In all academic sources, sucrose is identified as α−glucose (1-->2) β−fructose. However, I cannot find any explanation anywhere as to why the fructose monomer has to be in the β configuration. Maltose has both α and β anomers, same for lactose. Even trehalose, another non-reducing disaccharide with glycosidic linkage between two anomeric carbons, has α-α, α-β, and even β-β anomers. Why is sucrose special? And is there a disaccharide out there that has α−glucose (1-->2) α−fructose configuration?
There is no reason why the corresponding disaccharide where the fructose is in the α configuration couldn't exist, but nature has decided to make the β configuration.
Saying that "Maltose has both α and β anomers" is misleading. The glycosidic bond in maltose is always α, otherwise you'd have cellobiose which is a different component. However, it is right that the reducing end of maltose can be both α and β. Sucrose, in turn, is a non-reducing carbohydrate. Its glycosidic bond is α,β-1,2. Theoretically, there could be an α,α isomer, but it doesn't seem to be relevant.
Paul Körner "However, it is right that the reducing end of maltose can be both α and β."
Actually, that's what "α and β anomers of maltose" means in biochemistry. When we say "α-maltose", we understand that it's referring to the α configuration of the second glucose. The first glucose - as you said - always has to be α; if it's β it's called cellobiose. We also have α- and β-cellobiose by the way, as well as α- and β-lactose, α- and β-isomaltose, etc. That's why it's strange to me that I can't find anywhere in literature a mention of a so-called "α-sucrose".
It's because in sucrose the anomeric C atoms of glucose and fructose are both involved in the glycosidic bond. That's why they can't change their configuration without breaking the glycosidic bond. In reducing disaccharides like maltose the anomeric C of the "second" glucose is "free" (hemiacetal only). Therefore, the open-chain form and all possible configurations (alpha, beta, furanose, pyranose) of the second sugar are available without breaking the glycosidic bond.
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