What is the difference between d galactose and l galactose

Pairs of enantiomers are stacked together. We know, using the shortcut above, that the enantiomer of R R must be S S - both chiral centers are different. We also know that R S and S R are diastereomers of R R , because in each case one - but not both - chiral centers are different.

In general, a structure with n stereocenters will have a maximum of 2 n different stereoisomers. We are not considering, for the time being, the stereochemistry of double bonds — that will come later. For example, let's consider the glucose molecule in its open-chain form recall that many sugar molecules can exist in either an open-chain or a cyclic form. There are two enantiomers of glucose, called D-glucose and L-glucose. The D-enantiomer is the common sugar that our bodies use for energy.

In L-glucose, all of the stereocenters are inverted relative to D -glucose. That leaves 14 diastereomers of D-glucose: these are molecules in which at least one, but not all, of the stereocenters are inverted relative to D-glucose. One of these 14 diastereomers, a sugar called D -galactose, is shown above: in D-galactose, one of four stereocenters is inverted relative to D-glucose.

Diastereomers which differ in only one stereocenter out of two or more are called epimers. D-glucose and D-galactose can therefore be refered to as epimers as well as diastereomers. Draw the structure of two more diastereomers of D-glucose. One should be an epimer. Erythronolide B, a precursor to the 'macrocyclic' antibiotic erythromycin, has 10 stereocenters. We know that enantiomers have identical physical properties and equal but opposite degrees of specific rotation.

In addition, the specific rotations of diastereomers are unrelated — they could be the same sign or opposite signs, and similar in magnitude or very dissimilar. Since a molecule with n chiral centers can have 2 n stereoisomers…. You can draw an epimer by drawing D-galactose with 1 and only 1 of its chiral centers reversed. There are 3 other epimers that could be drawn as long as you only swap a single chiral center in the diastereomer that you use. Since the diastereomer above only varies from L-galactose by 1 chiral center, the above is an epimer in relationship to L-galactose.

Since it varies from D-galactose by 3 chiral centers, it is not an epimer but a diastereomer. Since not all of the chiral centers are swapped, it is not an enantiomer! Chemically, galactose is stereoisomeric to glucose, which means that the spatial arrangement of atoms is different in both molecules, without any difference in connectivity or bond multiplicity between the isomers. Soluted in water, galactose forms a cyclic hemiacetal like glucose; the open chain form and the cyclic form are at equilibrium.

Long chained galactanes also contain L-galactose which is a mirror-image isomer of D-galactose L means left-handed in the Fischer projection and D means right-handed.

Usage: Galactose is mainly used clinically; for instance in parenteral nutritions. The galactose free diet is critical only in infancy, since with maturation another enzyme is developed that can metabolize galactose. The chair form of galactose follows the same pattern as that for glucose. The anomeric carbon is the center of a hemiacetal functional group. A carbon that has both an ether oxygen and an alcohol group is a hemiacetal. In the chair structure this results in a horizontal projection Haworth - an upwards projection.

The Alpha position is defined as the -OH being on the opposite side of the ring as the C 6. In the chair and Haworth structure this results in a downward projection.