Molecule of the Day

Again, another artificial sweetener discovered by sloppy, sloppy chemists. God bless them (us?). It makes me wonder sometimes whether I’m working with a bench full of delicious chemicals that are just waiting for an accidental taste.

Aspartame was discovered in 1965 by James M. Schlatter, a chemist working for G.D. Searle & Company. Schlatter had synthesized aspartame in the course of producing an anti-ulcer drug candidate. He discovered its sweet taste serendipitously when he licked his finger, which had accidentally become contaminated with aspartame.

Here is the structure of aspartame. It is the methyl ester of a dipeptide of aspartic acid and phenylalanine. That’s it. It’s an incredibly simple structure:

Systematically, it is “Aspartylphenylalanine methyl ester.” The observant reader will notice the two chiral centers; however, these are natural amino acids and not hard to get at (in fact, I’m pretty sure you can buy homochiral aspartic acid or phenylalanine at the health food store).

Interestingly, there are calories in aspartame. It’s pretty much just a very small protein, so it has the 4 kcal/gram you get out of any protein (or carbohydrate). However, it’s a few hundred times sweeter than sugar, so for the same level of sweetness, you take in well under 1% of the calories associated with the sugar-sweetened product (usually low enough to round down to zero, as some beverages do, or sometimes up to one, because it looks cool).

Some people think aspartame is bad for you. This is open for debate. There is one group for whom we know it is terrible: the phenylketonuric. These individuals lack the ability to metabolize phenylalanine, which builds up in their body. In children, this can cause profound mental retardation (and they must take a special low-phenylalanine diet). In adults, it is deleterious, but not the end of the world. It used to be that avoiding phenylalanine intake was only indicated for phenylketonuric children; now, the prevailing wisdom is for PKU adults to avoid it as much as possible as well. This is difficult because the amino acids are pretty well-distributed in all foods, so cutting any one out completely is very hard. Today, infants are screened for PKU at birth.

Small contingents object to just about every part of the aspartame molecule: aspartate, phenylalanine (in healthy individuals), and methanol (derived from the methyl ester). It remains unclear whether it’s bad (we know, at least, it isn’t as bad as the far end of the anti-aspartame fringe claims, because many people drink diet soda with no apparent ill effects). This may become a moot point in the coming years, since sucralose (Splenda) has become so phenomenally popular, displacing aspartame from many products. A special Splenda-sweetened Diet Coke is available.

See you Monday!