Recently,
the sulfate radical (SO4
•
–) has been found to exhibit broad application
prospects in various research fields such as chemical, biomedical,
and environmental sciences. It has been suggested that SO4
•
– could be transformed into
a more reactive hydroxyl radical (•OH); however,
no direct and unequivocal experimental evidence has been reported
yet. In this study, using an electron spin resonance (ESR) secondary
radical spin-trapping method coupled with the classic spin-trapping
agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and
the typical •OH-scavenging agent dimethyl sulfoxide
(DMSO), we found that •OH can be produced from three
SO4
•
–-generating systems
from weakly acidic (pH = 5.5) to alkaline conditions (optimal at pH
= 13.0), while SO4
•
– is the predominant radical species at pH < 5.5. A comparative
study with three typical •OH-generating systems
strongly supports the above conclusion. This is the first direct and
unequivocal ESR spin-trapping evidence for •OH formation
from SO4
•
– over a wide
pH range, which is of great significance to understand and study the
mechanism of many SO4
•
–-related reactions and processes. This study also provides an effective
and direct method for unequivocally distinguishing •OH from SO4
•
–.
We have found recently that nuclear uptake of the cell-impermeable DNA light-switching Ru(II)-polypyridyl cationic complexes such as [Ru(bpy)2(dppz)]Cl2 was remarkably enhanced by pentachlorophenol (PCP), by forming ion-pairing complexes via a passive diffusion mechanism. However, it is not clear whether the enhanced nuclear uptake of [Ru(bpy)2(dppz)]2+ is only limited to PCP, or it is a general phenomenon for other highly chlorinated phenols (HCPs); and if so, what are the major physicochemical factors in determining nuclear uptake? Here, we found that the nuclear uptake of [Ru(bpy)2(dppz)]2+ can also be facilitated by other two groups of HCPs including three tetrachlorophenol (TeCP) and six trichlorophenol (TCP) isomers. Interestingly and unexpectedly, 2,3,4,5-TeCP was found to be the most effective one for nuclear delivery of [Ru(bpy)2(dppz)]2+, which is even better than the most-highly chlorinated PCP, and much better than its two other TeCP isomers. Further studies showed that the nuclear uptake of [Ru(bpy)2(dppz)]2+ was positively correlated with the binding stability, but to our surprise, inversely correlated with the lipophilicity of the ion-pairing complexes formed between [Ru(bpy)2(dppz)]Cl2 and HCPs. These findings should provide new perspectives for future investigations on using ion-pairing as an effective method for delivering other bio-active metal complexes into their intended cellular targets.
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