Introduction to Carbohydrates

Transcription

1 Introduction to Carbohydrates 1. A six-carbon aldose has four chiral centers as follows: 2R, 3R, 4S, and 5R. A. Draw the sugar in linear and cyclic form B. Draw the form that would predominate in solution. One of the rings shown above, hard to predict if alpha or beta would be favored. C. Indicate as alpha or beta, if necessary. Middle structure above is alpha and structure on far right is beta in part A D. Assign as furanose or pyranose, if necessary. Both rings are the alpha and beta anomers of the pyranose form, although the furanose form is also possible, but is not shown. E. Assign D or L. Draw the other form. The sugar shown above is D. The L sugar is shown below

2 F. Draw the sugars epimer at carbon A seven-carbon ketose has four chiral centers as follows: 3S, 4S, 5R, and 6R. A. Draw the sugar in linear and cyclic form B. Draw the form that would predominate in solution. Cyclic forms most likely to predominate in solution. C. Indicate as alpha or beta, if necessary. The middle structure is alpha and the structure on the far right is beta in part A D. Assign as furanose or pyranose, if necessary. Both rings shown are in the pyranose form E. Assign D or L. Draw the other form. The sugar shown is D. The L form (enantiomer) is shown below

3 F. Draw the sugars epimer at carbon Determine the absolute configurations of all chiral centers in N-acetylneuraminic acid shown below.

4 BCH 4053 Spring Instructor: Koculi- SI leader: Adam R. 4. Draw the mechanism for D-glucose cyclization. 5. Define the following in regards to biochemistry: A. Enantiomer: non-superimposable mirror images B. Diastereomer: all stereoisomers that aren t enantiomers. C. Epimer: diastereomers that differ by the configuration of one chiral center. D. Anomer: are diastereoisomers of cyclic forms of sugars or similar molecules differing in the configuration at the anomeric carbon (C-1 atom of an aldose or the C-2 atom of a 2- ketose). The cyclic forms of carbohydrates can exist in two forms, α- and β- based on the position of the substituent at the anomeric center. E. D vs. L sugars: enantiomers. D and L sugars have the same physical and chemical properties unless subjected to a chiral environment. F. Hemiacetal: Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half. These compounds are formed by formal addition of an alcohol to the carbonyl group. Hemiacetals started from an aldehyde whereas hemiketals started from a ketone. G. Hemiketal: see above H. Furanose: five membered ring used for reference for carbohydrates. I. Pyranose: six membered ring used for reference for carbohydrates. J. Aldonic acid: An aldonic acid is any of a family of sugar acids obtained by oxidation of the aldehyde functional group of an aldose to form a carboxylic acid functional group. Thus, their general chemical formula is HOOC-(CHOH)n-CH2OH. K. Uronic acid: any of a class of compounds that are derived from sugars by oxidizing a CH2OH group to an acid group ( COOH). L. Deoxy sugars: ugars that have had a hydroxyl group replaced with a hydrogen atom. Examples include: Deoxyribose, or 2-deoxy-D-ribose, a constituent of DNA. M. Glycosidic bonds: type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. A glycosidic bond is formed between the hemiacetal or hemiketal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some compound such as an alcohol. N. N-glycosidic bonds: same as above, except one of the atoms is a nitrogen instead of an oxygen. O. Reducing end: A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde group or a free ketone group. All monosaccharides are reducing sugars, along with some disaccharides, oligosaccharides, and polysaccharides. Please note, errors are possible and you should assume there is at least one since I complete these late at night. This answer key should be used for reference and not as your sole source of information.

5 BCH 4053 Spring Instructor: Koculi- SI leader: Adam R. 6. Indicate the glycosidic bonds in the antibiotic Esperamicin A1, shown below. Please note, errors are possible and you should assume there is at least one since I complete these late at night. This answer key should be used for reference and not as your sole source of information.