[poobear]

Hi!

We did an experiment where we followed a tryptophan residue in a protein with fluorescence (excision 280 nm, emission scan 300-440 nm). There was a drop in fluorescence, but if we paused the light source for a moment the fluorescence went back to normal.

I can understand irreversible photobleaching, too much energy from the light can simply destroy bonds in the fluorophore. But what causes are there for reversible photobleaching? 
I tried a pubmed search but didn't find anything good..

Thanks!

[Yggdrasil]

Here's a nice paper which explores the mechanism by which a carbocyanine dye can reversibly transition between a fluorescent state and a non-fluorescent dark state.  I'm not sure how relevant it is to tryptophan fluorescence, but maybe it can give you some ideas:

Dempsey et al. 2009 Photoswitching Mechanism of Cyanine Dyes.  J. Am. Chem. Soc. 131: 18192–18193.  doi:10.1021/ja904588g