Valine is an amino acid with a nonpolar side chain and serine ...

Question

Valine is an amino acid with a nonpolar side chain and serine is one with a polar side chain. Draw the two amino acids.

a. Why is the side chain for valine nonpolar, whereas the side chain for serine is polar?

Accepted Answer

All right. Hi, everyone. So this question is asking us to draw the fully ionized form of the amino acids given isoleucine nonpolar side chain and thine polar side chain give the reason for the difference in their side chain polarity. Now, recall the nice thing about amino acids is that they all have a very characteristics structure, right? They have the same general structure to them. The first component that I want to discuss right now is the alpha carbon in the center of the structure. So we've got one carbon that is connected to a hydrogen in addition to everything else in the amino acid. Now, amino acids consist of an amino group or an NH two when it's neutral as well as a carboxyl group or a carboxylic acid group or ac ooh when it's neutral. Now recall that the amino group is basic and the carboxyl group is acid because when we discuss the fully ionized form of an amino acid, what that means is that both the amino group and the carboxy group have to have a charge to them. Now because the amino group is basic, right? It accepts a proton. This means that in the ionized form the amino group, what have a positive charge due to the fact that nitrogen would gain a third proton. Now, on the other hand, right, the carboxyl group is acidic, it donates or loses a proton. And therefore, the car boxell group would end up having a negative charge. And so the final component to any given amino acid again attached to the alpha carbon in the center is the R group or the side chain because it is the side chain that is different depending on what amino acid you're talking about, right? So by recognizing the different side chains, you can distinguish amino acids from each other. So in order to draw the fully ionized form of these amino acids, we're going to use the same template that we talked about throughout this video. And because we're talking about two different amino acids, what I'm going to do here is copy this template and paste it once more, right? Because amino acids have the same general formula. And so what I can do is take the same backbone and simply change the R groups to distinguish between my different amino acids. So with that being said, let's go ahead and talk about the side chains starting off with that of isoleucine. Now recall that isoleucine has a three carbon chain as it's our group, right? Or it's side chain. But in addition to this on the carbon of the R group directing or directly connected to the alpha carbon is an additional methyl group. So that's three carbons and a methyl group attached to the side chain carbon that's directly connecting to the alpha carbon. Now, three mean on the other hand, has a two carbon side chain with a hydroxy or O group on the carbon directly connecting to the alpha carbon. So in this case, on the screen, here to the left side is ISOL Len and to the right side is 3d. So now to answer the final part of the question that discusses the difference in their side chain polarity by understanding what their side chains look like, we can understand the difference in polarity. Now recall that when two atoms connecting together have a difference in electron negativity that bond is said to be polar, right? Because electrons between those two atoms are being shared unevenly due to their difference in electron negativity. Now, when we compare side chains to each other, recall that hydrocarbons are nonpolar because carbon, carbon and carbon, hydrogen bonds have no significant electronegativity difference. On the other hand, when we have atoms that are not carbon or hydrogen in the side chain, like for example, oxygen, then that changes things right, because oxygen being highly electron is going to have a significant difference in electronegativity between itself and hydrogen. So that right there is the main difference between the side chain polarities of these two amino acids because it is the hydroxy group of iodine that makes the side chain of thine polar. So for the final answer, I would say that the oh group or hydroxy group in three makes it side chain poehler which is not present in the side chain of I. So losing and I recognize that it's cutting off a little bit. So I'm just going to move this text slightly upwards and there you have it. Here is your final answer, right. Weve identified the structures of the fully ionized forms of both isoleucine and thine. And so the the reason for their difference in inside chain polarity is because three andine contains a hydroxy group in its side chain. And so the difference in electronegativity between oxygen and hydrogen is what explains its polarity. Whereas isoleucine having a hydrocarbon side chain has no such difference in electron negativity or significant difference. And so with that being said, thank you so very much for watching. And I hope you found this helpful.

Valine is an amino acid with a nonpolar side chain and serine is one with a polar side chain. Draw the two amino acids.
a. Why is the side chain for valine nonpolar, whereas the side chain for serine is polar?

Key Concepts

Amino Acid Structure

Polarity of Side Chains

Hydrophobic and Hydrophilic Interactions

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