Briefly discuss stereoisomerism with reference to carbohydrates.
It is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms but differ in the three-dimensional orientations of their atoms in space.
Stereoisomers are molecules containing the same atoms bonded identically but the bonded atoms are oriented differently in space. That is to say, they have identical bonding constitutions but differ in how the atoms are oriented in the space around the atoms to which they are bonded.
The D- sugars and L- amino acids. The designations of D- and L- refer to how the pair of enantiomers differ in their bonding configurations. In biochemistry, D is a symbol used as a prefix to indicate the spatial configuration of certain organic compounds with asymmetric carbon atoms. It is used if an organic compound has a configuration about an asymmetric carbon atom (chiral center) analogous to that of D-glyceraldehyde (the arbitrarily chosen standard), in which the hydroxy (OH) functional group is on the right side of the asymmetric carbon atom. The number of possible stereoisomers depends upon the number of chiral centers in the molecule. Van't Hoffs rule states: number of stereoisomers = 2n , where n = number of chiral centers. For example, a molecule with 2 chiral centers can have 4 stereoisomers.
Mostly there are L-aminoacids in all living systems and D-carbohydrates.
D-Glyceraldehyde an aldotriose is the simplest carbohydrate. It has one stereogenic center. It is a sweet colorless crystalline solid, C3H6O3, that is an intermediate compound in carbohydrate metabolism. D-glyceraldehyde is the arbitrarily chosen standard for the assignment of the D configuration.D-Glucose is an aldohexose with four stereogenic centers stacked on top of one another. It is also referred to as dextrose, grape sugar, or blood sugar. It has the empirical formula C6H12O6 .