Glucose is a sugar molecule that can take many different forms in space.
Alpha glucose is a specific isomer of glucose. Glucose can exist in a variety of different molecular shapes, each one having unique properties. Some shapes occur naturally, others are the products of synthetic laboratory reactions. The different shapes have different names. However, regardless of shape, all glucose molecules have 6 carbon atoms, and are single monosaccharides.
Straight Chain Glucose Structure
Glucose is frequently written as a straight chain molecule. As a straight chain, there are six carbon atoms, labeled from 1 to 6. Carbon-1 is the carbon that is part of the aldehyde, or CHO group. Carbon-2, carbon-3, carbon-4 and carbon-5 are each attached to four different groups: a hydrogen atom, a hydroxyl group, and two different types of carbon atoms. For example, the four different groups carbon-2 is attached to include an aldehyde CHO group, a hydroxyl OH group, a hydrogen H atom, and a four-carbon CH(OH)CH(OH)CH(OH)CH2(OH) group. These four carbon atoms, C2 through C5, are called chiral carbons. The positioning of each of the four groups attached to them determines its overall three-dimensional shape in space.
Glucose Ring Structures
Alpha-glucose is a form of glucose that is in a ring shape. Ring-shaped glucose structures occur when the hydroxyl OH group on the carbon-6 atom reacts with the aldehyde group on the carbon-1 atom. A water molecule is removed, and the result is a hexagon whereby five carbon atoms make up five vertices, and an oxygen atom makes up the sixth vertex.
The chiral carbons of glucose can be oriented in space in a variety of ways, creating different isomers of glucose. Isomers are compounds having the same molecular formula, but different three-dimensional structures. There are two types of isomers, structural isomers and stereoisomers. Structural isomers have the same molecular formula but the atoms are bonded together in a different order. For example, glucose and galactose are structural isomers. Stereoisomers, on the other hand, are isomeric molecules that have the same molecular formula and the atoms are also bonded in the same order, but they differ in their three-dimensional orientation of their atoms in space. Alpha-glucose and beta-glucose are examples of stereoisomers.
Alpha Glucose Structure
Alpha-glucose is a specific stereoisomer of glucose. In the ring form, alpha-glucose exists when the hydroxyl group on the carbon-1 atom is below the plane of the ring, and in the same plane as the hydroxyl group on carbon-2 and carbon-4. In contrast, beta-glucose is the isomer where the hydroxyl group on the carbon 1 atom is above the plane of the ring and in the same plane as the hydroxyl group on the carbon-3 atom.
D- and L-Forms of Glucose
Glucose can also exist as D and L forms, which are simply glucose molecules that are mirror images of each other. The D and L designations refer to dextrorotatory and levrorotatory, right-rotating and left-rotating, respectively. Solutions of D-glucose rotate polarized light to the right, while solutions of L-glucose rotate polarized light to the left. Only D-glucose exists in nature.