Molecule of the Day

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A number of carboxylic acids are used as cosmetics. Familiar to many readers will be glycolic acid. It’s used in over-the-counter “cosmeceuticals” to improve skin tone. Also used in professional “chemical peels” is trichloroacetic acid. These acids are all closely related to the familiar acetic acid, but they have “electronegative” substituents.

In the aqueous systems we’re talking about, the strength of an acid is related to its tendency to liberate protons. An acid’s tendency to dissociate into protons is related to the thermodynamic stability of its “conjugate base” - the acid, minus a proton. These electronegative substituents withdraw some electron density from the negative charge on the conjugate base, giving a more potent acid. Read the rest of this entry »

Here’s one many will have heard of - creatine. Most people know it as a bodybuilding supplement; it is used to add lean mass (from working out harder, and it’s contended, some intracellular water retention), as well as allow harder anaerobic workouts before failure. It’s the latter we’re interested in.

Creatine actually occurs endogenously, and it is synthesized by the liver. An enzyme called creatine kinase exists in the body to move a phosphate from ATP to creatine, making N-phosphocreatine and ADP. This reaction is not very thermodynamically favorable - you will only make 1 molecule of phosphocreatine for every 1000 molecules of ATP or so. This is good, because your body needs the ATP for other things (you’ve probably heard of ATP as the “universal energy currency” of the body) as well. Phosphorylation of creatine only happens when your body has a large excess of ATP.

The reverse reaction takes place too, and, as you might guess, phosphocreatine still would prefer to transfer its phosphate to ADP, regenerating ATP. It’s favored in the low-ATP regime - such as during the last repetition of a weightlifting set. This latter reaction is shown below:

Read the rest of this entry »

Early readers will remember that one of my favorite things about artificial sweeteners is that they seem to often be discovered by accident. Apparently acesulfame is no exception:

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This one is a much larger molecule than we usually do, but it’s easy to forget that enzymes (and all proteins) are really just very large molecules. Taq polymerase is a DNA polymerase (DNA-copying enzyme) from the microorganism thermus aquaticus, a bacterium that lives in very hot water. Read the rest of this entry »

A catenane is a topologically locked two-ring system. Put more plainly, it is a molecular chain (with interlocking links). I used to doodle molecules like this before I knew they existed. As I was going through organic chemistry in undergrad, I always figured they were impossible to make. Read the rest of this entry »

This is a bizzare molecule. As I was discussing yesterday, structure-odor relationships are tricky. One very reliable predictor, though, is that the compounds of the later chalcogens (that is, compounds containing a sulfur, selenium, or tellurium atom) stink. Sulfur being the second most common member of this group after oxygen, this usually means thiols.

Thiols have a prodigous appetite for metal, especially mercury. This led to a second name for them: mercaptans (describing their ability to capture mercury).

All this leads up to a truly unusual thiol: grapefruit mercaptan. You’ll notice it looks like another terpene, like carvone and damascone. Grapefruit mercaptan is such a singular compound because it is a nice-smelling thiol. It’s another one I haven’t smelled, but I’m told it’s very complex and grapefruity. It’s also unique because not many things synthesize thiols on purpose (thiols being reactive and stinky - skunks are a notable exception here).

The Wikipedia article notes that this is a bit of a thorn in the flavor industry’s side - almost ALL thiols stink, and this is a notable exception. Thiols have a nasty habit of oxidizing to form dimers in the presence of oxygen (R-SH -> R-S-S-R), and even if grapefruit mercaptan doesn’t stink, its decomposition products probably do. All these leads to a not-so-hot flavoring agent.

Here’s the structure:

See you Monday.

I will admit to more than a casual fascination with smell. I am forever wafting vials of compounds for a whiff - which I get more and more flak for, as people get more and more cautious. I figure a few femtograms of small molecules here and there are the last thing that will do me in.

Perfumers (and biologists) still are looking for structure-odor rules, which prove to be continuously elusive. As I’ve mentioned before, one problem here is language; describing odor is hard! Read the rest of this entry »

But I’m just getting home. See you tomorrow.