Figure 4.

Fluorescence excitation spectra of fura-2 (free acid) in solutions containing 0–39.8 μM free Pb2+ in the absence and presence of Ca2+. Excitation spectrum was scanned between 300–400 nm, and emitted fluorescence was detected at 505 nm. When bound to Ca2+, Fura 2 the fluorescence spectrum shifts with an increase in the maximum and minimum fluorescence. These occur at ~ 340 and ~380 nm respectively. Note that the emitted fluorescence does not change upon altering free Ca for excitation at 360 nm. This “Ca-insensitive” wavelength is known as the “isobestic point”.