Valine is an amino acid with a nonpolar side chain, and serine is an amino acid with a polar side chain. Draw the two dipeptides that can be formed by these two amino acids. Identify the peptide bond.

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Welcome back, everybody. Let's take a look at the next problem. Cysteine and methionine are both sulfur containing amino acids. However, the side chain of cysteine is polar, while the side chain of methionine is nonpolar, write the structures of the two dipeptide produced using these two amino acids and indicate the peptide bond in each. So we need to think about our two R groups, what they look like and how you would make a dipeptide showing the peptide bond that's formed. So we will first talk about cysteine bys, it has an R group of C two sh while methionine has an R group of CH two C two sc three. So you can see the difference there that cystine has this terminal s bonded to a hydrogen. So we have that great difference in electron negativity between the sulfur and the hydrogen creating a polar bond. Whereas methionine, the sul the sulfur is internal, there's no hydrogen bonded to it. It's just bonded to two carbons. So there's just not as great an electronegativity difference between sulfur and carbon giving it its nonpolar nature sort of thinking about how this structure works. So think about the general structure of any amino acid. By convention, they're always written with the amino group first on the left side. So H two N bonded to a carbon which has a hydrogen on it. And this is where the R group of amino acid that distinguishes each amino acid from the other is bound. So that carbon is labeled as the alpha carbon, then that carbon is attached to the carbon group that is part of a carboxylic acid. So that carbonyl carbon also has an oh group on it. This can react with a second amino acid. So we'll draw another one, we'll say it's a different amino acid. So it's got R prime a different R ro and it can react by losing the hydroxy group from our carboxylic acid and losing a hydrogen from R NH two with the results that a molecule of water is lost and a new bond is formed between the carbon carbon of the first amino acid and the nitrogen of the second. And that bond linking this to is called a peptide bond. So if we want to draw the structure of the two dipeptide produced, we'll start with the one that has cysteine first and then methionine, when we say first, we mean a cysteine as its c terminal and the methionine as its N terminal. Oh Excuse me, it's exactly opposite from that that can led astray by the fact that syne starts with ac so be careful of confusion. Like that. So cysteine as its N terminal, since again, by convention, we write the N terminal on the left and the meine as its C terminal. So we'll draw this as a line structure just to make it a little more, less bulky here. So we'll start with the H two N of cysteine that will be bonded to our alpha carbon, which will then have a bond to the carbonel carbon. Draw that with ac double bond to an O that will then have the peptide bond, then nitrogen of the next amino acid methionine. So it's that our, our group for cysteine, I'm going to draw that in red just to keep it distinguished there. So we know we have a bond to a carbon. That's our CH two, that carbon has a bond to a sulfur which has a hydrogen on it. Right? S so there's our cysteine. Now to continue our structure, we had that nitrogen of the metia, it's bonded to an alpha carbon which then is bonded to the carbon carbon. For that DB O. This is the C terminal so that carbon carbon has a bond to an O group. Now we add an R group on the alpha carbon. In this case, we have CH two CH two then a sour drawing that s and that sulfur is bonded to a methyl group at that, we're just going to scroll up a little. So we don't run out of space at the bottom there. So that is in the order of cysteine methionine. And we want to indicate the dipeptide bond. So I'll highlight that in blue, that would again be that bond between the carbon carbon and the nitrogen. So we'll label that as the peptide bond. So there's our first structure Cine thinning highlight that in yellow there. Then we just need to draw it in the other order methionine cysteine. So again, we'll start with H two N. There's our N terminal that's bonded too are alpha carbon. Then as a bond to our carbon carbon that will then have the peptide bond. There's the hydrogen of our next amino acid going to add my R group of methionine. So that is our CH two CH two S and then C three, then continue our cysteine. So we have the nitrogen spot, an alpha carbon just bond to our carbon carbon. That's the C terminus. So that has a bond to an O and our group here being CH two 12 and S just have to indicate the polypeptide bond. So we'll highlight that in blue again, between the carbonyl carbon and the nitrogen. So there is our peptide bond between the two. And again, that's our structure. When we have the other order methionine as the N terminal and Sistine as the C terminal. See you in the next video

Valine is an amino acid with a nonpolar side chain, and serine is an amino acid with a polar side chain. Draw the two dipeptides that can be formed by these two amino acids. Identify the peptide bond.

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Key Concepts

Amino Acids

Dipeptides

Peptide Bond