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Having looked at the chemistry of monosaccharides
in some detail, we’re now ready to move onto
some more complex sugar chemistry
that involves the joining together of monosaccharides.
If we join two monosaccharides together
like the structures that you see on this page,
we’d be talking about disaccharides.
Two monosaccharides coming together makes a disaccharide.
Three or more monosaccharides coming together makes
the oligosaccharide and if we’re talking about [psk]
bringing together a large number of monosaccharides,
we’re talking about the polysaccharides.
Here you can see that basic structure of the disaccharides,
many of them are common to you,
such as maltose, cellulose, the cellobiose
– which is the building block of cellulose – and sucrose.
You can see basically what we’re talking about
is two monosaccharides that are joined together
by this glycosidic bond.
And notice that in one case what we have is
the acetal functional group,
that is the ano- anomeric carbon
of the sugar on the left-hand side of this disaccharide
has the functional group where at the C1 position
of the acetal functional group.
Whereas on the right-hand side,
that sugar has the hemiacetal
since it has the hydroxyl group there.
Really what we want to do in this webcast is set-up
[psk] the idea of structure elucidation of disaccharides.
And let’s do that with a common sugar – lactose.
We’ll return to the disaccharides
that are seen here in a later webcast.
But, for now, what we want to do is just take
and examine in great detail one disaccharide.
And that disaccharide is one you’re familiar with
– it’s lactose from milk.
So, basically lactose like many other disaccharides
is a sugar that has a multiple of six carbon atoms
and the reason is because disaccharides are often formed
from hexoses.
Hexoses, two of them come together or
two or more will have an integer number
of six carbon atoms involved in the formula
and lactose is a C12 disaccharide.
So, in other words, it’s a hexose dimer
and when we subject that to hydrolysis,
recalling that glycosidic bond
that acetal bond that I pointed out on a previous slide,
is going to be ah, broken under these hydrolysis conditions.
Then we’re going to end up making the two
monosaccharide building blocks, and in the case of lactose,
one obtains D-glucose and D-galactose.
Now, if you remember
[psk] these two sugars are actually very similar.
They’re identical structures in that they’re hexoses
and the only thing that differs about them
is their stereochemistry at C4.
They’re epimers at C4 and so these are basically two
ah, hexoses ah, differing only by that one
stereo chemical position.
Now, our question is
and what we want to set-up for the next webcast is,
what’s the structure of lactose?
We only know what its building blocks are
– we don’t know anything else about the structure.
And that’s what we’re going to determine in the next webcast.
And let me for you, ah, in the remainder of this webcast,
simply set-up the problem
that we’re going to have to examine in the next webcast.
So, we know uh, based on what I just showed you,
that the hydrolysis conditions break apart lactose
into two sugar molecules – glucose and galactose.
[psk] But, what we don’t know is which sugar is on the left
– that is, which one of the sugars is connected
as it’s anomeric carbon with that acetal functional group.
We don’t know specifically
whether this stereochemistry should be up or down,
but that would determine
whether it’s glucose or galactose here
on the left-hand side.
And then the other thing we don’t know
- the second point we don’t know
- is which of the sugar’s hydroxyl groups
on the right-hand side is attached.
There’s at least four pas- four hydroxyl groups,
which could be connected to that sugar on the left.
So, that’s the second issue that we don’t know.
And then the third issue that we don’t know,
would be this issue of the stereochemistry
at the α or β position.
Specifically, we don’t know whether or not
at that anomeric position on that left-hand side sugar,
whether is axial or equatorial.
Axial being α, equatorial being β.