Fig 6. GCaMP6m has a single pKa for the Ca²⁺-saturated state and two pK_a's for the Ca²⁺-free state.

Normalized 470 nm excited fluorescence intensity at 515 nm plotted against pH for Ca²⁺-saturated (solid circles) and Ca²⁺-free (open circles) states. A single-binding site model fit (green trace) was used for the Ca²⁺-saturated state and includes data points for one titration event from pH 4.3 to 8.39 with a pK_a of 7.10 ± 0.01. A two-binding site model fit (red trace) was used for the Ca²⁺-free state and includes data points for two titration events from pH 4.3 to 10.5, the first with a pK_a of 8.01 ± 0.01, and the second with a pK_a of 10.08 ± 0.02.