00:01
It’s a very important or it’s rather very
important to consider carbohydrates in this
context because we’re now going to be looking
at the biological application of hemiacetal
formation. Up here, shown here a brief equation
that shows the conversion of carbon dioxide
and water into oxygen and carbohydrate. Note,
how I’ve actually restricted the term and
called it (CH2O)n. This refers to the fact
that, of course, the one we are familiar with
is C6H12O6, which is glucose, but let’s
have a look at where we derive the name carbohydrate
from.
If we take water away from that equation (CH2O)n,
we are left with carbon. Hence, the term carbohydrate
or hydrates of carbon. And as you should be
aware, this transformation is photosynthesis
which occurs via the chlorophyll co-factor
in plant enzymes. So, those carbohydrates
D-glucose in the sense C6H12O6 which is dextrorotationary,
remember what we said about stereochemistry
right at the beginning in Module I, we talked
about stereochemistry. So, D-glucose will
rotate the plane of polarised right of light
to the right. This is D-glucose and most sugars,
naturally occurring sugars, are D.
01:29
Now, this is the structure. We see we have
here the six carbons and we have, as you can
see, multiple OH groups offered. Crucially
though, the functional group which I want
to draw your attention to is that which is
in the 1 position and that is our aldehyde
and remember what we said about aldehydes
reacting.
01:48
The straight line projection I’ve shown
there, otherwise known as a Fischer projection,
is actually not necessarily a true representation
of the glucose in space. And so, I’ve redrawn
in the centre and it’s a in a better sort
of arrangement of how it folds around on itself
shown there.
Note and pay attention to the alcohol in the
5 position on the Fischer structure and how
it appears in the cyclised structure there.
02:19
What happens, in this instance, is that it’s
possible to form so called pyran rings or
pyranose rings. So, if you look at the bottom
part and also to the left hand side, you will
see that there are two possible cyclic structures,
okay? And so, pyranose or pyran ring structures
are the usual way in which glucose is shown.
Now, it’s important to bear in mind that
when you’re comparing or when you’re showing
these that it’s possible when you get the
pyranose ring structures for the 5 OH to react
either at the bottom end of the carbonyl carbon
or at the top end of the carbonyl carbon.
So, let me try and show you that in a little
bit more detail. Here we have our planar sp2-hybridised
aldehyde. What then happens is you get nucleophilic
attack by the alcohol in the 5 position on
that chain which can attack the carbon, open
up the carbonyl double bond and then plonk
the H+ to cap off the negative charge bond.
03:26
But, depending on whether the... if there’s
attack from the bottom or the top will give
rise to either so called alpha or beta types
of the pyranose ring structure.
03:42
Notice also, if we look in the 1 position,
as shown here on the pyranose ring structure,
so those cyclic structures between which is
a line saying mutarotation, we can see that
what we have is what I showed you before:
an alkyl oxygen to a carbon to an OH group.
04:01
These are hemiacetals and these are naturally
occurring.
04:04
And indeed, what you see, as a consequence
of either attack from the top or attack from
the bottom, is these two different pyranose
ring forms which are either alpha or beta.
04:13
Beta is where the OH is technically in the
up position and alpha is actually in the down
position and these exist in a state of equilibrium.
And so, if you were to take, for example,
a single... a single type of hemiacetal, for
example, the alpha form and dissolve it up
in water, eventually over a period of time,
it would equilibrate, so they would contain
35.5 percent of the alpha form and around
64.5 percent of the beta form and only about
0.003 percent of the open-chain aldehyde.
So, as I say, these pyranose ring structures
are crucial and these pyranose ring structures
do vary depending on whether or how this equilibrates.
05:01
You can actually trap these pyranose in their
beta or alpha forms by derivatising the OH
groups that are bristling around that 6-membered
ring structure.