AIEEE and JEE Main Questions (to 2021): Amino Acids and Peptides

Transcript

0:00Introduction

Hello everyone! Here is a real question from JEE Main twenty twenty one. Four amino acids are joined in a chain and placed in a strongly alkaline solution, at p H twelve point five. How many negative charges does the chain carry? Your first guess may be just one, for the free carboxyl group at the end of the chain. But the real answer is bigger than that, and by the end of this video you will count every charge yourself. Today we solve three past questions from AIEEE and JEE Main on amino acids and peptides, with four simple ideas. So, let's begin.

0:40Idea 1: the α-carbon and optical activity

So, idea one: the general formula of an alpha-amino acid. H two N, with a small lowered two, is the amino group, COOH is the carboxyl group, and the short dashes are bonds. Both sit on the middle carbon, the alpha carbon, named by the Greek letter on screen. It carries four groups: the amino group, the carboxyl group, a hydrogen, and capital R, the side chain. Okay? Now let me ask you this. When do we call a carbon atom asymmetric? Yes, exactly, when it carries four different groups. The double arrows mean so: four different groups, so an asymmetric carbon, so the compound is optically active, which means it rotates plane-polarised light. Right?

1:28Idea 2: know the side chains

Now idea two: the side chains. Aspartic acid, with its code ASP in brackets, has the side chain C H two COOH; the lowered two counts hydrogen atoms. Glutamic acid, GLU, has one more C H two. So both carry an extra carboxyl group, and they are the acidic amino acids. Clear? Tyrosine, TYR, has C H two joined to C six H four, a benzene ring, with the lowered six and four, and an O H on that ring. That is a phenolic O H, and it decides Example two. Got it? Now, JEE Main twenty twenty one asked this about tyrosine. Between the alpha carbon and the ring, is there one C H two group, or two? And is the O H ortho, on a ring carbon next to the one that carries the chain, or para, on the ring carbon opposite it? Let us read the answer from tyrosine's name; the numbers are positions, and the brackets hold the ring group. Propanoic acid has three carbons, and carbon one is the carboxyl carbon. Two-amino puts the amino group on carbon two, the alpha carbon, and the ring group sits on carbon three. With me? So only carbon three lies between them, which means one C H two group. And four-hydroxyphenyl puts the O H at ring position four, opposite the chain, which is para, not ortho. Okay?

2:58Idea 3: the charge depends on pH

Now idea three. In water, an amino acid is a zwitterion, a dipolar ion, because the carboxyl group passes its proton to the amino group. So we get H three N plus, with a raised plus sign, at one end, and COO minus, with a raised minus sign, at the other, as in the middle box of the figure. Right? The p H where it has no net charge, and does not move in an electric field, is the isoelectric point; the long arrow at the bottom shows the p H rising from left to right. Below it, in acid, COO minus picks up a proton, the H plus on the lower arrow at the left, giving a cation, the left box, which moves to the cathode. Clear? Now we add alkali, O H minus, on the upper arrow at the right. Which group gives up its proton now? Yes, exactly, the N H three plus group, which becomes plain N H two. So above the isoelectric point we get an anion, the right box, which moves to the anode. Got it? And in a strongly alkaline solution, like p H twelve point five, the amino group is free N H two with no charge, and the carboxyl group is COO minus. With me?

4:15Idea 4: what stays free in a peptide?

Now the last idea. In the figure, the COOH of glycine and the N H two of alanine join; the boxed O H and H leave as water, H two O, with a minus sign beside the down arrow. The shaded amide link, C O, N H, is the peptide bond, and the product, glycylalanine, is named at the bottom. Okay? In a linear chain there is one peptide bond fewer than the number of residues, so four residues are held by three peptide bonds. Right? Now, these bonds use up the alpha amino and alpha carboxyl groups inside the chain. So which groups are still free to carry a charge? Yes, the two ends. The N-terminal residue on the left keeps a free N H two, and the C-terminal residue on the right keeps a free carboxyl group. Side-chain groups are not used in peptide bonds, so they stay free too. Clear?

5:11Example 1: all amino acids but glycine are optically active

Time for Example one, an easy question from the AIEEE paper of twenty twelve. Which statement is correct? Option A says all amino acids except lysine are optically active, and option B says all amino acids are. Option C says all except glycine, and option D says all except glutamic acid. Pause the video and try it first. Done? Okay, let's check. Every alpha carbon carries the amino group, the carboxyl group, a hydrogen, and R, the side chain. These are four different groups as long as R is not hydrogen; the slashed equals sign means is not equal to. Got it? Now, lysine has an extra amino group in its side chain. So is lysine optically inactive? No, it is not, and options A and D are both traps. The extra groups of lysine and glutamic acid sit on the side chain, so their alpha carbons still carry four different groups. Right? And glycine? Its side chain is a hydrogen atom, so its alpha carbon carries two hydrogens and only three different groups. No asymmetric carbon, so glycine is not optically active, which rules out option B too. So the answer is option C, all except glycine. With me?

6:34Example 2: negative charges on Gly-Glu-Asp-Tyr at pH 12.5

Now Example two, our hook, from JEE Main twenty twenty one. Find the total number of negative charges in this tetrapeptide at p H twelve point five. The three-letter codes stand for glycine, glutamic acid, aspartic acid and tyrosine, and each short dash between them is a peptide bond. Pause the video and try it first. Done? Okay, let's check. Glycine, on the left, is the N-terminal residue, with the free N H two. At p H twelve point five that amino group stays free, with no charge, and glycine's side chain is just a hydrogen. Okay? Now the carboxyl groups: one in the side chain of glutamic acid, one in aspartic acid, and the C-terminal one on tyrosine. In strong alkali all three lose their protons, giving three COO minus groups, as the double arrow shows. Right? So, three negative charges, and we are done, aren't we? That is the trap, because we forgot one side chain. At p H twelve point five, the phenolic O H of tyrosine also loses its proton; the arrow shows it turning into O minus. That is one more negative charge. Clear? So the total is three plus one, which is four negative charges. Our hook is solved, with one charge at the C-terminal end, two from the acidic side chains, and one from the phenolic O H of tyrosine. Got it?

8:05Quick revision: glycine, tyrosine, charge at a given pH

So, quick revision. All alpha-amino acids except glycine are optically active. Tyrosine has one C H two group and a para O H, while aspartic acid and glutamic acid carry an extra carboxyl group. And for the charge at a given p H, check the two free ends and every side chain that can ionise. You have now cracked three real past questions, so well done! Try the practice problems on this page, and keep going. All the best!