Structure of Carbs

• Carbohydrates


  A major class of naturally occurring compounds that are key for much of life on Earth with multiple roles and forms
• Functions of carbohydrates

  • Form cell walls in bacteria and plants and exoskeletons of arthropods - cellulose is the most abundant organic compound in the biosphere
  • Energy carriers: ATP and coenzymes contain phosphorylated sugars
  • Energy storage: starch and glycogen are broken down into glucose - fuel
  • Framework for DNA/RNA - the codes for life
  • Carbohydrates on cell surfaces allow cell-cell recognition (e.g. sperm binding to a specific oligosaccharide on the egg)
• What are carbohydrates?

  Hydrates of carbon and a group of natural products related to simple sugars
• Structural diversity of carbohydrates


  Ranges from glyceraldehyde, which is a simple carbohydrate with 3 C, to cellulose, which is a large polymeric structure

  
• Sugars (saccharides)


  The simplest types of carbohydrate: they are polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses)

  
• Sugars


  • Glucose (an aldose)
  • Fructose (a ketose)

  
• Glyceraldehyde


  The simplest sugar, a triose and an aldose
• D/L convention for sugars


  Based on the conformation of a sugar with respect to glyceraldehyde: the sugar being looked at is compared to glyceraldehyde and this is used to determine whether its D or L

  
• Fischer projections


  Used to represent the structures of carbohydrates in an easy and accessible manner, with the backbone running top to bottom and the carbonyl towards the top

  
• Linear vs. cyclised sugars


  4 and 5 carbon hydroxyaldehydes or ketones can react with themselves to give stable, cyclised forms, containing the hemi-acetal functional group

  
• Anomeric carbon


  The C1 carbon with 2 O attached in cyclised sugars

  
• Pyranose


  A 6-membered ring sugar
• Furanose


  A 5-membered ring sugar
• Haworth projections


  • Represent cyclised sugars with the ring in a planar form, with the oxygen to the top and right, and substituents included either above or below the plane of the ring

  
• Converting from Fischer to cyclic structures


  Draw the 5 or 6-membered ring, with oxygen in the top right and C1 in the top left. All the groups on the left in the Fischer go above the ring and all the groups on the right go below the ring.
• Mutarotation


  The equilibrium between the α and β anomers of a cyclised sugar in aqueous solution, which can be catalysed by acid or base

  
• Acid catalysed mechanism of mutarotation

  1. Lone pair on ring O is protonated by the acid, forming a positively charged O species

  2. A pair of electrons from the bond between the anomeric C and O is transferred to the O, breaking their bond and opening the ring

  3. This allows for rotation - the double bonded O goes from pointing down (axial position) to pointing up (equatorial position)

  4. Nucleophilic attack from the lone pair on the OH group on the carbonyl C, with electrons from the double bond going to neutralise positive charge on O, giving a closed ring

  5. Hydrogen is lost from ring O to give cyclic glucose
• Conformations of cyclic sugars


  Furanose rings adopt an envelope conformation, with four ring atoms in a plane and one raised. Pyranoses normally have the CH2OH group in the equatorial position.