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Hi Josef,

It's much more likely that you are denaturing the fluorescent protein.  Simple synthetic organic fluors like Alexa Fluors or Cy dyes, are often reconsistituted entirely in DMSO in their reactive form, for that purpose to protect them from oxygen.  DMSO does not hurt the actual fluorescent molecule itself.  I would absolutely imagine it can hurt protein, though.  With fluorescent proteins, the chromophore at the center of the beta barrel of a GFP requires the protein component to remain rigid.  Should the protein component relax or denature, the chromophore at the center of the molecule is no longer rigid itself and cannot resonate the energy it absorbs efficiently (quantum efficiency).  The same effect is seen with methanol or other denaturing solvents and fluorescent proteins.  The effect is not photobleaching, just a loss of quantum efficiency.  

Your cells also can't be so happy with the presence of the DMSO.  What most people would do in this instance is ask themselves 1. Can the drug be dissolved in something besides DMSO and 2. If not, can you concentrate the drug more so as to deliver the least amount of DMSO as is possible.

Good luck,
Kelly

-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]] On Behalf Of Josef Gotzmann
Sent: Thursday, March 14, 2013 9:42 AM
To: [log in to unmask]
Subject: Is DMSO photobleaching autofluorescent proteins?

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Dear List,

I have a technical questions: 
When we live image cells (in buffered saline, pH7.4) expressing GFP- and
mCherry-labeled proteins, respectively, on our confocal and we add a drug
(dissolved in DMSO) we rapidly (within seconds) lose fluorescence of both
dyes. After a very sharp drop down to 50% of intensity within the first 2
minutes, we reach a plateau (2-5minutes), followed by another less sharp,
though steady, decrease in fluorescence for another 2 minutes.
This is the effect we see with an end concentration of 1% DMSO, which is
completely absent when are scaling down the DMSO concentration to 0,1% !
We have already ruled out that it's simple bleaching due to lightning
conditions.
My understanding was that the OH-radical scavenging properties of DMSO
should more stabilize fluorescence!?
Anyone an idea how DMSO can act like that?

thx
Josef