Carbohydrates Part 1: Simple Sugars and Fischer Projections


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. Thanks for watching guys. Subscribe to my channel for more tutorials, and as always, feel free to email me:

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