point six zero gives eleven point nine four, so the isoelectric point is one half times eleven point nine four. Right? That is five point nine seven, close to six, just as expected for a neutral amino acid. If you got five point nine seven, very good! Clear?

5:44Example 2: pI of aspartic acid, an acidic amino acid

Now a harder one. Aspartic acid has three p K a values: one point eight eight for the alpha COOH, with alpha the Greek letter on screen, three point six five for the side chain COOH, and nine point six zero for the N H three plus. Find its isoelectric point. Pause the video and try it first. Done? Okay, let's check. Many students average the first and the last, as for glycine: one point eight eight and nine point six zero, giving five point seven four. Is that right? That is the trap, so be careful here. Aspartic acid has an extra acid group, so it is acidic, and its isoelectric point must be near three, so five point seven four cannot be right. Okay? Follow the charge as the p H rises. In strong acid we have N H three plus and two COOH groups, so the charge is plus one. Past one point eight eight, the alpha COOH loses its proton, so the charge is zero. Past three point six five, the side chain loses its proton, minus one, and past nine point six zero, the N H three plus does, minus two. The number over each arrow is the p K a. With me? So the neutral form, with zero charge, lies between one point eight eight and three point six five, the two values on either side of it. Right? So the isoelectric point is one half times, one point eight eight plus three point six five, which is one half times five point five three. That gives two point seven six five, about two point seven seven, near three, as expected. Clear?

7:30Quick revision: zwitterion, pI and movement in a field

So, quick revision. An amino acid exists as a zwitterion, so it is salt-like; its acidic group is N H three plus, and its basic group is COO minus. At the isoelectric point there is no net charge and no movement; below it, a cation goes to the cathode, and above it, an anion goes to the anode. And to find it, average the two p K a values on either side of the neutral form. Now go and try the practice problems on this page. All the best!