Hey it's professor Dave, let's talk about carbohydrates.
We've all heard of carbohydrates and sugars, especially if we are loading up
with energy for the big game, or if we're on a diet. But many people don't know
that these words refer to the same class of molecules. Carbohydrates got their
name because they are literally hydrates of carbon, molecules with several carbon
atoms that each bear a hydrogen and a hydroxyl group. When they are small they
are also known as sugars and their names and in "ose", like glucose or sucrose, which
is the sugar that we commonly refer to as simply, sugar. Let's learn about
monosaccharides first.
These are the monomeric units that polymerize to form polysaccharides.
We can name these according to the number of carbon atoms they have, which will be
between three and six, so a three-carbon monosaccharide would be a triose, if it
has four carbon atoms it's a tetrose, five carbons would be a pentose, and six would
be a hexose. In addition, all carbohydrates will have either an
aldehyde or a ketone functional group in the molecule, so we would refer to those
as an aldose or a ketone respectively. Combining these conventions we could
refer to this monosaccharide as an aldohexose and this one as a ketopentose.
Each of these carbons, the ones that bear both hydrogen and hydroxyl, will be a
chiral center, and the convention for drawing linear monosaccharides is to use
Fischer projections. When we look at these it is important to understand that
each vertex is a stereogenic carbon and we show the hydrogens and hydroxyls on
either side. With Fischer projections, horizontal lines are implied to be wedge
bonds and the vertical lines are implied to be dash bonds. To remember this
just imagine a bowtie on each carbon like it's going to the chemistry prom.
Fischer projections are just a different way of
visualizing a molecule, which we can see if we draw this molecule in line
notation and view it from the side. The Fischer projection is what we would see
if we were in the plane of the screen looking directly at the chiral center.
However we choose to view it, it's important to realize that Fischer
projections do not imply a flat molecule with 90 degree bond angles. These carbons
are still tetrahedral, we just save time by drawing sugars this way. When we draw
a linear monosaccharides we always put the aldehyde or ketone at the top and
draw the rest of the molecule downwards, and we report the stereochemistry of the
molecule by looking at the chiral center at the bottom farthest away from the
carbonyl. If on this carbon the hydroxyl points right it's a D sugar, if it points left
it's an L sugar. So we would call this D-glucose and this would be L-glucose.
This terminology is a bit more outdated than the R and S we use to assign absolute
configuration but the convention has stuck around, and it remains the way that
we differentiate between enantiomers of sugar molecules. We got this from
glyceraldehyde, the simplest sugar and an aldotriose. This has only one chiral
center and we developed D and L terminology according to the two enantiomers of
this molecule. For some reason nature has selected to work with D sugars so unless
otherwise stated, we can assume that we are talking about D sugars.
Let's also know that since a molecule with n chiral centers has 2^N
stereoisomers, there are two aldotrioses, the two glyceraldehydes, since
there's only one chiral center on the molecule, there are four aldotetroses
these two and their L enantiomers, there are eight aldopentoses, these
four and their L enantiomers, because of the three stereocenters, and a total of
sixteen aldohexoses, these eight and their L enantiomers. These differ only
in the stereochemistry of these chiral centers. Lastly, with Fischer projections
let's be sure to understand that
swapping the position of two groups on a chiral center results in the inversion
of that stereocenter, and a totally different molecule, and while 90 degree
rotations are forbidden because they invert stereochemistry, 180
degree rotations are allowed. Monosaccharides exist in an equilibrium
between a linear and a cyclic form, with the cyclic form being highly preferred.
They cyclize by an intramolecular hemiacetal formation mechanism we
learned in organic chemistry. We know that when hemiacetals form it's because
an alcohol attacks an aldehyde or ketone, in this case it's an intramolecular
cyclization because the hydroxyl group and the carbonyl are on the same
compound. Now let's orient this linear molecule in a way that we can see the
cyclization take place. We know that carbonyl carbons are sp2 hybridized and
therefore exhibit planar geometry so when the hydroxyl attacks the carbonyl
it can do so from either side, thus generating two different stereoisomers
which are called anomers. It can either attack from this side, pushing the
new hydroxyl down, resulting in the alpha anomer, or it can attack from this side
pushing the new hydroxyl up, resulting in the beta anomer, that makes the resulting
hemiacetal carbon the anomeric carbon, because we can either get the alpha
anomer or the beta anomer, depending on which side the hydroxyl attacks from.
When we draw a cyclic monosaccharides we sometimes use Haworth projections.
With these we look at the ring from the edge
meaning this part of the ring is closer to us and this part is further away from
us, and we show the functional groups projecting straight up and straight down.
This does not accurately display the true geometry of the molecule because we
know six-membered rings like these prefer to be in a chair conformation but
Haworth projections have other practical uses. When we draw these the
convention is to place the anomeric carbon on the right
and the CH2OH on the other side of the ring pointing up. If the new hydroxyl group
that was generated during hemiacetal formation points down and is trans to
this group, we have the alpha anomer, and if it is pointing up, cis to this group
we have the beta anomer. If a cyclic monosaccharide has a six-membered ring
we will call it a pyranose, but sometimes the hydroxyl on carbon four can
do the attacking, resulting in a five-membered ring which we will call a
furanose. Rings smaller than this are unlikely to form due to ring strain.
We should also note that monosaccharides don't always remain in the alpha or beta
form, we can have an equilibrium between them, since the hemiacetal formation is
completely reversible. This alpha glucose might go back to the linear form and
then cyclize again to form the beta anomer. In fact, for glucose, the beta
anomer is preferred because the hydroxyl on the anomeric carbon will be in the
equatorial position versus axial in the alpha anomer. For this reason a sample of
glucose that has more of the alpha form than the beta will undergo mutarotation
which is a shift towards equilibrium values for the two anomers
which in the case of glucose is about two-to-one in favor of the beta anomer.
Sugars will not always prefer the beta anomer, in some sugars like mannose, the
alpha anamur is preferred due to reasons of hyperconjugation that we call the
anomeric effect. So we now know a bit about monosaccharides, their linear form
and the way they cyclize, and it is the cyclic form that is able to polymerize
to form long polysaccharide chains. Let's learn about those next.
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