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The difference between thiazole orange and TO-PRO-1 is that thiazole orange
has a methyl group on the nitrogen in its quinoline ring, while the nitrogen
in TO-PRO-1 is connected via a propyl (PRO would stand for propyl rather
than protein) chain to a quaternary trimethylammonium. It is presumably the
extra positive charge of the quaternary ammonium which makes TO-PRO-1 less
facile at crossing membranes than is thiazole orange. In the wide world of
bacteria, there may indeed be species with enzymes which could chew this
off, but garden variety proteases wouldn't do the job. Note that propidium
differs from ethidium, which gets into cells more readily, by having an
extra positive charge, also a quaternary trimethylammonium hooked via a
propyl chain to the nitrogen in the phenanthridine ring. Dick Haugland and
company created YO-PRO-1, TO-PRO-1, TO-PRO-3, etc. by adding the same group
to the permeant dyes oxazole yellow, thiazole orange, thiazole blue, etc.
Although I don't have solid enough experimental evidence to publish, I
strongly suspect that oxonol staining of cells changes with changes in
phospholipid packing (this is known to happen with merocyanine 540) as well
as with membrane potential. Cells which exclude propidium but stain more
brightly than "live" cells with oxonol may have undergone some change in
phospholipid packing; interestingly enough, such changes, detected using
merocyanine 540, accompany apoptosis in eukaryotes.
-Howard
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CD-ROM Vol 3 was produced by Monica M. Shively and other staff at the
Purdue University Cytometry Laboratories and distributed free of charge
as an educational service to the cytometry community.
If you have any comments please direct them to
Dr. J. Paul Robinson, Professor & Director,
PUCL, Purdue University, West Lafayette, IN 47907.
Phone: (765)-494-0757;
FAX(765) 494-0517;
Web