Laurdan Generalized Polarization (GP)

The emission of the amphipathic probe 6-dodecanoyl-2-dimethylaminonaphthalene (laurdan) is sensitive to the polarity and physical state of the membrane and has been used to monitor membrane fluidity and lipid phase transitions [54]. Fluorescence measurements were performed on a Cary Eclipse spectrofluorometer (Agilent Technologies, USA) by excitation at 340 nm and dual emission readings at 440 nm and 490 nm with 5 nm bandpass each for an average of 4 measurements. GP measurements were conducted at 35°C ±0.1°C using a circulating water bath controller. GP was calculated according to the following equation [54]:

GP values range between +0.6 to -0.4 depending on a lipid phase state. Under equivalent experimental procedure any changes in laurdan GP upon the addition of metals will correspond to a change in membrane polarity, water accessibility and lipid packing. The change in GP (ΔGP) is calculated as the difference between metal-free controls and fluidity changes in samples containing metals. A decrease (negative values) in ΔGP is an indication of a fluidization effect while an increase (positive values) is an indication of a rigidification effect. The laurdan measurements are highly reproducible within ±0.005, well below the GP increases reported in this manuscript (Table S1).

Control experiments with the water soluble analogue Prodan or Laurdan were performed to exclude spectral changes (such as quenching) upon metal addition and no interferences were observed.