Table 1 Photophysical characterization of FR-GECO1a, FR-GECO1c, and mKelly2. Source data are provided in the Source Data file

Excitation maxima λ_ex (nm)

596

596

598

Emission maxima λ_em (nm)

642

646

649

Ca²⁺-dependent ΔF/F₀

5.8 ± 0.2

18.1 ± 0.4

n/a

K_d (nM)

29

83

n/a

Apparent Hill coefficient (n_H)

1.49

1.98

n/a

Apparent pK_a (Ca²⁺ free)

8.92

9.47

5.6

Apparent pK_a (Ca²⁺ bound)

7.96

7.88

n/a

k_on (M⁻¹ s⁻¹)

9.98 × 10⁵

5.31 × 10⁶

n/a

Tau_on (μs)

1.00

0.188

n/a

k_off (s⁻¹)

3.71

6.39

n/a

Tau_off (ms)

269

156

n/a

Extinction Coefficient ε (M⁻¹ cm⁻¹) (Ca²⁺ free)

5710

4850

43,000

Extinction Coefficient ε (M⁻¹ cm⁻¹) (Ca²⁺ bound)

27,000

26,500

n/a

Quantum Yield Φ (Ca²⁺ free)

0.19

0.077

0.18

Quantum Yield Φ (Ca²⁺ bound)

0.34

0.35

n/a

Brightness ε×Φ (×1000) (Ca²⁺ free)

1.1

0.34

7.7

Brightness ε×Φ (×1000) (Ca²⁺ bound)

9.2

9.4

n/a

2P Brightness F₂ (GM), λ_max in parentheses (Ca²⁺ free)

1.6 (1112 nm)

0.48 (1108 nm)

n/d

2P Brightness F₂ (GM), λ_max in parentheses (Ca²⁺ bound)

9.2 (1112 nm)

7.4 (1112 nm)

n/d

ΔF/F₀ (2P), λ_max in parentheses

5.9 (1112 nm)

15.4 (1112 nm)

n/a