Ah, summer! Perhaps you have been enjoying the great outdoors, hiking through the lush forests of the Pacific Northwest, biking through the Appalachian wilderness, or relaxing in the backwoods of the deep South. Also, perhaps you forgot (or never learned) the old Boy Scout adage "leaves three - let it be, berries white - poisonous sight." A few days after your escapades maybe you returned home to find an itchy red rash spreading over your arms and legs. Blast! You just became another victim of urushiol!

The irritating chemicals in plants such as poison oak, poison ivy, and poison sumac are called urushiol oils, and collectively are part of a group of biologically active molecules called catechols. Figure 1 illustrates catechol, a benzenediol (or, if it pleases you, o-hydroxyphenol), which is the parent compound of all catechols, and heptadecylcatechol, the particular species found in poison oak. The catechols found in various plants generally vary only in the length of the hydrocarbon chain hanging off the benzene ring¹.

The oily catechol derivatives found in poisonous plants can be prepared through a variety of reactions and from a number of starting materials, the details of which will not concern us here. For those students studying organic chemistry we will say the following: one relatively straightforward approach (though perhaps not the most industrially practical) to preparing heptadecyl catechol is to begin with 3-bromo-benzene-1,2-diol, protect the hydroxyls, perform a Grignard-style synthesis, and deprotect the hydroxyls to give the desired product. In reality, however, researchers sometimes find it easier merely to purify the urushiol oils from plants.

Clearly, urushiol oils have a potent effect on biological systems (especially human skin and mucous membranes). As with all true allergies, the observed effect (itching, redness, pain, et cetera) are not a direct result of any damage caused by urushiol but by the immune response of the unlucky victim. Whenever a foreign substance enters a person, that person's immune system has cells that can detect the substance, recognize it as foreign, and recruit a microscopic retaliatory assult team. The most well-known response is the production of antibodies that bind to the substance and either inactivate it or flag it for destruction. However, additional responses include the secretion of speciality biomolecules such as pyrogens (which cause local heating of tissues or even full-scale fever) and prostaglandins (which are known, among other functions, to cause inflammation). The localized accumulation of these immune molecules in response to the presence of urushiol trigger the nasty allergic effects.

Urushiol oils are particularly difficult to remove by conventional means (such as washing with water) due both the their lack of polarity (read: water insolubility) and affinity for cell membranes (which largely consist of molecules known as phospholipids which distantly resemble urushiol oils). Hence the oils can be quickly absorbed into the skin and prove impossible to remove even with thorough washing.

As the final rub (no pun intended) catechols are known to be prone to oxidation to related species known as quinones (interesting in their own right and probably deserving of a future Molecule of the Week page). Quinones are significantly more reactive than catechols (due to a disruption of the benzene ring and introduction of carbonyl² groups), and can react with proteins in cells to cause additional problems beyond the basic immune response.