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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."
Most of this is correct, except that cellulose is actually a polymer made up of glucose. More specifically, cellulose is beta(1,4)-linked glucose. This online lecture has a picture of cellulose both as a linear chain and as how linear chains can form a more complex sheets (not pictured: the sheets can then stack). We can't use cellulose because we don't have cellulase, a series of enzymes that can hydrolyze the chain. It's been a while, but I think it pops off glucose pairs, not monomers but I could be mistaken.
This is a pretty good comment, but I'd like to add that the reason why a certain enantiomer of a sugar would not convey calories is because it would fail to react with digestive enzymes. However, any chemical that is altered to remove specificity for these enzymes is also likely to cease its specificity for the taste receptors that register "sweet". While I believe that taste receptors might be a little bit more lenient, I would bet that this would be one of the problems. The extreme difficulty/cost of seperating enantiomers being another, as noted in the parent. which is why other approaches are yielding results more quickly, such as the undigestible "fat" of Olestra.
and btw IAABOC (i am a bioorganic chemist)
well, in training. give me a few years ; )
Mod me down, and I will become more powerful than you can possibly imagine!
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.
Aspartame never reaches the organs? Well I guess we don't have to worry about it causing cancer then, since all cancers happen in the organs.
Right?
And you mean isomer, not isotope.
OTOH, you do have a point. After all, the thalidomide disaster was caused by optical isomers, that's never a good thing to associate with.
I am one of many. My idea is not unique, nor do I expect my voice alone to sway you. I speak in a chorus of opinion.
As far as separation goes, try this:
Take a small peptide analogous to the binding sequence of a glucose transporter, high affinity metabolic enzyme, etc. There are quite a few you can chose from. Select for enantiomer specificity. Synthesize a large quantity. Mass production of a small peptide isn't terribly expensive, e.g. aspartame. Bind it to your matrix of choice and pack a column. Run your mix through it and now, the biologically active form is gone, leaving you with the inactive forms coming out of the column. When it gets "full", wash it all out and reuse.
and btw IAABOC (i am a bioorganic chemist)
Me too. In fact, the reason I came up with this is because I'm collecting fractions of an enzyme by an affinity column right now.
There is a reason for everything. Sometimes that reason just sucks.