Invitrogen - Molecular Probes

<1>	Spectra of hydroxycoumarins are pH-dependent. Below the pK_a, Abs shifts to shorter wavelengths (325–340 nm) and fluorescence intensity decreases. Approximate pK_a values are 7.8 (H189, C2111), 7.5 (H185), 7.3 (T659) and 4.9 (D6566, C12881).
<2>	H185 Abs = 339 nm (EC = 19,000 cm^-1M^-1), Em = 448 nm at pH 4.