Photosensitization : causes?

Gabriel Tojo qogatojo at
Wed Jan 15 07:46:20 EST 1997

Martin Witchard (Cat) wrote:
> About a year back I developed a strange, patchy dark 'rash' on my hands.
> This rash only occurred on the back of my hands and more on one hand than
> the other. Finally, I traced the culprit.
> I had read that some people can develop sensitization to rue (Ruta
> graveolens) and sure enough, this was the case. In some sort of
> photosensitization it reacted with the skin exposed to sunlight after I'd
> touched the plant.
> Can someone tell me what causes photosensitization from plants? There are
> also several weeds that can cause the problem in livestock if they are
> ingested. How does this occur?
> Answers?
> --
> ====================================================
>    Martin Witchard

Some chemicals are able to get electronically excited (high energy) by 
the absorbtion of a quantum of light, and they transmit this energy to 
other molecules which became more reactive. In this way it is possible 
to generate, for instance, singlet oxygen, which is a form of oxygen 
much more reactive than the normal triplet oxigen.
Something of this kind must be happening. I think coumarins use to be 
the culprits in some plants.

Gabriel Tojo
University of Santiago de Compostela

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