Fig 4. A four state model requires the quantitative characterization of each of the four distinct Ca²⁺-dependent states of GCaMP6m.

This model illustrates a simple four state equilibrium that includes all possible combinations of the Ca²⁺-saturated and Ca²⁺-free states of the sensor protein, as well as anionic and neutral forms of the chromophore. Indices: Anionic form (A); Neutral (N); Ca²⁺-free (0); Ca²⁺-saturated (+); Quantum yield (φ); extinction coefficient (ε); Gibbs free energy of the protein (G); dissociation constant for calmodulin in GCaMP6m with anionic chromophore (K_d^(A)); dissociation for calmodulin in GCaMP6m with neutral chromophore (K_d^(N)); Acid dissociation constant for chromophore in Ca²⁺-saturated state of GCaMP6m (K_a⁽⁺⁾); Acid dissociation constant for chromophore in Ca²⁺-free state of GCaMP6m (K_a⁽⁰⁾).