JEE Advanced and IIT-JEE Questions: Amino Acids and Peptides

Transcript

0:00Introduction

Hello everyone! Let us start with a puzzle from a real IITJEE question, from twenty eleven. One mole of a decapeptide, a chain of ten amino acids, weighs seven hundred and ninety six grams. We break it completely into amino acids and weigh all the pieces. Together they weigh nine hundred and fifty eight grams! So the pieces weigh more than the whole. Where did the extra mass come from? Today we solve past JEE Advanced and IITJEE questions on amino acids and peptides with just three ideas. They come up often, so let's begin.

0:40Idea 1: the zwitterion and the pH

So, idea one, the zwitterion. In water, and even in the solid, a proton moves from the COOH group of an amino acid to its N H two group. So one end becomes H three N plus, where the raised plus is a charge and the lowered three counts hydrogens, and the other end becomes COO minus. In the middle, R is the side chain, and the short dashes are bonds. This dipolar ion is the zwitterion, in the middle box of the figure. Okay? Now, its plus and minus cancel, so at the isoelectric point it has no net charge and does not move in an electric field. But it still reacts with acids and bases. So tell me, which group can give a proton away, and which group can take one? Yes, exactly, the N H three plus group holds an extra proton and can give it away, so it is the acidic group. And COO minus can take a proton, so it is the basic group. Remember this one, because Example one depends on it. Clear? Now add acid, H plus, so the pH is low. The COO minus takes the proton and becomes COOH; the arrow means becomes. So we get the cation in the left box, and it moves to the cathode. Right? And add alkali, O H minus, so the pH is high. Now the N H three plus loses its proton and becomes N H two. We get the anion in the right box, and it moves to the anode. The long arrow at the bottom shows the pH rising. Got it?

2:16Idea 2: the peptide bond and its direction

Now idea two, the peptide bond. Look at the figure. The O H of glycine's COOH group and a hydrogen from alanine's N H two, inside the little box, leave together as a water molecule, H two O, okay? What is left joins up, and this amide bond, C O, N H, is the peptide bond. We always write a peptide with the N-terminal residue, the one with the free N H two, on the left, and the C-terminal residue, with the free COOH group, on the right. So the figure shows glycyl-alanine, written Gly-Ala. With me? Now join them the other way round, alanine first and glycine second, and we get alanyl-glycine. Same two amino acids, so is it the same dipeptide? No, it is a different peptide. In alanyl-glycine the free N H two is on alanine, and the free COOH group is on glycine, so the order matters. Clear? That is exactly an IITJEE question from two thousand and three. Lysine and glutamine can join in two ways, so they give two dipeptides, lysyl-glutamine and glutaminyl-lysine.

3:31Idea 3: count the bonds, the mass and the water

Right? Now idea three, some simple counting. Our decapeptide has ten residues in one chain. So how many peptide bonds does it have? It must be ten, one for each residue, right? Careful here, because the answer is nine, not ten. The bonds sit between the residues, so a chain of n residues has n minus one peptide bonds. Only a cyclic peptide, closed into a ring, has n bonds. Okay? Forming each bond removes one water molecule, eighteen grams per mole. So the molar mass is the sum of the amino acid molar masses, minus eighteen times n minus one; the Greek capital sigma means the sum. For glycyl-alanine, glycine seventy five plus alanine eighty nine, minus eighteen, gives one hundred and forty six grams per mole; the raised minus one means per. Right? And complete hydrolysis is just the reverse, so it adds back n minus one molecules of water. So our puzzle is solved: the pieces weigh more because water joined them. Clear?

4:36Example 1: basic groups in lysine (IIT-JEE 2010)

Now Example one, from IITJEE twenty ten. Look at this form of lysine. An N H three plus sits on a chain of four C H two groups; the lowered four outside the brackets means four of them. Then comes a carbon carrying a free N H two, and last, COO minus. Find the total number of basic groups. Pause the video and try it first. Done? Okay, let's check. Lysine is a basic amino acid, and this form has two nitrogen groups and one COO minus. So that makes three basic groups, right? No, that is the trap, so be careful. The N H three plus on the side chain already holds its proton. Remember idea one: this group can only give a proton away, so it is acidic, not basic. Okay? Now the N H two on the alpha carbon is free; alpha is the Greek letter on screen. Its nitrogen can take a proton, so it is basic. Right? And COO minus can take a proton too, just as in the zwitterion, so it is basic. The double arrow means gives, so the answer is two basic groups. Got it?

5:55Example 2: glycine units in a decapeptide (IIT-JEE 2011)

Now Example two, from IITJEE twenty eleven, our puzzle from the start. Here, a decapeptide of molar mass seven hundred and ninety six, on complete hydrolysis, gives glycine, with molar mass seventy five, alanine and phenylalanine. Glycine makes up forty seven point zero percent of the total mass of the products. Find the number of glycine units. Pause the video and try it first. Done? Okay, let's check. Many students take forty seven percent of seven hundred and ninety six, which is about three hundred and seventy four; the wavy equals sign means about. Then they divide by seventy five, the division sign, and get almost exactly five. So the answer is five, right? No, that is the trap from our puzzle. The forty seven percent is of the products, and the products weigh more than the peptide. So start with idea three. Ten residues means nine peptide bonds. Okay? Hydrolysis adds back one water for each bond, so nine molecules of water. Nine times eighteen gives one hundred and sixty two grams. Right? So the products weigh seven hundred and ninety six plus one hundred and sixty two, which is nine hundred and fifty eight grams. That is the extra mass from our puzzle, and it is just water. With me? Glycine is forty seven percent of this. Zero point four seven times nine hundred and fifty eight gives about four hundred and fifty grams. Clear? And one mole of glycine is seventy five grams, so four hundred and fifty over seventy five gives six. So the decapeptide has six glycine units, not five. The water decided the answer, and that closes our puzzle from the start.

7:48Quick revision: zwitterion, N-terminal, n minus 1 bonds

So, quick revision. In the zwitterion, N H three plus is acidic, while COO minus and a free N H two are basic. Peptides are written with the N-terminal residue on the left, so glycyl-alanine and alanyl-glycine differ. And n residues have n minus one peptide bonds, so hydrolysis adds back n minus one waters. Now go and try the other past questions on this page. All the best!