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α-d-Mannose is a sweet-tasting sugar. β-d-Mannose, on the other hand, tastes bitter. A pure solution of α-d-mannose loses its sweet taste with time as it is converted into the β anomer. Draw the structure of β anomer and explain how it is formed from the α anomer.

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The conversion of α-d-mannose to β-d-mannose is an example of mutarotation. Mutarotation refers to the spontaneous interconversion between different anomers of a sugar in solution.

The structure of α-d-mannose and β-d-mannose are slightly different due to the orientation of the hydroxyl group (-OH) attached to the anomeric carbon (C1). In the α anomer, the -OH group attached to C1 is in the opposite direction of the -CH2OH group (i.e., axial position), while in the β anomer, the -OH group attached to C1 is in the same direction as the -CH2OH group (i.e., equatorial position).

The conversion of α-d-mannose to β-d-mannose occurs via a hemiacetal intermediate. In solution, the α anomer undergoes a ring-opening reaction to form an open-chain aldehyde intermediate. This aldehyde intermediate can then react with water to form the β anomer via a hemiacetal intermediate.

The structure of β-d-mannose is shown below:

          HOCH2OH

             |

    HOCH2OH-CH-OH

             |

          HOCH2OH

             |

             H

As for the bitterness of β-d-mannose, it may be due to the orientation of the hydroxyl group at C2, which is different from that in the α anomer. The different orientation of the -OH group may affect the way the sugar interacts with taste receptors in the mouth.
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