NBD derived diphenyl(aminomethyl)phosphane – A new fluorescent dye for imaging of low pH regions and lipid membranes in living cells

This work presents the comparative characterization (X-ray crystallography, NMR, solvatochromism and pH dependence of electronic absorption, steady-state and time resolved fluorescence properties) of a novel NBD based phosphane: 7-nitro-4-(4-(diphenylphosphinomethyl-piperazin-1-yl)-2,1,3-benzoxadiazole (1) and an analogous compound: 7-nitro-4-(4-methyl-piperazin-1-yl)-2,1,3-benzoxadiazole (2). Protonated forms of the compounds are highly luminescent, while the neutral ones are not. The pKa values in water solutions are in the range 6.8–7.0 for 1 and 6.4–6.6 for 2. However, we showed that these values do not reflect the luminescence properties in more complex environments, where other equilibria may occur. Performance of 1 and 2 as potential molecular probes for biological imaging of living cells was evaluated towards mammalian (HEK 293T) and fungal cells (two Candida albicans strains: CAF2-1 - parental strain, and DSY1050 - mutant with no functional CDR1, CDR2 or MDR1 genes). For the HEK 293T cells, both compounds showed an intracellular punctuated bright staining typical of acidic compartments (late endosomes/lysosomes), as confirmed by co-localization with LysoView 633. In C. albicans the fungal cells, 1 labeled both organelle and plasma membranes, with similar behavior in both strains. For 2 the results indicated a different intracellular localization and an export of 2 by the cells through MDR transporters. In conclusion, our studies show that 1 and 2 can be the efficient cellular stains. Moreover, we expect that 1 can be also successfully used as a transition metal complexes fluorescent label, being a substitute for PPh3 or related phosphane ligands.