monosaccharides structures cyclic

1. PRESENTED BY
SYEDA JAWERIA REHMAN
BIO CHEMISTRY

2. Introduction
Derived from a latin word
 Also called simple sugars
Sweet in taste
Can not hydrolyse
Examples
o glucose
o fructose
o glactose

3. Cyclic Structures of
Monosaccharides
we have represented monosaccharides as linear
molecules
many of them also adopt cyclic structures
conversion occurs because of the ability of aldehydes
and ketones to react with alcohols

5. In some cases, OH and carbonyl groups on
the same molecule are able to react with one
another in an intramolecular reaction
 monosaccharides larger than tetroses exist
mainly as cyclic compounds

6. QUESTION?
 why the aldehyde reacts with the OH
group on the fifth carbon atom rather
than the OH group on the second
carbon atom next to it?

7. ANSWER
 Recall that cyclic alkanes containing five or
six carbon atoms in the ring are the most
stable.
 The same is true for monosaccharides that
form cyclic structures: rings consisting of
five or six carbon atoms are the most stable.

9. Stereoisomers
 When a straight-chain monosaccharide
forms a cyclic structure, the carbonyl oxygen
atom may be pushed either up or down,
giving rise to two stereoisomers known as
anomers
• OH group on the first carbon
atom projected downward
alpha
• OH group on the first carbon
atom pointed upward,Beta

11.  comparison for glucose
α form melts
at 146°C β form melts at
150°C
specific
rotation of
+112°,
specific
rotation of
+18.7°.

12. Mutaration
 possible to obtain a sample of crystalline glucose in
which all the molecules have the α structure or all have
the β structure
 dissolved in water
 they open to form the carbonyl group
 then reclose to form either the α or the β anomer
 interconversion between anomeric forms
 At equilibrium, the mixture consists of about 36% α-
D-glucose, 64% β-D-glucose, and less than 0.02% of
the open-chain aldehyde form

13. Refrences: