SY-21 Azide (QSY®-21 equivalent) is a nonfluorescent acceptor dye often used for preparation of peptide and oligonucleotide FRET probes. A broad and intense quenching range from 580 nm to 680 nm makes it useful as an acceptor in fluorescence resonance energy transfer (FRET) applications in conjunction with fluorescent dyes at 580 nm to 680 nm, such as Fluorescein, Alexa Fluor® 568, 594, 633, 647, TAMRA, ROX, Texas Red and Cy5. SY-21 Azide can be reacted with terminal alkynes via a copper-catalyzed click reaction (CuAAC). It also reacts with strained cyclooctyne via a copper-free "click chemistry" reaction to form a stable triazole and does not require Cu-catalyst or elevated temperatures. QSY®-21 is registered trademarks of ThermoFisher Scientific.
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