John,
	From the non physicist's point of view the answer could go 
something like this. If you have a power surge it can fry your 
computer. But if your computer is not plugged into the mains then it 
would take a very big power surge indeed to do the damage. A molecule 
in the ground state can of course be damaged by free radical attack 
but no more or less than other molecules. But once a molecule has 
absorbed a photon then it is not in the ground state any more. To 
continue my hoaky analogy, a chromophore in light is like your 
computer plugged in to the mains.

	Hope this helps. The physicists (and musicians) can go for 
the triplets.

	Tobias


>Hi Everyone,  this question follows on from a helpful discussion 
>that we had about photobleaching back in November.  I have recently 
>tried to explain to a group of colleagues about the mechanism of 
>photobleaching.  The answer is based on the transition of molecules 
>from the excited singlet state (S1) to the triplet state (T1) which 
>is long-lived and therefore more susceptible to bleaching by free 
>radicals (my entire discussion of this is below).
>
>My question that arises from my attempted answer is: why are excited 
>molecules more susceptible to oxidative attack than ground state 
>molecules.  I hope I'm not completely mucking up the mechanism here. 
>Would the physicists out there please help.
>
>Thanks, John.
>
>The original answer: When excited, fluorophores generally transition 
>from singlet ground state (S0) to singlet excited state (S1). 
>Relaxation from S1 to S0 results in emission of heat and light 
>(fluorescence). Lifetime in S1 is in the nano to pico second range 
>and allows very little time for the excited molecule to interact 
>with free radicals. Periodically, however, an excited molecule will 
>do a transition from S1 to the triplet excited state (T1 - the 
>physics of this is a bit difficult to understand). T1 is a very 
>long-lived state - molecules can remain in T1 for up to the 
>microsecond range - i.e. a thousand to a million times longer than 
>for normal S1 state. It is during this long T1 state that molecules 
>are attacked by free radicals and destroyed.
>
>--
>Runions signature
>
>(Sent from my cra%#y non-Blackberry electronic device that still has wires)
>
>
>
>*********************************
>John Runions, Ph.D.
>School of Life Sciences
>Oxford Brookes University
>Oxford, UK
>OX3 0BP
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