The structure of some simple sugars is shown in Fig. 1. They all contain five or six carbon atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume., and each carbon atom, except for one, is attached to a hydroxyl groupThe functional group of an oxygen atom bonded to an hydrogen atom, -OH; found in alcohols. (OH). The remaining carbon is double-bonded to oxygen, forming a carbonylThe functional group consisting of a carbon atom doubly bonded to an oxygen atom; found in aldehydes and ketones. group ). Sugars are thus all aldehydes and ketones and are usually referred to as aldoses or ketoses. A moleculeA set of atoms joined by covalent bonds and having no net charge. like glucose is called an aldehexose since it contains six carbon atoms. The difference between different sugars can be very subtle. Note, for instance, that mannose and glucose differ only in the geometrical arrangement of the OH group on one carbon atom. Subtle as this difference is, molecules in a living cell can tell the difference.
In Fig. 1 we have indicated the structure of the sugars in linear or chain form, but sugars usually occur in one of several ring or cyclic structures. We will only consider two of these, the α and β form of glucose. Because of the flexibility of the carbon chain, the linear form of glucose can easily adopt a conformationIn an ion or molecule, one of many spatial arrangements of atoms that differ by rotation about single bonds, but that do not differ in which atoms are attached to each other. in which carbon atom 1 lies adjacent to the oxygen atom on carbon atom 5. When this happens, a protonThe positively charged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom. can be transferred and a carbon-oxygen bond formed:
A careful consideration of the geometry of this structure reveals that not one but two cyclic structures are possible. These are called α- and β-glucose and are shown in Fig. 2. In the β form the C—O bond on carbon atom 1 (shown in dark color) is parallel to the C—O bond on carbon atom 4, while in the α form these two bonds are at an angle of 180° – 109.5° = 70.5°. This geometric difference may seem relatively trivial, but it turns out to be important when glucose molecules are used as building blocks to form larger entities.