M.Ed.,San Francisco State Univ.
Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.
Identifying Amino acids can be difficult. So here are some tips for figuring out if an R-Group in an Amino acid is polar or non-polar.
This first set that I'm going to show you, are all the non-polar R-Groups. As you know from way back when you studied about polar and non-polar, non-polar means that they have all the same charge or basically they don't have different charges on different ends.
In our first one,if you count them up we have, 9 total Amino acids that have non-polar R-Groups. What I'll do is I'll box the R-Groups for you, and then I'll show you how it's non polar. In the first one that we have here we have Glycine and the R-Group is just a Hydrogen. So simply that's one charge. That's one way to obviously to figure out that that's non-polar. Remember non-polar means a Hydrophobic which means they don't attract to water.
We go to the right to the next one, Alanine. And we take a look here we have CH3 which is some where to Methane and if you notice Carbon only bond with the Hydrogens around it that the Hydrogens are in the back. They all have basically similar charges they have there and what other bond is the Carbon. Since they all have the Hydrogens there, they all have the same charge, so they're non-polar.
Then now we take a look here and in Proline, you have basically these right here. And you have basically a Hydrocarbon chain again this time it's a ring. So Hydrocarbons would tell you that it's also non-polar.
Go to Valine, here's your R-Group, also Hydrocarbons. You go to Phenylalanine you have a Hydrocarbon plus you have the Benzene ring which is also a type of Hydrocarbon. Just in case you forgot a Hydrocarbon, just made up of a Carbon and a Hyrdrogens. The whole of the Hydrogens are on the outside they're all non-polar because they have the same charge.
Go to Leucine, also Hydrocarbons. You go to Tryptophan which you find happy turkey, and basically you have pretty much all Hydrocarbons with the exception of the NH that you have here. But the NH is embedded in a ring here, and so what happens is, this is mainly non-polar especially since the Benzene ring is super bulky.
Then Isoleucine is an Isomer of just Leucine and so also have Hydrocarbons, so that's non-polar. Then over here you have a Sulphur here in Methionine but all the outsides end up having Hydrocarbons and so that's why that's non-polar. If you think off Hydrocarbons in your R-Group basically that tells you that it will be non-polar. Here we'll write external Hydrocarbons because we'll go and on the next page I have here, I have all the polar R-Groups.
There are 11 of them, because remember there are 20 total Amino acids. So our R-Groups here our first one is Serine and our R-Groups here. We do have a Hydrocarbon, but on the end we have an OH a Hydroxyl group. Basically Hydroxyl groups are bait and so they are polar just like in water. So that should tell you that, that would be polar.
Let's take a look at all the other ones. So let's take a look at Aspartic acid has a Hydroxyl group, so that would work. Going down you have Glutamic acid, which also has a Hydroxyl group on the end. We also have Threonin which also has a Hydroxyl group. And then if you'll follow me all the way to the upper hand corner, Tyrosine we have a hydroxyl on the end for part of the R-Group.
You have a Benzene and a Hydroxyl group so that makes a phenyl, but a Hydroxyl on the end makes it so that it's polar. That's one tip for looking for things that have Hydroxyl groups, they would be polar.
The other tip, there's 3 of these, would be to look for an Amino group. So let's take a look at Asparagine. Let's take a look here and an Amino group you know has a Nitrogen, so NH2. These have Amino groups. And so the Nitrogen is also Polar, because it has that lone pair on there. Let's take a look at Glutimine, it also has an Amino group here, and then Lysine on the end has an Amino group. Also Arginine on the end has an Amino group. And then we have Histidine and Histidine has this Nitrogen with the double bond, and that double bond acts as way for making a bond. And so they have this lone pair here and so that part would make it polar, and so that's Histidine.
Then the last thing that you would look for, is called a SulfHydryl group. And so I'll show you that right here, SH it has a Sulfur. This is called a SulfHydryl group and that works similar to along the lines of Oxygen, because remember, they're in the same group. So basically you would have this pen shape along the Sulfhydryl group and then you would also have a polar compound.
Just a recap, if you have on the end a Hydroxyl group, so OH. You have an Amino group, like an NH2, or you have a Sulfhydryl group, like an SH group on the end, then that would tell you that you have a polar R-Group for that particular Amino acid.
Just a recap, some tips; non-polar R-Groups would generally have Hydrocarbons, Carbon, Hydrogens involved in it, just like the 9 that we showed you before. Then Polar R-Groups would have on the end, a Hyroxyl group an OH, and Amino group with a Nitrogen NH2 or a Sulfhydryl group with a Sulfur SH.
Hopefully these tips will help you in identifying whether an R-Group and an Amino acid is polar or non-polar. Have a good one.