Unravelling the correlation between metal induced aggregation and cellular uptake/subcellular localization of Znsalen: an overlooked rule for design of luminescent metal probes

Unravelling the unique effects of metal coordination on biological behaviours is of importance to design metal based therapeutic and diagnostic agents. In this work, we chose luminescent Znsalen (ZnL1) as a case study to demonstrate that metal induced aggregation arising from the intermolecular Zn center dot center dot center dot O interaction influences its cellular uptake and subcellular localization. Comparative studies with the free bases (L-1 and L-2) show that ZnL1 undergoes cellular uptake through caveolae-mediated endocytosis and internalizes in endosomal/lysosomal compartments, in contrast to the localization of L-1 and L-2 in the mitochondria. Further studies of photophysical properties, TEM imaging and DLS analysis suggest that ZnL1 tends to form large sized fibrous structures in aqueous media. To investigate the relationship between ZnL1 aggregation and the biological behaviour, we used pyridine to tune the aggregation-to-deaggregation transition and found that, in the presence of pyridine, ZnL1 could localize in the mitochondria and internalize into cells through the passive diffusion pathway. Such distinctive biological behaviours resulting from the different Znsalen species clearly point out the importance of metal induced aggregation or metal speciation analysis in designing metal complexes as biological probes.