Heterocyclic Compounds
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Heterocyclic Compounds

Other elements such as nitrogen and oxygen can be included in the rings. When they are, the compounds are called heterocyclic compounds. The hetero- part of the name means that more than one kind of element is included within the ring and -cyclic, of course, indicates that there is at least one ring present in the compound.

Heterocyclic Bases

One particular group of hetero cyclic compounds are the heterocyclic bases. These examples are extremely important compounds. Look at the names of these compounds. If you have studied any biology, you will probably recognize these names as being very important parts of DNA and RNA molecules. The ones shown here are uracil, thymine, and cytosine.

In the biochemistry lesson on nucleic acids, you will learn about how these heterocyclic bases bond to sugar molecules and phosphate groups to form DNA and RNA. You will also learn about how they can bond to one another to pull the two strands of the DNA together.

Structre of uracil. [str5u.jpg]
Structure of thymine. [str5t.jpg]
Structure of cytosine. [str5c.jpg]


For now, we will limit our consideration of these compounds to three things.

The first is why they are called bases. Essentially, it is the presence of the unbonded pair of electrons on each nitrogen atom that gives these compounds their basic properties. Their ability to provide a pair of electrons in order to attract a proton makes these compounds basic.
Second, you should recall from when we dealt with amines, the presence of this unbonded electron pair also allowed for hydrogen bonding to occur to the nitrogen atom. This, of course, can happen only if there is another polar molecule nearby which has a hydrogen available for hydrogen bonding.
str5u.jpg (2459 bytes) str5t.jpg (2987 bytes) str5c.jpg (2498 bytes)

If you look carefully at these diagrams, you can see that each of these compounds has nitrogen or oxygen available to receive hydrogen for hydrogen bonding. Remember: unbonded electron pairs are also available on the oxygen atoms that are contained in these compounds. You should also notice that each of these compounds contains hydrogen atoms bonded to nitrogen. These hydrogen atoms are the ones that can form hydrogen bonds to other molecules. For future reference you should remember that there are groups on each of these heterocyclic bases which can form hydrogen bonds both by having the hydrogen available for the hydrogen bond and having a bonding site available to hydrogen from other molecules. We will go into the details of the hydrogen bonding patterns of these molecules in one of the biochemistry lessons.

Third, you should notice that each of these compounds has an amino group which can form amide bonds to other types of compounds. Look for such amide bonds in the structural formulas of such substances as DNA, RNA, and ATP in the biochemistry lesson on nucleic acids.


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