Ring Structures

Although the bond angles force the carbon atoms of sugars not to be in the same plane, it is not easy to represent this when drawing the structures of sugars on a sheet of paper. Many modern text books do show the sugar structures in perspective by adopting a drawing convention, but that is beyond the scope of this presentation. The older Haworth representations will be used here.

The previous page about formation of a hemiacetal of a sugar made no attempt to represent bond lengths and merely tried to show what goes where. The following sketch shows glucose in a ring structure :

The numbers can be omitted. They are to help you see what goes where. Think of the ring as a tilted plane with the OH groups shown as lines that project above or below. The #1 carbon was the aldehyde group, and the new OH that came from acetal formation can be above or below the plane. The alpha form is shown. There is an equilibrium between the open chain, the alpha ring, and the beta ring. The proportions of each depend on the particular sugar.

To understand and appreciate sugar structures, the rings of D- and L- sugars should be compared. The rings should be flipped and rotated to see which groups end up above or below the plane. The structures are often arranged to get reacting groups close to each other as one sugar reacts with another to form disaccharides, trisaccharides, and sugar polymers such as cellulose. However, these pages are designed for engineers who need some introduction to sugar structures and do not need to know everything. This should be enough about sugar rings to move on to other topics.

  • Structures of sugars and polymers
    while on sabbatical leave, ESB, Porto, Portugal July 1996