Sodium Green as a Potential Probe for Intracellular Sodium Imaging Based on Fluorescence Lifetime

We characterized the use of the fluorescent probe Sodium Green for measurements of intracellular free sodium using frequency–domain, phase–modulation fluorometry. The intensity decays were found to be strongly Na⁺dependent, with mean lifetime increasing from 1.13 ns in the absence of Na⁺to 2.39 ns in the presence of 140 mmNa⁺. Detailed analysis of the intensity decays in the presence of Na⁺and K⁺in the concentration range from 0 to 500 mmis provided. Sodium sensing using data measured at a single modulation frequency is described. Phase and modulation data showed high sensitivity to Na⁺and substantially lower sensitivity to K⁺. Additionally, exposure of Sodium Green to intense illumination indicated that Sodium Green is much more photostable than its precursor, fluorescein. These results indicate that lifetime-based measurements with Sodium Green can be used for imaging of intracellular free [Na⁺] in the range from about 0.5 to 50 mmwith high accuracy.     

Viscosity and sedimentation study of sonicated DNA–proflavine complexes

The intrinsic viscosity of sonicated calf thymus DNA (molecular weight 4–5 × 10⁵) increases and the sedimentation constant decreases, with increasing binding of proflavine at 0. 2 ionic strength and at 25°C. The measurements correspond to a linear increase in length of the almost rodlike DNA molecules with the amount of proflavine bound; independent calculations from viscosity and sedimentation measurements yield almost identical results. Over the range of r (moles of proflavine bound per moles of nucleotides) equal to zero to r = 0.13, the length increases by about 20%. This extension is compatible with the intercalation hypothesis proposed by Lerman. Density increments at various values of r, at constant chemical potential of diffusible solutes, were determined. It was also found that, in addition to the known isosbestic point of DNA-proflavine complexes at 455.5 mμ, an additional isosbestic point exists at 225.5 mμ; this proved extremely useful for the evaluation of binding studies.