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Better Living Through Chiral Chemistry
Atario writes "A long time ago, I remember reading in some science magazine that someone had the bright idea of using enantiomers (the two forms of asymmetrical molecules, like left- and right-handed versions of the same one) to make zero-calorie sugar -- turns out, in general, that sugars are all asymmetrical, and everything generates and uses only one of the two chiralities (handednesses) for each one. If we consume a mirrored version of, say, sucrose, we might get all of the sweet but none of the calories. Sweet-tooths rejoice! But nothing ever came of it. Now, however, Wired has an interesting article that explains what the holdup has been and indicates the logjam may break soon, but, as it turns out, in a non-synthetic, albeit nonzero-calorie, way."
To say that stereochemistry is limited to 2-handedness is a ridiculous oversimplification of reality. If I'm not mistaken, C6-H12-O6 "glucose"; has D-glucose has 4 chiral carbon atoms (2^4 = 16 possible stereoisomers) - I believe only one of which is able to provide calories.
It isn't as neat as that.
In general N chiral carbons means 2^N possible configurations. Half of these are mirror images (enantiomers) of each other. That means there are 2^(N-1) distinct epimers. These are distinct chemicals in their own right. They don't just taste different and rotate light backwards. They have different melting points, solubilities, IR spectra, etc.
So glucose, with its 4 chiral carbons, is a member of a family of eight related epimers. Any two of the 8 will differ from each other in that between 1 to 3 (1 to N-1) carbons have flipped chirality, making them diastereomers of each other- some carbons are flipped, some aren't. And each has a D and an L form.
The 8 epimers of glucose (including glucose itself) have names: allose, altrose, glucose, mannose, gulose, idose, galactose, and talose. Collectively they are referred to as aldohexoses. As far as I know, most are digestible and taste more or less sweet. Gulose is nonfermentable. Galactose is less sweet than glucose but certainly has calories since it's a component of lactose. Finding information on what the rare ones taste like is difficult. They don't appear in many cookbooks and the people who work with them don't seem to be terribly interested in telling us what they taste like.