Oregon Green™ 488 Maleimide (Invitrogen™)

The thiol reactive Oregon Green™ 488 maleimide can be used to can be used to create green fluorescent bioconjugates with excitation/emission maxima ∼496/524 nm.
This fluorinated analog of fluorescein overcomes some of the key limitations of fluorescein, including greater photostability and a lower pK_a (pK_a ∼ 4.7 versus 6.4 for fluorescein), making its fluorescence essentially pH insensitive in the physiological pH range.