Welcome to another Tutor Short provided by the Educational Support Services
Department uf Lehigh Carbon Community College in Schnecksville, Pennsylvania
which is just outside of Allentown. These videos review key learnings for the
science courses provided here at LCCC. Please remember that the Educational
Support Services Department does provide walk-in tutoring five days a week.
In today's video we're going to continue with our
video presentation on carbohydrates.
This slide was used in previous videos and today's
focus will be with the
Disaccharides. Remember that
monosaccharides are the simple carbohydrates
take can't be split or hydrolyzed into smaller carbohydrates.
The disaccharides (di- meaning 2)
are two monosaccharides combined together
by a glycosidic bond.
We'll discuss these today.
In our Chem 106 course here at LCCC,
we only deal with three disaccharides. There are many
but we're just going to deal with
these three: maltose, lactose and sucrose.
These three disaccharides are important in this course.
2 (maltose & lactose) are reducing sugars
and sucrose is not (more on that later).
The maltose disaccharide is formed
by combining
2 glucose molecules
to form maltose and water is given off.
Similarly for lactose,
glucose combines with galactose
to form lactose and water comes off
when the glycosidic bond is formed.
Finally, when you combine
glucose with fructose, you'll form sucrose and water.
Let's now look at this reaction more closely.
We'll start with maltose.
Maltose is formed from
2 glucose molecules.
But it's actually a unique combination
since one glucose must be an alpha glucose
(the OH (green) on carbon #1 must be located down or trans from the CH2OH)
The second one can be either alpha or beta.
In this example, the OH is on the bottom or alpha position.
but the OH could have been up here in the beta position
and it would have been fine.
But the glucose on the left needs to be
an alpha glucose (OH situated down).
(give me a few seconds to switch pens here)
What happens between these two ring structures to come together
is the green OH on carbon #1
of the Alpha glucose is going to combine with the hydrogen
on carbon #4 of the other glucose
to form a bond & release water.
The same occurs with the other two disaccharides.
When the water comes off,
that leaves behind this red Oxygen
that will form the bond between these two ring structures
and that's called a glycosidic bond.
Because the position of the 2 OH groups
are both on what I call the bottom side
of these two ring structures,
your glcosidic bond will look like this.
Because these hydrogens are side by side (they're both located up
on the ring structures) this is called an
alpha glycosidic bond.
(let me erase some things here)
It's further called a 1,4 glycosidic bond because it is
between carbon #1 and carbon #4
In summary, maltose is formed
by an alpha glucose combined with an alpha
or beta glucose & you'll always form
an alpha-1,4-glycosidic bond.
However, the maltose you make
will either be alpha or beta depending on
whether you started with an alpha glucose
or beta glucose on the right side.
So depending on this glucose will tell you whether you
have an alpha or beta maltose.
There are two other sugars that are important
in our Chem 106 course
(there are a lot of other sugars but we're only dealing with these three)
One is lactose,
that is a combination of
galactose & glucose & again there's uniqueness.
It must be a beta-galactose, which
has the OH up on this Carbon #1
to be beta-galactose.
(Remember this OH tells you have Galactose)
And the glucose again can be either alpha or beta.
This OH tells you that you have glucose
but the OH for the alpha or beta distinction
is either up or down on carbon #1.
So we need a beta-galactose combining with an alpha or beta glucose.
You should notice the OH on carbon #1
on galactose will combine with the hydrogen
on carbon #4 of glucose and form water.
(I should show that down here)
That leaves you the oxygen
to make the glycosidic bond.
This glycosidic bond
has a hydrogen here and hydrogen there.
The hydrogens are on opposite sides
(one up & one down) so this
is a beta-bond and again a 1,4
(carbon #1 to carbon #4) glycosidic bond.
So that doesn't change with lactose,
you always form a beta-1-4-glycosidic bond.
It must be a beta-galactose and
either an alpha or beta glucose.
The choice of either alpha or beta glucose,
decides whether to have alpha or beta lactose.
(where the OH is located
on carbon #1 of the glucose)
The OH doesn't move when the bond forms.
If it's down above, it'll be down below.
So that reviews lactose.
The final disaccharide is Sucrose.
The uniqueness here is that you only have one
variation (essentially no variation),
just one structure. It must be an alpha-glucose
combining with a beta-fructose.
You're alpha-glucose structure
has an alpha position on the carbon #1.
and your fructose with a beta position
on carbon #2.
(OH in the up position)
So if you have alpha-glucose & beta-fructose,
you'll combine them with a glycosidic bond.
The OH on carbon #1 on glucose
combines with the hydrogen
on carbon #2 of fructose and releases water.
(I don't show the water on the slide)
Then the oxygen here
on carbon #2 of fructose will make the glycosidic bond.
There's only one variation.
You're connecting carbon #1 with
carbon #2 so it's a 1,2
glycosidic bond in this case.
Because this hydrogen is up (beta)
on this structure and
the CH2OH group is down (alpha),
this is called an alpha-beta-1,2-glycosidic bond.
That's the nomenclature they came up with and
you'll only form an alpha-beta-1,2-glycosidic bond
whenever you make sucrose.
(Let me clean this up)
Remember, only one variation.
Therefore there are no isomers for sucrose
and it is not a reducing sugar like the other two disaccharides.
This is because carbon #1 on the glucose part of sucrose
does not have an OH on it
and carbon #2 on fructose part
does not have an OH either.
so it's not a reducing sugar.
This distinction will be important for some teachers.
Looking back on the other two disaccharides,
they are reducing sugars since they have
an OH on carbon #1
of the right-hand ring structure.
So hopefully this review
of these three common disaccharides
will be helpful. Thank you.