A platinium(II)-based molecular light switch for proteins

The platinum(II) complex [Pt(bzimpy)Cl](+) (1) [bzimpy = 2,6-bis(benzimidazo-2-yl)pyridine] has been synthesized and characterized by FAB mass spectrometry and UV/Vis, NMR, and emission spectroscopy. Complex I emits weakly from its (MLCT)-M-3 state in aqueous solution but strongly in nonaqueous media. Complex 1 exhibits a molecular light switch effect with a drastic enhancement in its emission intensity and a 72-nm blue shift in the emission maxima in the presence of bovine serum albumin (BSA). Denaturation of BSA in the presence of 6 m urea leads to a decrease in the emission intensity of the complex, which suggests its binding to the hydrophobic pockets of the protein. In the presence of hemoglobin, however, quenching of the emission of the complex is observed due to its binding to Cys104 in subunit C and the resultant fluorescence resonance energy transfer from the platinum(II) complex to the heme. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).