The ribonucleosides that make up ribonucleic acid (RNA) a...

Question

The ribonucleosides that make up ribonucleic acid (RNA) are composed of D-ribose (a sugar) and four heterocyclic “bases.” The general structure of a ribonucleoside is shown here.

The four heterocyclic bases are cytosine, uracil, guanine, and adenine. Cytosine and uracil are called pyrimidine bases because their structures resemble pyrimidine. Guanine and adenine are called purine bases because their structures resemble purine.

b. Predict which nitrogen atoms are basic.

Accepted Answer

All right. Hello everyone. So this question says that ribonucleotide which constitute ribonucleic acid or RN A consist of four heterocyclic bases and D ribose, the sugar, the four heterocyclic bases are guanine, adenine, cytosine and uracil, which are featured on the screen here. Guanine and adenine are referred to as purine bases due to their structural similarity to purine cytosine and uracil are referred to as perimeter bases due to their structural similarity to perimeter encircle, the basic nitrogen atom in each heterocyclic base. So first and foremost, just talking about the general structures of these bases recall that both purine and pyrimidine are aromatic molecules, which means that guanine, adenine, cytosine and uracil should also be aromatic. Now, these molecules are unique in that they're known as aromatic hetero cycles. So as mentioned before, these molecules are aromatic, but we do have hetero atoms as part of the aromatic ring. In this case, nitrogen is that hetero atom. So the question is really to understand which nitrogen atoms in each heterocyclic base are basic. Now, in this particular case, nitrogen only has one lone pair of electrons when it's neutral, right. So we have two possibilities for a nitrogen inside of the ring, either it's basic, which means that it's available to establish a covalent bond with something else or the lone pair of electrons on nitrogen is part of the aromatic system. So what that means is that there's a possibility that a nitrogen of the ring must contribute its lone pair of electrons to the aromatic system to maintain that stability. If that's true of a particular nitrogen, then it's not going to be very basic because it's not going to be available to react, referring to that lone pair. So with that being said, there's a little tip that I like to use when trying to distinguish between nitrogens that are part of the aromatic system and nitrogens that are not, lets consider Guan Yin for a second guanine here on the far left. So first, any nitrogen atom that is not part of the ring is going to be basic. So I'm just going to go ahead and circle the nitrogen that we have on the outside here. So now this leads us to the remaining nitrogens that are actually inside the ring. Now, what you'll notice about the structure of guanine is that some of these nitrogen atoms are part of a double bond between itself nitrogen and a carbon adjacent to it whenever that's the case, right, that nitrogen is not going to donate its lone pair of electrons to the aromatic system. So just to go ahead and rephrase that statement to clarify if a nitrogen is part of a carbon nitrogen double bond inside of a ring, that lone pair of electrons is going to be basic and available to react. So in this particular case, nitrogen, excuse me, guanine actually has two nitrogen atoms that are part of a carbon nitrogen double bond inside of the ring. So both of those nitrogens are basic because the their loan pair of electrons is available to react with something else. Now, nitrogen atoms that are not part of a carbon nitrogen, double bond inside of the ring are going to donate their lone pairs to the aromatic system, which is why I'm not circling the other two nitrogens of the guanine ring. OK. So now let's move on to adding and a ay does have another nitrogen that's outside of the rink. So that one's going to be basic. Now, in this particular case, adenine like guanine, hence their structural similarity has four nitrogens inside of the ring. Three of those nitrogens are part of a double bond between themselves and an adjacent carbon. What that means is that all three of those nitrogens are going to be basic because they don't need to donate their lone pair to the aromatic system. Water nitrogen is not part of such a double bond. So it is going to donate its loan pair of electrons to the aromatic system, meaning it is not basic. Alright. So now cytosine, now cytosine like uracil is a derivative of pyrimidine. And in pyrimidine, we have two nitrogens inside of the ring. Cytosine has a nitrogen outside of the ring. So that one's going to be basic, but one of the two nitrogens of the ring is part of a carbon nitrogen double bond. So that's the one that I'm going to circle because that's the one that's basic. The other nitrogen of the cytosine ring is not part of such a double bond, which means that it is going to donate its long pair to the aromatic system. This leads me now to your. So now Uracil doesn't have any knife fusions outside the ring. So there's nothing to circle in that sense. And what's worth mentioning is that both nitrogens inside of the ring €4. So are not part of a double bond between itself and an adjacent carbon. What that means is that both of these nitrogens are not basic because they are part of the aromatic system. So there's nothing to circle for yourself and there you have it. So now we've gone through all four bases for RNA. And with that being said, thank you so very much for watching. And I hope you found this helpful.

Key Concepts

Basicity of Nitrogen Atoms

Heterocyclic Compounds

Pyrimidine and Purine Bases

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