Cyclic Carbohydrates


Another one of the learning objectives
for the carbohydrate chapter is to be able to describe the cyclic structures.
Here I’ve got a a monosaccharide, but you’ll also want to be able to
describe the structures of the disaccharides and the polysaccharides. We
don’t expect you to memorize these structures, but we’d like you to be able
to describe them in terms of what units make them up and what kind of linkages
they have. Here’s the open chain structure of glucose, we call it.
This is a typical carbohydrate. There are six carbons here. It’s got an aldehyde at
the top, so it would be an aldose. Six carbons would be a hexose. This is an
aldohexose. And we’ve numbered those carbons from the top to the bottom,
starting with the end where the C double bond O is located, or as near as we can
to where the C double bond O is located. The aldehyde group, as you know from the
aldehyde chapter in organic, can react with alcohol groups. And what happens is
that glucose, as it’s twisting and turning and flexing bonds and so forth, this
aldehyde group can react with the alcohol at position 5, and as it does so,
then a cyclic structure would come together. Here I’ve redrawn the structure
of glucose now. The CH2OH and the OH’s have specific directions. If they did not,
it would not be glucose. All of them have specific directions except here at
position 1, which was originally this aldehyde carbon up here. Because as the
ring closes, the OH can be trapped in the up position, which is called beta, or
it can be trapped in the down position, which is called alpha. So we’ve got two
new glucoses, in a sense. They’re both D-glucose, but we have Beta-D-glucose, OH
up, and Alpha-D-glucose with the OH down. One other thing I’d like to point out is
that this position right here, that carbon number one, where we had the
aldehyde carbon originally. This is now a hemiacetal. It’s got an OH. There’s an H
there that we haven’t shown. The carbon is attached to two oxygens, and those are
the requirements for a hemiacetal. So actually if we were to look at a
solution of glucose, what we’d find is that the open chain form is in
equilibrium with these two cyclic forms, and the cyclic forms are opening and
closing, so we really have a mixture of all three. The ring forms predominate, but
we’d have a little bit of this open chain form too. Let’s look at a
disaccharide. Maltose is malt sugar. It’s comprised of two glucose units, and again,
we don’t expect you to memorize the structure of maltose, to be able to draw
it. What we’d like you to know is that it’s a glucose linked to a glucose and
in a specific way. They’re linked by an alpha 1 to 4 linkage. Let’s see if that
makes sense. Here’s position 1 of the glucose unit on
the left, and remember it can be up or down. And if it’s down, it’s alpha, and
it’s linked to the 4 position of the neighboring glucose unit, so it’s an
alpha 1 to 4. We didn’t have to specify the direction at position 4 because if
we say it’s glucose, then it has to be down at position 4. We’ve got a
hemiacetal group over there on the right hand side because it’s got an OH and an O attached to this carbon. There’s a hydrogen that we haven’t shown. That
hemiacetal group is opening and closing. Here I’ve shown it in the alpha position,
but the beta position would exist too. So really there are two forms of maltose:
alpha and beta maltose. This functional group back here on the left, well here’s
our carbon. There’s a hydrogen that we haven’t shown.
Two oxygens attached to this carbon. It’s an acetal, and acetals are a
lot more stable than a hemiacetal, so this isn’t breaking and opening. It’s
fixed right there. In carbohydrate chemistry there’s another name for an
acetal group. It’s called a glycoside. So this is an example of a glycoside. Or
sometimes we point to that specific bond, that carbon oxygen bond, and call it a
glycosidic linkage. Let’s see, seemed like there was, oh. I know. There was one more
thing I wanted to tell you. That when we take into our digestive system maltose,
the body hydrolyzes this linkage, and we get two glucose units. They’re absorbed
into the bloodstream at that point. Let’s look at the structure of lactose. Lactose
is also called milk sugar, and the three things that we’d like you to know are
right here at the top. It’s alternate name; the fact that it
consists of of two monosaccharide units, a galactose linked to a glucose; and
they’re linked by a beta 1 to 4 linkage. And when we take lactose into our body,
the digestive system is able to cleave this bond, and we get a unit of galactose
and a unit of glucose. These enter the bloodstream. Glucose as you know becomes
blood sugar. Galactose enters the same kinds of reactions and and eventually
increases the blood glucose level as well. They’re linked by a beta 1 to 4.
Let’s have a look. Here’s a galactose unit on the left. It differs only at one
position from glucose, here at position 4, the OH is up rather than down. But at
position 1, that oxygen is pointing up, so it’s beta at position 1. And it’s down,
it has to be down if this is a glucose unit. So it’s beta 1 to 4 linkage. Here’s
an acetal or a glycoside over here on the left. Over on the right with
that OH, it’s a hemiacetal. And rather than indicating alpha or beta,
I used a wiggly line to represent that it’s a mixture of the alpha and beta.
Both are possible, so there are two forms of lactose. This ring is fixed, but this
ring is opening and closing, opening and closing. Let’s look at the structure of
sucrose. Sucrose is household sugar. When we use the word sugar around the home,
it’s sucrose that we’re referring to. If we use the word sugar in a clinical
setting, of course, it would be glucose. The sucrose, or the, I’m sorry, the sugar
in sugar beets is sucrose. Sugar cane is sucrose. Nectar in flowers is sucrose.
Consists of a glucose linked to a fructose. An alpha 1 to a beta 2
linkage. So those are the three things that’s that we’re concerned about right
there. We’ll look at the structure, but again, you don’t have to worry about
being able to reproduce this structure. Here’s a fructose unit at the,
I’m sorry, glucose unit at the top. Here at position 1, it’s alpha. Down at the
bottom is the fructose ring system, a 5 membered ring rather than 6.
This is pyranose. This is furanose. This is position 2. It goes 1, 2, then works our
way around the ring. Up here we have an acetal linkage. Here we have a ketal.
And neither one of these are hemi, so there’s only one form of sucrose. It’s
not opening and closing to an aldehyde form. And so sucrose, when you talk about
the reactions of disaccharides, it’s not a reducing sugar, because the ring form
just stays that way. There’s no open chain form. In the eighth
minute of this video, I misspoke. When we were looking at this structure, I
called it a triose. And that, obviously has four carbons that would
make it a tetrose. So don’t let my mistake mislead you in
this video. Thank you.

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3 thoughts on “Cyclic Carbohydrates

  1. Sir thanks for valuable videos which have been increasing my knowledge day by day , I would like to thank everyone who are making videos on YouTube and other social networking sites so as to spread knowledge. I like approach of every one of you. Teaching is one of elite careers,and you guys have proven it, and all those who are involved in spreading knowledge without expectation of money are True Teachers. Shaping knowledge of generation next is also service towards nation. India's generation next is in right hand, I am proud of you guys for providing knowledge without any selfish attitude. Thanks a lot.

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