Is there a protein that bonds with NaCl with a resulting structural change that can be identified with a marker? Can you establish if a protein has bonded to NaCl by the addition of a marker(?), that results in a change in colour?
The closest idea I can think of is to use an enzyme for which there is a profound change in catalytic activity depending on NaCl concentration. For example, I worked with the purified alpha subunit of DNA polymerase III (DnaE) from Haemophilus influenzae bacteria. It's activity was severely inhibited by NaCl in the mM range. One could use the readily measurable activity of this enzyme as an assay for NaCl, but it would not be selective because the effect was of ionic strength, not NaCl specifically.
John,
I have 3 suggestions for you, first you may want to look at the protein tryptophan fluorescence itself. Remember that protein tryptophan fluorescence is exquisitely sensitive to its environment. 2nd you may consider using ANS as a probe, ANS binds to hydrophobic patches of a protein. Any exposed hydrophobic patches from NaCl binding would be revealed and result in a large fluorescence increase. These 2 suggestions are not specific to NaCl binding so having appropriate controls will be of the utmost importance.
Finally, you may consider using surface plasmon resonance, this will give you specific binding of NaCl to a protein.
Some mutant forms of YFP have been engineered as fairly sensitive halide sensors, with one mutant (H148Q/I152L) being best for I- but usable for Cl- sensing in CFTR screens. FRET based halide sensing using these proteins has also been done.
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