Table 1. Spectroscopic and coordination properties of MGH, HaloTag-MGH and magnesium green ^a .

	λ abs [nm]	λ em [nm]	ε [cm–1 M–1]	Φ free b (Φbound)	K d (Mg2+) [mM]	K d (Ca2+) [μM]
MGH	515	538	77 000	0.19 (0.56)	1.3	12
HaloTag-MGH	517	540	78 000	0.21 (0.61)	0.67	7.5
Magnesium green	509	534	77 000	0.20 (0.56)	0.88	12

^aMeasured at 37 °C in 100 mM HEPES buffer, 115 mM KCl, 20 mM NaCl, pH 7.4.

^bRelative fluorescence quantum yield determined using fluorescein (Φ = 0.85 in 0.1 M NaOH aq.) as a standard. Φ_free and Φ_bound denote the relative fluorescence quantum yield in the absence and presence of 100 mM Mg²⁺, respectively.