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.
|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.
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
|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.|
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
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