Starch is a natural polymer, or more precisely: it is a polysaccharide. The term polysaccharide is not a very specific one. It just indicates that a polysaccharide contains many “sugar molecules”. It doesn’t say, however, what kind of sugar molecule or even different sugar molecules it is constructed of.
So in order to understand the structure and chemistry of starch we have to dig a little deeper into carbohydrate chemistry. As the denomination carbohydrate indicates these molecules contain carbon atoms plus Hydrogen and Oxygen atoms in the same ratio as in Water (H2O) which is 2 : 1. The most abundant carbohydrate is glucose, having the sum formula C6H12O6. One could thus say glucose contains six carbon atoms + 6 water molecules. Glucose, sometimes also called dextrose, more specifically is a monosaccharide or simply said a sugar.
The last mentioned terms maybe confusing because normally sugar describes the white crystalline powder used for sweetening purposes in our kitchens etc.. Chemically household sugar from sugar beet or cane is a disaccharide, composed of glucose and fructose. So the term sugar is the most confusing, as there are many “sugars” and some are not sweet at all.
Let us thus keep in mind that there are monosaccharides (= one “sugar” molecule), di-, tri-, tetra-, …, poly-saccharides that are composed of two, three, four or many “sugar” molecules.
But let us go back to glucose C6H12O6. Glucose is synthesized by green plants. With chlorophyll as catalyst they take six molecules CO2 from the air and six molecules of water from the soil. With the help of sunlight they convert these into one molecule of glucose releasing six molecules of oxygen to the air.
You may be astonished by the glucose formula in fig. 1 because a) there it is called D-Glucose and b) because it is shown not in the linear form (or Fischer projection) that you may be more familiar with.
To explain the D-: this comes from the latin word dexter for right and was chosen because a solution of glucose turns the plane of polarized light sent through this solution clockwise. So D-Glucose is dextro-rotatory.
The effect that solutions of natural molecules turn the plane of polarized light comes from the fact that those molecules have optical active carbon atoms. There are also molecules that turn the plane of polarized light anti clockwise, these thus get the prefix L-, like for instance L-lactic acid. L- thus stands for left, without any latin.
When is a carbon atom optically active?
The answer is quite simple: Whenever a carbon atom has four different binding partners it becomes optically active. Lactic acid is one of the most straightforward optically active molecules. The formula is CH3-CHOH-COOH. The central carbon atom is bound to one CH3-group, one COOH group, an H-atom and an OH-group, it has thus four different partners. Fact is that the four bonds of a carbon atom direct away from the center into the corners of a tetraeder. This allows constructing two molecules having the same chemical formula. However, the arrangement of the groups allows two variants which behave like mirror images of each other. So the molecules have a different stereochemistry.
Such molecules are generally called enantiomers. They have the same properties like our right and left hand. Our hands are constructed of identical components, however, the thumb of the right hand being on its left side (looking from the upper side) and vice versa for the left hand. So they are not superposable. Due to this similarity this effect is called chirality a term based on the greek word “chiros” for hand.
Out of two stereoisomers or chiral molecules normally only one is physiologically active or at least more active than its mirror molecule. Natural processes are specifically producing only one of the enantiomers. For example lactic acid bacteria produce L-lactic acid. Chemical processes in contrast are unspecific in this respect and produce a 50 : 50 blend if the possible stereoisomers.
Why this excursion into stereochemistry? When looking again at the fischer projection of D-Glucose and count the number of optically active carbon atoms you should come to a number of four. These are the carbon atoms No.s 2, 3, 4 and 5 (counted from the top as is the convention).
So in addition to the mirror molecule (enantiomer) of D-Glucose which would be L-Glucose, there are in total 16 stereoisomers possible all having the chemical formula C6H12O6. Out of these 8 are pairs of enantiomers. The most abundant in nature are D-Glucose, D-Mannose, and D-Galacose.
Focussing on glucose there is an additional complication. The Fischer projection is not reflecting that in solution the oxygen from the OH-group on C-5 forms a bond with C-1 and transfers its hydrogen to the oxygen on C-1 transforming the Aldehyde into an alcohol.
According to what we’ve seen so far, the clue of this reaction becomes obvious: a new optically active carbon atom is now created on C-1. The reason is that now C-1 is bound to 4 instead of 3 atoms. Furthermore the oxygen on C-5 can attack C-1 either from the top or from the bottom, meaning that two stereochemically different molecules can be created.
These new isomers are called the anomers of glucose and get the prefixes alpha and beta respectively (Please note that the hydrogen atoms attached directly to the carbon atoms are not shown to keep it simple).