Hey guys. Now let's talk about an oocyte reaction of monosaccharides called N glycosidation. So guys, monosaccharides have the ability to react at the O position or the oxygen position in several different ways. In acidic conditions, monosaccharides can substitute selectively at the anomeric position, only at the anomeric position to produce what's known as glycosides. Remember that glycosides would put basically like an R group or some kind of substituent over in the anomeric position. Okay? Well, it turns out that when nitrogenous, so I'm just going to put n for nitrogenous nucleophiles are used, meaning nucleophiles that get their lone pair or their nucleophilicity from a nitrogen, that substitution product is called an N glycoside. Okay? Because instead of forming an O glycoside meaning that you have a glycoside with an oxygen here, instead, we're going to form a glycosidic bond with a nitrogen-containing compound, okay? Now the term I'm going to use most of the time for it is called an N glycoside. That's very similar to O glycoside so it's very easy to remember. But another nomenclature term that you should just know is that N glycosides are also referred to as glycosyl amines, okay? So if you hear about glycosyl amine or glycosyl amine, That is basically an amine being attached to, to a ring, to a furanose or a pyranose, and it means the same thing as N glycoside. Okay? So as you can see in this reaction that I'm about to show you, this molecule could either be named 1 Amino D Glucopyranoside, because I'm saying that the glycoside is an amino group, 1 amino, or it can be referred to as D-glucopyranosylamine because what we're saying is that it's the same as a glucopyranose, except that it happens to have an amine in the anomeric position which is a pyranosylamine, okay? Guys, I'm not going to make you're not going to be accountable for this nomenclature, but you should still recognize it, okay? Awesome. So let's go through the general reaction. The general reaction says that if you take a cyclic carbohydrate and then you react it with a nitrogen-containing nucleophile. In this case, I'm using the simplest one which is ammonia, but it could have been a complicated one. In acid, what we're going to get is that the nitrogen attaches to the anomeric site and only the anomeric site, and you're going to get a mixture of anomers with that nitrogen in the glycosidic bond, this is called the N glycosidic bond. Glycosidic link or bond. Okay? I just want to make some points here. So just you know, why can it only react at the glycosidic position and no other positions? Why would you only substitute the n here and not for example at C2? Guys, for the same exact reason, same exact mechanism as O glycosidation. Remember that in order for O glycosidation to take place, you need to pass through the oxocarbenium intermediate that stabilizes the positive charge right here. The same exact thing is going to happen with N glycosidation Glycosidation where acid is going to come in, kick out the water, I'm going to form a double bond, and but all that happens with N glycosidation is that at the last step nitrogen comes and attacks the positive charge here and then gets deprotonated. And that is why we would get a nitrogen-containing compound that looks very similar, just at the anomeric position. Cool? Awesome. So that is that so far. In the next video, I'm going to talk to you guys about specific types of N glycosides that are called Ribonucleosides.
Monosaccharides - N-Glycosides - Online Tutor, Practice Problems & Exam Prep
Monosaccharides have the ability to react at the –O position in several different ways. In acidic conditions, monosaccharides can substitute selectively at the anomeric position to produce glycosides.
General Reaction
Video transcript
General Reaction:
Creating Ribonucleosides
Video transcript
So an N-glycoside that specifically contains a ribose monosaccharide along with a heterocyclic nitrogen base, meaning it needs to be like an aromatic nitrogen-containing heterocycle. If you have those things connected through an N-glycosidic bond, which we just talked about, that is actually referred to as a ribonucleoside or also just like to be called a nucleoside. Okay? Now keep in mind that's a very different word than the one we had up here, not glycoside. Glycoside is this, but if it's specifically a ribose with a heterocyclic nitrogen base, it's called a ribonucleoside which happens to be the r and n portions of RNA. RNA genetic material, nucleic acids are actually just made out of a sugar backbone along with the nitrogenous base. So remember that RNA stands for Ribonucleic acid. Right? And what we're doing here is we're forming the ribo and the nucleoside part. Okay? The only thing is that later on they add phosphate groups, you add phosphate groups to get to the acid part. Okay? So how does this work? Well first of all guys, I'm going to be showing you the 4 base pairs of RNA because that's the easier one to talk about first.
So you guys might remember this from biology, you may not remember this from biology which is fine, I'm just going to teach it from scratch. But basically, the 4 heterocyclic bases that you could have for RNA are guanine, cytosine, adenine, and uracil. And these are represented by the letters g, c, a, and u. Now you might remember that there's a letter called t, you might be more familiar with t but T works with DNA and it's very similar to uracil, it's actually just missing it has an extra methyl group, that's it. But just knowing these bases you pretty much have the big picture of what a base looks like, okay? And the way that RNA works, the way that it's built is that you have a ribose sugar, remember ribose is alright. So we have D-ribose and D-ribose cyclizes to create alpha D-ribofuranose, which it doesn't always have to be alpha, but alpha happens to be the prevalent one for this one, okay? So alpha Dribofuranose. By the way, the reason this one's alpha is because notice that the stereo descriptor is facing up in this case because it's a D, so it's facing up. So then since it's trans that would be alpha. Okay now guys what happens is you take that furanose, that ribofuranose, and you attach it with an N-glycosidic bond to any one of these heterocyclic bases either guanine, cytosine, adenine or uracil.
In fact, just you know the nitrogens that we would use would be these. It's either going to be this nitrogen for guanine and adenine or it's going to be this nitrogen for cytosine and uracil. Okay, so these are the nitrogens specifically that attach to the anomeric position. And if you make that beta N1 linkage, what you're going to wind up getting is what's called a ribonucleoside. In this case, ribose comes from the fact that it's a ribosugar. Nucleoside because it's now attached to a heterocyclic base, okay? Now let's just break this down a little bit so you guys know exactly what I'm talking about. Why do they call it a beta N linkage? Well the N comes from the fact that it's an N-glycoside, right? Where does the beta come from? Why is it beta? Because specifically beta is the direction, is the anomer that faces cystowards our stereo descriptor, right. Notice that my stereo descriptor is facing up and this one is facing up and actually to be RNA, you need to be the beta anomer. The alpha anomer doesn't work, it needs to be the beta anomer. They need to both be facing up. So in this case, this ribonucleoside is the basics of the base code G. G later on, all it needs is a phosphate group, phosphate groups attached to the O and you're actually going to have RNA. Okay. Specifically, it's called guanosine once you attach the ribosugar to guanine. Isn't that cool? So guys, even though this seems a little bit like a little bit advanced, like it seems like, oh wow we were just talking about sugar and now I have like a whole RNA molecule that I'm dealing with. You guys know this whole mechanism. There's no reason that you can't build one of these from scratch now because we know how to use the anomeric position oxycarbenium ion that then can get attacked by the nitrogen. So really this is just a cool application of a mechanism you already know, alright? So let's go ahead and move on to the next video.
Note: The Adenine and Guanine structures should be switched.:)
Propose an acid-catalyzed mechanism by which cytosine can form a β-1 N-linkage with 2-deoxy-β-Dribofuranose to produce a deoxynucleoside (DNA) called deoxycytidine.
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