A rhodamine based turn-on chemosensor for Fe3+ in aqueous medium and interactions of its Fe3+ complex with HSA

A novel hexa-coordinating rhodamine-based chemosensor, HL6, with N4O2 donor atoms, selectively and rapidly recognizes Fe3+ in the presence of all biologically relevant as well as toxic metal ions, numerous anions (except for F- and I-, which quench FI) and amino acids. The lower detection limit (110 nM) along with cytoplasmic cell imaging applications with negligible cytotoxicity provides a good opportunity towards in vitro/in vivo cell imaging of Fe3+ ions. Scanning electron microscopy (SEM) studies reveal a spherical microstructure for the free ligand, HL6, which melts upon increasing the applied voltage (>= 2 kV); however, this is reduced in size in the presence of SDS, morphology remaining the same. [L-6-Fe](2+) gives hexagonal rod-like microstructures due to pi center dot center dot center dot pi stacking extended in a one dimensional fashion, which, in the presence of SDS, turn into cubic microstructures. However, in the presence of HSA both HL6 and [L-6-Fe](2+) agglomerated severely. A plot of FI (or r) vs. [SDS] at constant [L-6-Fe2+] (20 mu M) shows a rapid decrease in FI (or r); FI or r reaches a minimum at similar to 2 mM SDS and then increases and becomes saturated at >= 8 mM SDS. The initial rapid decrease in FI or r with the increase in [SDS] arises due to the breakdown of the self-assembled microstructures of [L-6-Fe](2+) formed probably through pi center dot center dot center dot pi stacking in pure aqueous solution in the presence of SDS (<= 2 mM). The increase in FI or r with [SDS] beyond 2 mM may arise due to trapping of the [L-6-Fe](2+) moiety inside the micellar cavity thereby giving it a thermodynamically rigid structure. Again, fluorescence anisotropy measured as a function of HSA concentration (0-70 mu M) at a fixed concentration of HL6 and Fe3+ (20 mu M each) at 555 nm showed a gradual increase in r with the increase in [HSA]. Site selective binding and molecular modeling studies revealed that [L-6-Fe](2+) predominantly binds in the subdomain IIA of HSA by both hydrophobic and electrostatic forces. A circular dichroism (CD) study clearly points towards the reduction in alpha-helical organization of the protein and subsequent increase in the coiled structure upon binding with [L-6-Fe](2+) indicating a small but definitive partial unfolding of the protein.