Carbon monoxide (CO) is an endogenous signaling molecule
with demonstrated
pharmacological effects. For studying CO biology, there is a need
for sensitive and selective fluorescent probes for CO as research
tools. In developing such probes, CO gas and/or commercially available
metal-carbonyl-based “CO-releasing molecules” (CORMs)
have been used as CO sources. However, new findings are steadily emerging
that some of these commonly used CORMs do not release CO reliably
in buffers commonly used for studying such CO probes and have very
pronounced chemical reactivities of their own, which could lead to
the erroneous identification of “CO probes” that merely
detect the CORM used, not CO. This is especially true when the CO-sensing
mechanism relies on chemistry that is not firmly established otherwise.
Cu2+can quench the fluorescence of an imine-based fluorophore,
DPHP, presumably through complexation. The Cu2+-quenched
fluorescence was restored through the addition of CORM-3, a Ru-based
CORM. This approach was reported as a new “strategy for detecting
carbon monoxide” with the proposed mechanism being dependent
on CO reduction of Cu2+to Cu1+under near-physiological
conditions (Anal. Chem.2022,94,11298−1130635926081). The study only used CORM-3 as the source
of CO. CORM-3 has been reported to have very pronounced redox reactivity
and is known not to release CO in an aqueous solution unless in the
presence of a strong nucleophile. To assess whether the fluorescent
response of the DPHP-Cu(II) cocktail to CORM-3 was truly through detecting
CO, we report experiments using both pure CO and CORM-3. We confirm
the reported DPHP-Cu(II) response to CORM-3 but not pure CO gas. Further,
we did not observe the stated selectivity of DPHP for CO over sulfide
species. Along this line, we also found that a reducing agent such
as ascorbate was able to induce the same fluorescent turn-on as CORM-3
did. As such, the DPHP-Cu(II) system is not a CO probe and cannot
be used to study CO biology. Corollary to this finding, it is critical
that future work in developing CO probes uses more than a chemically
reactive “CO donor” as the CO source. Especially important
will be to confirm the ability of the “CO probe” to
detect CO using pure CO gas or another source of CO.