Class Notes (835,241)
Canada (509,043)
Microbiology (500)
MBIO 2360 (22)
Lecture 13

MBIO 2360 Lecture 13: Lecture 13 – MBIO

3 Pages
Unlock Document

MBIO 2360
Mc Kenna Sean

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
More Less

Related notes for MBIO 2360

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.