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: