Trans Fats (Kinky fat)

There are two kinds of fatty acids: saturated and unsaturated. Saturated fatty acids have no carbon-carbon double bonds (-CH=CH-), they have only (-CH₂-CH₂-) single bonds. Notice that carbons only involved in single bonds have one more hydrogen than those involved in a double bond. They are "saturated" with hydrogens.

Double bonds come in two forms: cis and trans. A cis double bond has both large substituents (the carbons in the fatty acid chain) on the same side of the double bond. A trans double bond has large substituents on either side of the double bond. Below is a picture of three 18-carbon fats: oleic acid, a monounsaturated (one double bond) cis fat, elaidic acid, a monounsaturated trans fat, and stearic acid, the saturated 18-carbon fat.

Trans fatty acids occur naturally to a small extent, but for the most part, we make them by artificially modifying natural unsaturated fats. There is an energy difference between cis and trans double bonds; trans double bonds tend to be lower in energy. This is because the substituents in trans configured double bonds aren't as crowded as those in cis double bonds. This saves trans a bit of energy. So why does just about every living thing in the world make cis fatty acids? It actually costs us energy, and EVERYTHING does it! So there must be a potent evolutionary benefit.

One important contributor to whether something will be a liquid or a solid is how well it can pack together. Saturated fats pack very well. Trans unsaturated fats pack very well. Cis unsaturated fats pack terribly. This is why most unsaturated fats are oils (and highly saturated fats, like lard, are actually somewhat crystalline).

So what does this have to do with cis fats? Here is the explanation I was always given: cis fats, packing more poorly, allow for a more fluid cell membrane. Since every cell in every living thing has a double envelope of fatty acids, membrane integrity is important. Unsaturated fatty acids, though, wind up in all sorts of structures, such as the prostaglandins, so who knows what else they do. More likely they just get treated a lot like saturated fats, because the shape of the trans linkage is so different from the cis linkage most fatty-acid-processing enzymes use. If anyone has a more detailed answer than membrane fluidity, I'd like to hear it.

The melting point difference in fats is dramatic: oleic acid, at top, melts at 13C, elaidic acid melts at 44C, and stearic acid melts at 72C.

So where do these oddballs come from? A few are made naturally, but most are synthetically produced from highly unsaturated fats. If you heat fats to deodorize them (very hot, >200C), some double bonds will isomerize. Most, though, are produced in this mysterious "partial hydrogenation" process. Here, fats with multiple double bonds are held under high pressure with a metal catalyst (nickel and platinum are two common ones) and hydrogen gas. The metal allows the hydrogen to add right onto those double bonds, producing waxy saturated fats if you let it go long enough. Since most people want something melty on their toast, they don't let it completely finish, leaving some double bonds behind (and therefore, a more fluid, gooey, delicious, mouth-pleasing fat).

However, the process is partially reversible. Some fats will stop somewhere between binding to the metal and release with a hydrogenated double bond, and go back to being a doubly-bonded (unsaturated) fat. As you'll remember if you've stuck with me, trans double bonds are lower in energy. Once you break the double bond and make it again, you end up with - yep, mostly trans.

As you've probably surmised, the concern with trans fatty acids is that they look (and act, remember the melting points above) a lot more like saturated fats than unsaturated fats. More worrisome is just what they'll do when they're standing in for their geometric isomers (the cis fats).

I close with this fun fact. Cis and trans are actually deprecated language; we're supposed to be using E (from German entgegen) for trans, and Z (from German zusammen) for cis. They now teach you to remember "Z" as being on "zee zame zide of zee double bond." I didn't go in for such glib borderline-Teutophobic mnemonics in undergrad, and I much preferred to think of the double bond substituents poised on either side as "entgegen in combat," and drew pictures such as the below in the margins of my notes.

I suppose it wasn't so much it being better as it being mine.

See you tomorrow.