Open chain structure and cyclic structures of D-glucose

 D-glucose exists in two main forms: the open-chain (linear) structure and the cyclic structures (which include the α and β forms). Below is a breakdown of each structure:

1. Open-chain structure of D-glucose (Fischer projection):

In its linear form, D-glucose is an aldohexose, meaning it has an aldehyde group (-CHO) at one end and six carbon atoms.

Structure:

     CHO

       |

H - C - OH

       |

HO - C - H

       |

H - C - OH

       |

H - C - OH

       |

  CH2OH

• The aldehyde group (-CHO) is at carbon 1.
• Carbons 2 to 5 each have a hydroxyl group (-OH) and a hydrogen atom (-H).
• Carbon 6 has a -CH2OH group.
• The orientation of the hydroxyl groups defines D-glucose. For D-glucose:
• OH on carbon 2 is on the right.
• OH on carbon 3 is on the left.
• OH on carbon 4 is on the right.
• OH on carbon 5 is on the right.

2. Cyclic structures of D-glucose (Haworth projections):

D-glucose cyclizes to form a six-membered ring known as a pyranose. This reaction involves the aldehyde group at carbon 1 reacting with the hydroxyl group at carbon 5, forming a hemiacetal linkage.

There are two cyclic forms, α-D-glucose and β-D-glucose, which differ in the orientation of the hydroxyl group on carbon 1 (the anomeric carbon).

α-D-glucose (Haworth projection):

• In α-D-glucose, the hydroxyl group on carbon 1 points downward (below the plane of the ring).

      O

     / \

HO - C   C - OH

     |     |

  H - C   C - OH

     |     |

OH - C   C - H

     |     |

    CH2OH

β-D-glucose (Haworth projection):

• In β-D-glucose, the hydroxyl group on carbon 1 points upward (above the plane of the ring).

 

      O

     / \

HO - C   C - OH

     |     |

  H - C   C - OH

     |     |

OH - C   C - H

     |     |

    CH2OH

Key Differences Between α and β Forms:

• α-anomer: OH on carbon 1 is down (opposite to CH2OH on carbon 6).
• β-anomer: OH on carbon 1 is up (same side as CH2OH on carbon 6).

These cyclic forms can interconvert through a process called mutarotation when dissolved in water.