So, if you read a bit during the week, you might have guessed that I was alluding to the purple line on our chromatograph being an optical brightener. If not, here’s the idea:

You know the stereotype of the “blue-haired little old lady”? This comes from the fact that they used to use certain dyes, such as Prussian blue, to color their hair. This is because grey hair is often not grey but a little bit yellow. For whatever reason, some decided that a whitish tinge was aesthetically a bit better. Color mixing isn’t quite as simple as it seems. Adding a hint of blue to something yellow can actually make it look whiter. This was the idea with bluing hair - too much, though, and it was obvious. As anyone who’s owned a white undershirt for longer than a week can attest, these get a bit dingy too. Bluing to the rescue. Such dyes were commonly added to the laundry.
At least until quite recently. While I have no idea what the state of the art is in hair-dying, I know that laundry scientists have largely switched to a class of compounds called optical brighteners. These are compounds that absorb light in the UV and near-UV (a little bit of which is available even inside), and re-emit it at lower energy, in the blue end of the spectrum. This ends up making things look even whiter. I had a hard time picking an example; there are many dyes that do this. One subclass of optical whiteners is the coumarins, though, and I love coumarin:

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The last, fastest spot on the chromatograph is FD&C Blue #1, or Brilliant Blue FCF:

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The next spot on our chromatograph is Red 40, or Allura Red AC. Here is its structure:

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Here’s one of our paper chromatography spots - the yellow dye is the beloved Yellow #5, or tartrazine:

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The previous post about DEAE cellulose reminded me of a classic experiment in chromatography: the separation of dyes. This is where the name comes from - even though most of what we separate anymore isn’t colored, it stuck. Mikhail Tsvet, a Russian chemist, used it to separate chlorophylls and carotenes, two classes of brilliantly colored biomolecules. Lucky for you, when I have nothing to do on a Sunday afternoon, I do…well, the exact same thing I do during the week, only for free. In my kitchen. It gets messy. Today wasn’t so bad.

I went to the grocery and bought the regular food coloring everyone gets, the four little plastic bottles with red, blue, green, and yellow. If you look at the ingredients, my box says there is: water, propylene glycol, yellow 5, red 40, and blue 1. Propylene glycol, mentioned on the fomepizole entry, is just a nontoxic alcohol that probably makes the dyes a bit more soluble. Notice there are only three dyes. This is probably not so surprising since we have yellow, red, and blue, and the only one without its own dye is green. Presumably the venerable principle of “yellow and blue make green” is at work here. We set out to verify the YABMG theory.

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You have probably heard of chromatography. This is a chemical technique for separating mixtures of compounds. Various “stationary phases” can be used to separate compounds based on different characteristics. Probably the most common is silica, which is just pure, clean sand (speaking a little loosely. It’s so pure you wouldn’t recognize it as sand. It’s a fine white powder and very homogeneous in size.) It separates compounds based on their polarity. It is the most common medium used by organic chemists because it’s relatively cheap and it works on a wide variety of substrates.

Biochemistry makes it trickier. First of all, everything’s dissolved in water, which is the most polar solvent most people will ever encounter. Silica chromatography with water just doesn’t work. It’s done with organic solvents like ethyl acetate and methylene chloride. Biomolecules, as a rule, don’t take well to being dissolved in anything but water. A lot of the time (especially with proteins), you’re worried about a specific three-dimensional structure. Organic molecules, as a rule, don’t really care. This is largely because most of them are too small to have enough freedom to fold into anything interesting. This is why I can show the daily molecule as a stick drawing and not have to worry about 3D structure.

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