Carbohydrate Molecules vary in length.
So some carbohydrate molecules are very long and some are very short. Now long
carbohydrate molecules are also known as
polysaccharides, and that's because they consist of many individual monosaccharides.
So the simplest sugars are called monosaccharides. They consist of a single sugar monomer. Now,
why do we call sugars carbohydrates?
Well, if we take a look at the empirical formula for carbohydrates this is what we're going to find. We see that we have
carbon we have H and we have oxygen. In fact we see that we have carbon, and we have water. And
another name for water is hydrate, and so we have
carbohydrates, these molecules that contain carbon as well as water molecules.
So the empirical formula is C H2 O,
multiplied by n. Where n is basically any
positive integer any positive N value that is equal to 3 or greater. So the simplest
monosaccharide contains an N value that is equal to 3, and that means the simple monosaccharides
consists of three carbon atoms, six H atoms and three oxygen atoms. Now
generally speaking, there are two types of categories of sugars.
We have those sugars that contain aldehyde groups, and these are known as
aldoses, and we have those sugars that contain a ketone group, and these are known as
ketoses. And to demonstrate what the difference is, let's take a look at the simplest
monosaccharides that contains and value that is equal to 3. So this is the ketose
that consists of 3 carbons,
six H atoms, and three oxygens. And notice it's a ketose, because on both sides of this carbonyl group
we essentially have these carbon atoms and the specific name for this is dihydroxyacetone.
Now what about the aldose version of this same sugar molecule?
So this is what we basically have. And the reason we have two of these
aldoses is because we have a single stereogenic carbon
We have a chiral carbon and that means we're going to have two different isomers. These are known as enantiomers.
Enantiomers are mirror images of one another.
So we essential, on this particular molecule,
we don't have a mirror image because we don't actually have a stereogenetic carbon. Remember a stereogenic carbon
is a carbon that is chiral, and that means it is attached to four different groups.
So on this particular molecule
We don't have that but in this particular
molecule, if we examine this carbon we have four different groups attached to this carbon.
So we have the hydroxyl, we have the H atom, we have this group here, and this group here.
And so what that means is, there will be an enantiomer that will exist with respect
to this carbon, and this is the enantiomer shown here. so the d-glyceraldehyde simply means
that it's one of the enantiomers and the L simply means it's the other type of an enantiomer
and we'll see exactly what the D and L will present in just a moment.
So, once again the simplest sugars are monosaccharides.
And we call in(it?) carbohydrates because the empirical formula tells us they consist of carbon as well as water molecules.
So monosaccharides can either be aldehydes or they can be ketones?
Aldehyde-containing monosaccharides are known as aldoses.
And ketone-containing monosaccharides are known as Ketoses so the smallest functional
monosaccharide as we said in just the moment contains an N value of 3,
so we have three carbon atoms six H atoms and three oxygen atoms.
So what exactly is the meaning of the D and the L and what does the d represent and and what does the L represent?
Well to demonstrate the meaning behind the D and L letters, let's take a look at another type of
carbohydrate molecule namely erythrose. So let's compare D-
erythrose and L-erythrose. Notice what the d and the L
mean is these will be mirror images with respect to one another, these are known as
enantiomeres. So sugars that are mirror images of one another are known as
enantiomers, and so if we draw a line this will be a mirror image of this molecule,
and so these OH groups which point to this side will point onto the opposite side on
this molecule here. And so in the D isomer we see that this green
hydroxyl group, which is the hydroxyl group that is found on the stereogenic carbon that is farthest away from
this aldehyde group,
this is the carbon that we have to worry about. And if this
hydroxyl group on the final stereogenic carbon points to the right side, then that is the D-
isomer, the D-enantiomer, but if it points to the left side, so that is called the L-
monosaccharide, in this case the L-erythrose.
So the symbol D or L designates the absolute configuration
of the stereogenic carbon, the chiral carbon that is
farthest away from the aldehyde or ketone group and in this particular case it
happens to be the aldehyde group.Now in D-isomers
the OH points to the right side and in L isomers it points to the left side.
So if this is the left side,
and this is the right side
Then that means in this particular case it points [on] the right side and in this particular case it
points to the left side. so L
you can remember as left and D is the opposite of left, so that's right.
Now the problem with drawing, ah, the problem drawing our monosaccharides in the following manner is the fact that the
stereochemistry is not actually seen, because all these bonds seem to lie along the same plane.
But we know that sugar molecules are three-dimensional
molecules, they have stereochemistry. And
to describe the stereochemistry of sugar molecules
correctly the most common mechanism
we use are the
Fischer projection. so, Fischer projections are commonly used to describe this stereochemistry, the
three-dimensional arrangement of the atoms and the bonds within the sugar
molecule. And to see what the Fischer projection actually looks like, let's take a look at the following molecule.
So this is the same molecule that we essentially described here. It's the D-erythrose.
So in this particular diagram, the problem with this diagram is all the atoms essentially lie
along the same plane, and all the bonds also lie along the same plane.
But we know that is not true. And
to basically go from this diagram to a Fischer projection, all we have to do is we take these horizontal
lines and we bring them out of the board, and we take the vertical lines and we place them into the board. And this
is basically what we get. This is our Fischer projection.
And then if we take this, and we actually try to draw a three-dimensional diagram,
this is what we're going to get. So notice that this carbon and this carbon,
they lie along the same plane. These two will point downward as we see in this diagram.
They go into the board or into the page, and these four groups are coming out of the board as shown here,
they're going to come out of the page. So
horizontal lines are drawn in front of the plane (coming out of the board) while
vertical lines are drawn in the back of the plane (going into the board). So a Fischer projection
is nothing more than a way to basically describe the three-dimensional
stereochemistry of the sugar molecule.
Now the final thing I'd like to discuss are diastereoisomers and epimers.
And as we'll see in just a moment an epimer is basically a specific type of
diastereoisomers. So let's think back to organic chemistry for just a moment.
So what exactly is an isomer? Well from organic chemistry we know that
isomers are basically those molecules that contain the same exact molecular
formula. So they have the same exact number of atoms, and the same type of atoms.
But they differ in their arrangement of atoms in the
three-dimensional space.
Now we have two types of isomers when it comes to sugars. we have
the diastereoisomers, and we have enantiomers. So just a moment ago
we said enantiomers are those isomers that are mirror images of one another.
Now if it's not an enantiomer, then it is
a diastereoisomer. So we have two categories of isomers either in enantiomers or diastereoisomers.
So
essentially, if two sugar molecules have the same exact molecular formula
and they are not enantiomers then that means they must be diastereoisomers,
and these are two examples of diastereoisomers.
So if we take a look at D-erythrose, this is the same molecule that we basically have here, this in a ex, ah,
this is an example of a diastereoisomer
with respect to D-erythrose.
Notice it contains the same exact molecular formula, it has the same exact number and type of atoms.
So here we have one two three four carbon atoms one two three four carbon atoms,
we have 1, 2, 3, 4, 5, 6, 7, 8 H atoms,
and we have 8 H atoms here and finally we have 1, 2, 3, 4 oxygens. 1, 2, 3, 4 oxygen
So essentially they have the same number and type of atoms,
but they differ in their arrangement of those atoms in the
three-dimensional space. And notice that these are not examples of enantiomers, because if we draw
line here, they will not be mirror images, and so what that means is, if they're not mirror images
They're not enantiomers. They must be diastereoisomers.
And so these are two examples of diastereoisomers.
And notice that these are both the D-type molecule, the D-isomer because this (tsk)
green hydroxyl group found on the final stereogenic carbon to farthest away from this
aldehyde group points to the right side. Remember in the D isomers, the hydroxyl group found
on the last stereogenic carbon points to the right side. If this would have been the L
This would have pointed to the left side and we would have had this L-erythrose. So these are two examples
of diastereoisomers. So
when two sugars have identical molecular
formulas, have different arrangement of atoms, and are not enantiomers, they're not mirror images,
then they must be diastereoisomers.
Now a specific subgroup a specific subcategory of diastereoisomers are
epimers, so what exactly is an epimer?
Well, two Molecules are set to be epimers
if or two sugars are set to be epimers if they differ in
stereochemistry only at a single
chiral carbon atom. And it doesn't matter which carbon atom it is as long as it's a chiral carbon
atom, it's stereogenic. So to see what we mean let's compare D-Glucose and D-Mannose.
Again, these are both D,
and what that means is, these two Green hydroc, ah,
hydroxyl groups found on a final stereogenic carbon will b, ah, will both point to the right side of
the board.
Now.
Let's basically describe all the stereogenic carbons found on each one of these molecules.
[I've] Let's use a color that,
let's use orange.
So,
This is a stereogenic carbon. This is a stereogenic carbon.
This is a stereogenic carbon, and this is a stereogenic carbon. Likewise one two
three four. So these are all stereogenic carbons.
Now these are epimers because they only differ, and they're stereochemistry at a single one of these
chiral carbon atoms. And let's see which one that is.
So if we begin at the first carbon, the first chiral carbon, notice that the hydroxyl group points to the right and the
hydroxyl group here points to the right as well. Now if we move on to the second stereogenic chiral carbon,
we'll see that the hydroxyl actually points to the left in D glucose,
but it points to the right in D-Mannose. If we
move on to the third stereogenic carbon, we'll see that once again,
they point to the same side and the last one they also point to the same side.
So these are epimers, a specific type of diastereoisomers,
because they essentially differ in their stereochemistry only at a single carbon atom.