July 3, 2008 at 12:52 pm | | everyday science, great finds, science@home, tutorial

I’m on vacation with my financier fiancée for a week in South Carolina: a few days at our friends’ wedding, a few days at Hilton Head, and a few days with my soon-to-be in-laws. The weather has been beautiful: heat and humidity reminding of my UNC days. The shark that swam past me gave me the creeps, but otherwise Hilton Head was perfect.

One strange event: a friend got a sunburn and a strange rash. The dermatologist asked if she had been making Mojitos or drinking Coronas. Huh? Diagnosis: Phytophotodermatitis. This is really cool: limes (and various other plants) contain furocoumarins (particularly psoralen, structure below), coumarin-type chromophores that absorb strongly in the UV.

Psoralens act as photosensitizers: absorbing UV light and releasing reactive triplets or radicals. With fluorescence quantum yields only around 1-2%, psoralens transition to the triplet state (via El-Sayed’s rule, I expect, from the Jablonski diagram below) and phosphoresce strongly. Chromophores stuck in their triplet state can return to the singlet ground state by coupling with triplet O2, producing a highly reactive singlet O2 species. This may be one mechanism of the photosensitizing properties of psoralens. Alternatively, a psoralen molecule in its triplet state can react directly with DNA or other biomolecules with electron-donating capability. Various other photosensitizing reactions are discussed in an interesting review (Kitamura, N.; Kohtani, S.; Nakagaki, R. J. Photochem. Photobiol. C 2005, 6, 168-185).

So, basically, my friend was spraying tan accelerator on her skin, then sitting in the sun for hours! That equals strangely shaped splotches of sunburn. In fact, psoralens have been used in photochemotherapy (also called PUVA) for certain skin ailments, such as eczema and psoriasis. So be careful squeezing limes on the beach, or picking parsnips or playing with celery in the sun.

Check out some doctory stories in this article: Weber, I. C.; Davis, C. P.; Greeson, D. M. J. Emer. Med. 1999, 17, 235-237.


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  1. Wasn’t at all aware of El-Sayed’s rule. Wonder if that could be exploited in designing materials for triplet exciton-harvesting devices.

    Comment by psi*psi — July 3, 2008 #

  2. prolly.

    Comment by sam — July 3, 2008 #

  3. Who invited the nerd to the party:) Geez, I’m just trying to make a gin and tonic with lime and you go all “triplet exciton-harvesting” on me.


    Comment by Chemgeek — July 3, 2008 #

  4. There goes the licensing viability of my patent on parsnip infused skin cream.

    Comment by Kendall — July 4, 2008 #

  5. Interesting stuff! I’m going on vacation this week myself…I’ll keep this in mind. Who knows, the ladies might dig it…?

    Comment by mevans — July 5, 2008 #

  6. funny thing that you came across psoralens…when i made my brief stop in biology, i studied transcription in dna with cross-links and monoadducts. cool stuff ;)

    Comment by John — July 8, 2008 #

  7. Way over my head!

    Comment by Eczema & Psoriasis — July 10, 2008 #

  8. Just returned from a trip to a dermatologist to have some weird burn-like lesions checked out on my fingers….after looking at my fingers he asked “Have you been out in the sun drinking margaritas?” I replied “No, but I’ve been in the sun drinking corona and lime.” BINGO!

    Comment by Suzanne — July 15, 2008 #

  9. Hello guys.

    I just clicked on Your site after a quick search and – guess what – found that nice Kitamura publication that even Scifinder did not spit out.

    You have got Yourself a new reader..



    p.s.: .. cause I like the style of Your blog. Of course, if I can get some more time from my main plan, that is, the escape from the Ivory Tower of Max Planck.

    Comment by Piotr — December 7, 2008 #

  10. the redness and itchiness of Psoriasis can be relieved easily by corticosteroids.:.,

    Comment by Anthony Wilson — May 3, 2010 #

  11. So, I was helping out my dad in his garden, working in close contact with a fig tree. Got some sap on me, and it itched a bit, but I thought “whatever.”

    I proceeded to spend the next 9 hours in the sun. 48 hrs later – my arms and a spot on my leg are all blistered up.

    Cause of it: furocoumarins and exposure to the sun.

    Comment by Texasguy — July 20, 2010 #

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