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Lecture 13

MBIO 2360 Lecture 13: Lecture 13 – MBIO

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MBIO 2360
Mc Kenna Sean

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Lecture 13 – MBIO/CHEM 2360 Cyclic Carbohydrates  In solution, many carbohydrates exist in cyclic form  This is because of a tendency or carbonyl groups to react reversibly with alcohols: o Draw  Since each of the carbohydrates we have discussed has both a carbonyl and one or more alcoholic hydroxyls, an intramolecular hemiacetal or hemiketal is possible, giving a cyclic structure  The main restriction is steric, small rings (4 or fever atoms in the ring) are unstable, so the stable rings will be those containing 5 or 6 atoms (6-membered are more stable than 5 membered)  The reactions create one additional chiral centre o The anomeric carbon  Formation of cyclic forms is reversible  Glucose can form two cyclic structures involving a 6-membered ring o C1 is the carbonyl carbon o They are different because the new chiral centre (at C1) can be created in two different ways o These are called Haworth diagrams - a method for showing the stereochemistry of the molecules in two dimensions o Draw the two forms (alpha and beta)  This 6-membered ring is preferred over the less stable 5-membered ring o The structure is called a pyranose because of the ring pyran: o Draw  C1 is a new asymmetric C: isomers that differ only at the hemiacetal or hemiketal C are called anomers and the C is the anomeric C o C1 is where the new stereochemic centre is formed o Beta-anomer - OH on C1 is on the same side of the ring as the C6 o Alpha-anomer: OH on C1 is opposite to C6 o OH on asymmetric C on the "right" in the Fischer diagrams are "down" in the Haworth Diagrams  Fischer structures (non-cyclic) are correct for C3 or C4 sugars in solution, otherwise ring structures are more stable o In water - all three forms exist in equilibrium  Alpha-chain-beta  36% - 0.01% - 64% o The alpha-anomer and b-anomer can interconvert through the trace amounts of non- cyclic form  The interconversion is called mutarotation and can be measured by the rotation of the plane- polarized light o Convert between alpha and beta o Pure alpha-D-Glc rotates light at +112 degrees o Pure B-D-Glc rotates at +19 degrees o At equilibrium the mixture rotates light at +53 degrees  A weighted average of the contributions from the two forms: 1. 0.36(112) + 0.64(19) = 53 degrees  The pyranose rings are not entirely planar o Each configuration (alpha, beta) can exist in 2 conformations o draw o E = equatorial: group in plane with planar part of ring - less steric hindrance for bulky groups o A= axial: group perpendicular to planar part of the ring  The chair form is slight more stable than the boat form o Slightly less crowding of the bulkier groups around the ring  Here are two possible chair conformations of B-D-Glc o Draw o Only B-D-Glc can put all bulky substituents in the equatorial position so it is a very stable and abundant molecule (64% of total D-Glc in solution, as measured by rotation of light)  The
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