cis-ruthenium(II) complexes present differential fluorescent staining in walled-cell models (yeasts)

What is it about?

Thus, we wondered whether Ru(II) complexes, substituted with different dinitrogenated ligands (i.e. 2,2′-bipyridine or 1,10-phenanthroline), are suitable as biomarkers in walled cell such as bacteria (Gram-negative bacteria: Salmonella enterica; Gram-positive bacteria: Staphylococcus aureus and Bacillus cereus) and yeasts (Candida albicans and Cryptococcus spp.).

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Why is it important?

cis-Ru(deeb)3 2+ (R1; where deeb is 4,4′-diethanoate-2,2′-bipyridine) and cis-Ru(phen)3 2+ (R2; where phen is 1,10-phenanthroline) were synthesized. Although the presence of the cell wall (a structure that is present in yeasts and bacteria,) was previously described as a natural barrier that hampers the uptake of d6-based luminescent complexes, we previously demonstrated that rhenium(I) tricarbonyl complexes were useful to stain both yeasts and bacteria. Even though several studies of classical ruthenium(II) complexes can be found, none of those studies aimed to determine the potential of these compounds as biomarkers for walled cells, testing only cell lines that lack this permeability barrier. Walled cells exhibit a relatively rigid structure, mainly constituted by carbohydrates and proteins, and surround the plasma membrane. In this manuscript, we observed that both R1 and R2 exhibited very low cytotoxicity in different walled-cell models (including bacteria and yeasts). More importantly, we found that both R1 and R2 were able to fluorescently stain Candida albicans (yeast), with a simple and fast procedure, without the need of additional permeabilizer molecules and antibodies. Interestingly, R1 remained retained in a discrete central structure consistent with the cell nucleus, whereas R2 seemed to be accumulated in the cell wall. These results show that these two complexes can be used as biomarkers for walled cells as differential staining, supporting the fact that, as well as with rhenium(I) complexes, biomarkers properties can be modulated by changing the substituents in ruthenium(II)-derivative luminescent stains, even for walled cells.

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