Dihydroxyacetone is a very, very simple sugar. It is produced by the splitting of hexoses (glucose and fructose) in glycolysis. Most people are interested in it, though, for its role in sunless tanners:

Many sugars, DHA included, can participate in a reaction called the Maillard reaction. In this reaction, sugars react with amino acids and proteins. This can result in brown pigments. DHA (sugar) reacts with skin proteins (amino acids/proteins), providing a rich brown pigment, if you’re lucky, or an orange pigment, if you’re not.

The same reaction responsible for sunless tanning results in meat browning (and flavor). Bizzare.

Night!

Just about anyone who’s ever been in a GNC knows melatonin by now. People know it best as a sleep aid. However, it is responsible for a diverse range of biological processes, and you might be surprised to know that it is a sufficiently ancient compound that it seems to be found in quite a wide range of species.

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This is one of those boring-seeming ones: cysteine. However, biochemistry really wouldn’t work without it. Cysteine is the sulfur analogue of the amino acid serine. I say “sulfur analogue” because oxygen and sulfur are in the same group (column) of the periodic table. They are in group 6B, also known as the “chalcogens”, from the greek for “ore formers.” (Oxygen and sulfur, as well as, to a lesser extent, selenium and tellurium being ubiquitous in ores of metals). Many oxygen compounds have a sulfur analogue. In many ways members of the same group are very similar to each other, because their electrons are configured in analagous ways.

Oxygen can form a structure called a peroxide. R-OH (R representing something else attached to the oxygen) is an alcohol that has a peroxide brother in R-O-O-R. Peroxides are potent oxidizers (witness hydrogen peroxide and benzoyl peroxide’s use as disinfectants in low concentrations). At very high concentrations, they can be initiators for explosives (or explosives themselves!).

The sulfur analogue of a peroxide is a disulfide. This is a dimer of R-SH (a “thiol”), or R-S-S-R. This is an oxidized compound as well, but R-S-S-R is a much milder oxidant than R-O-O-H. Mild is the name of the game in biochemistry, since you only have cells, enzymes, blood, etc. to hold these things in. Peroxides are a bit much outside of our glass and plastic vessels. So it’s fortunate we have something mild and easily reversed, like disulfide bonds. Cysteine is what forms just about every disulfide bond in your body. The disulfide oxidized cysteine dimer is called cystine:

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This drug is one that really has had a profound effect on the medical landscape, but you don’t hear much about it in popular culture. Naloxone, commercial name Narcan, is an opioid antagonist. An antagonist is a drug that binds to a receptor, without activating it. Contrast an agonist, which binds a receptor and activates it. Opioid agonists are drugs like heroin, morphine, codeine, etc. Read the rest of this entry »

Cubane, unsurprisingly, is a cube-like structure with formula C₈H₈:

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PABA, or para-aminobenzoic acid, was used as a sunscreen some time ago. Most sunscreens are simply organic molecules that aren’t too soluble in water (so they don’t wash away) that absorb UV light (and convert it into heat). There are other kinds, like zinc oxide creams, that just provide an opaque/reflective layer. PABA, however, is of the first type.

It is also a biosynthetic precursor to folic acid in many bacteria. This is a profoundly essential nutrient, especially for rapidly dividing cells. This applies to pregnant women (hence the inclusion of extra folate in prenatal vitamins) and most bacteria, which tend to be pretty busy dividing. Since people don’t depend on the PABA-Folate conversion system (because we don’t have it), PABA has been used as a drug target for antibiotics. These are the ancient sulfa drugs, which predate even penicillin. Here is the structure of PABA:

Today, PABA is not in sunscreens because of allergy and carcinogenicity concerns (close analogues are safe and included, though). Sulfa drugs have also fallen by the wayside, largely because of allergy and poor efficacy compared to the modern stuff.

Interestingly, furosemide (Lasix) contains a sulfa-drug like structure, but works on a completely different pathway (it’s a diuretic).

See you tomorrow!

Here’s a quickie, phenolphthalein:

Phenolphthalein is known as an acid-base indicator because it gives a colorimetric change depending on the pH. Above a certain critical pH, called its pKa, phenolphthalein adopts mainly the lower structure shown above . Below this pH, it is mainly the upper structure. The lower structure is pinkish-purple, the upper is colorless.

As you can imagine, molecules like this have all sorts of uses. You can figure out the concentration of an acid or base, using the color as an indicator of whether your solution is acidic or basic (8.2 is sufficiently close to 7, neutral pH, that you can get away with this). It apparently is used to test for the presence of blood. It used to be used as a laxative, but its carcinogenicity has not been completely ruled out. It is also used in Barbie Hollywood Hair (scroll down to “uses”).

The broad class of dyes are used in “pH papers” for lab use; papers soaked with one or more dyes will change color depending on the pH; a series of dyes can give resolution below even 1 pH unit, which is pretty good. “Litmus” paper is the best known of these, containing a dye extracted from lichen.

You can extract your own pH indicator from cabbage. If you never did this, you had a deprived childhood and should try it over the weekend. If you are lazy, you can just use red wine or grape juice; there is a similar pigment with red acidic and blue basic forms. This broad class of pigments is known as anthocyanins. Many of the deep reds and blues in nature are due to this class of pigments, and they make great pH indicators.

Inspired by yesterday’s entry, which wasn’t really a molecule proper, today’s entry is about the diamminesilver (I) complex, better known as Tollens’ Reagent. In practice, this is usually generated by taking a solution of silver (I) nitrate and adding a drop of NaOH solution. This generates some Ag₂O, or silver (I) oxide. Addition of aqueous ammonia will dissolve the silver oxide, generating the diamminesilver (I) complex. Why do we care? Read the rest of this entry »

As we’ve discussed, gold is among the so-called “noble metals,” named as such for their lack of reactivity. Gold won’t dissolve in concentrated solutions of nitric acid or hydrochloric acid. Both are strong acids, and nitric acid is a potent oxidizer, which tends to help quite a bit in dissolving metals. It turns out chloride and an oxidizer are the necessary and sufficient conditions to dissolve gold.

Enter aqua regia, which is just a mixture of the two acids (providing both the chloride plus the oxidizer). Typically you use ~25% concentrated nitric acid and ~75% concentrated hydrochloric acid, but other proportions are known (the other one I see some people use is 75/25, which I think is quite a bit nastier.

Like so many colorfully named classics, aqua regia derives its name from alchemy. As you have no doubt figured out, it is from the Latin for “royal water,” from its gold-eating superpowers. It was discovered in 800 AD by the alchemist Abu Musa Jabir ibn Hayyan.

One unique thing about aqua regia is that it decays after being mixed up, so you always have to make it fresh. The nitric acid slowly works on the chloride ion, generating chlorine gas, leading to a pleasant swimming-pool aroma if you just catch a whiff, or choking fumes if you get more than that (Chlorine is really a violent poison in much higher concentrations than you run into at the pool, and it has been used as a war gas).

Also generated is the toxic nitrosyl chloride (NOCl), which is a beautiful reddish-orange. So you mix concentrated nitric acid (colorless, or maybe tinted just barely yellow, but mostly clear), concentrated hydrochloric acid (colorless), and you get a red-orange, bubbling, smelly solution that can dissolve gold. Can you imagine what the alchemists must have thought of this?

My very favorite story about aqua regia is this: during World War II, a Hungarian chemist living in Denmark, George de Hevesy, dissolved two fellow scientists’ Nobel Prizes in aqua regia literally as the Nazis stormed into Copenhagen so they wouldn’t be stolen (he assumed, correctly, that the Nazis would just leave the chemicals alone). After the war, he recovered the gold, and the Nobel committee recoined the prizes. You can read more about it here.

See you tomorrow.