Biological Molecules Flashcards
What is a carbohydrate?
A carbohydrate is an organic compound with a backbone of at least 3 carbons that typically has an empirical formula with a C:H:O ratio of 1:2:1. Carbohydrates are either aldehydes or ketones with other carbons in the chain bound to hydroxyl groups.
Exceptions to the C:H:O, 1:2:1 ratio are for deoxy sugars where a hydroxyl group has been replaced by a hydrogen.
How are straight chain carbohydrates named?
Straight chain carbohydrates are first classified as either aldoses, containing an aldehyde group, or ketoses, containing a ketone group. Then, the number of carbons is counted (di, tri, quatr, pent) and included after the aldo- or keto- prefix. Finally, the -ose suffix, denoting a carbohydrate, is attached to the name. There are no spaces between the prefix, numerical identifier, and the suffix.
Sometimes, carbohydrates are named without the aldo or keto prefix. Names like these would only indicate the carbohydrate carbon length.
According to simple carbohydrate nomenclature, what is the name of this molecule?
[image of straight chain aldopentose]
An aldopentose.
This carbohydrate has an aldehyde group which is denoted by the prefix aldo-. It is a five carbon sugar so it should be named by the Greek pent. Finally, the -ose suffix identifies the molecule as a carbohydrate.
How are cyclic carbohydrates named?
Cyclic carbohydrates are named based on the number of members in the ring. There are two main categories of cyclic carbohydrate, five membered furanoses and six membered pyranoses.
To denote a carbohydrate in the ring form, remove -se from the carbohydrate name and add the name to furanose or pyranose, as is appropriate.
What sort of cyclic carbohydrate is this molecule?
[image of a pyranose]
A pyranose.
The carbohydrate is in the five membered ring form, so it is a pyranose.
What is the common name of this molecule?
[image of straight chain D-glucose]
Glucose.
Glucose is a common hexose that can be identified in the Fischer projection by the orientation of the hydroxyl groups. From top to bottom, the hydroxyl groups will be oriented either right, left, right, right or left, right, left, left.
What is the common name of this molecule?
[image of D-fructose]
Fructose.
Fructose is a common pentose that can be identified in the Fischer projection by the orientation of the hydroxyl groups. From top to bottom, the hydroxyl groups will be oriented either left, right, right or right, left, left.
What are the three main structural ways in which carbohydrates are classified?
- As aldoses or pentoses based on whether the carbohydrate contains an aldehyde or a ketone group.
- As furanoses or pyranoses based on whether the carbohydrate is in the five membered or six membered ring form, respectively.
- As a regular sugar or a deoxy sugar based on whether the carbohydrate has each carbon bound to at least one oxygen or one or not.
What is the difference between a monosaccharide and a disaccharide?
A monosaccharide is composed of one cyclic sugar and a disaccharide is composed of two cyclic sugars bound together.
How is the absolute configuration of a carbohydrate determined?
The absolute configuration is determined by the chirality of the carbon atom furthest from the carbonyl carbon of the straight chain carbohydrate.
What determines whether the absolute configuration of a carbohydrate is D or L?
For a carbohydrate in the straight chain, Fischer projection form, it is determined to be a D sugar if the -OH is to the right on the chiral carbon furthest from the carbonyl group. Conversely, the carbohydrate is an L sugar if the -OH is to the left.
What is the absolute configuration of this carbohydrate?
The carbohydrate is a D sugar.
The -OH on the chiral carbon furthest from the carbonyl carbon is to the right on the Fischer projection.
What are epimers?
Epimers are two carbohydrates that differ in configuration at just one chiral carbon.
Epimers are a special type of diastereomer.
What is the relationship between the following carbohydrates?
The carbohydrates are epimers. They differ in configuration at only third carbon from the carbonyl. Thus, the carbohydrates are also diastereomers.
What is the stereochemical relationship between carbohydrates 1, 2 and 3?
1 and 2 are epimers differing at the chiral carbon closest to the carbonyl. Therefore 1 and 2 are also diastereomers. 2 and 3 are enantiomers, nonsuperimposable mirror images of each other. 1 and 3 have no formal relationship.
What is the stereochemical relationship between carbohydrates 1, 2 and 3?
1 and 2 are epimers differing at the chiral carbon closest to the carbonyl. Therefore 1 and 2 are also diastereomers. 1 and 3 are epimers differing at the chiral carbon second from the carbonyl. 1 and 3 are also diastereomers. 2 and 3 have no formal relationship.
What is an anomeric carbon?
An anomeric carbon is the most oxidized carbon in the ring form of a carbohydrate. The stereochemical configuration of the anomeric carbon is another way by which to classify carbohydrates.
The anomeric carbon is bound to a hydroxyl group and to the oxygen that is a member of the ring. The anomeric carbon in the ring form derives from the carbonyl carbon in the straight chain form.
How is it determined whether a carbohydrate is an alpha or a beta anomer?
The alpha anomer has the anomeric -OH trans to the carbon coming off of the ring in the Haworth projection. The beta anomer has the anomeric -OH cis to the carbon coming off of the ring in the Haworth projection.
What sort of linkage does the following disaccharide contain?
An alpha linkage.
The anomeric carbon of the left carbohydrate links to the right carbohydrate via an oxygen . Since the configuration of the linkage-forming anomeric carbon is alpha, the linkage is classified as an alpha linkage.
What sort of linkage does the following disaccharide contain?
A beta linkage.
The anomeric carbon of the left carbohydrate links to the right carbohydrate via an oxygen. Since the configuration of the linkage-forming anomeric carbon is beta, the linkage is classified as a beta linkage.
How is the linkage bond between carbohydrates broken?
The bond is broken via hydrolysis.
Water can nucleophilically attack the anomeric carbon that is forming the link and the other carbohydrate will act as the leaving group. Since the anomeric carbon is like the carbon of an acetal, the hydrolysis of the linked carbohydrates is similar to the hydrolysis of an acetal.
What new functional group is created by this reaction?
An ester.
The hydroxyl groups nucelophilically attack the anhydride, resulting in the acylation of all hydroxyl groups.
