FRET quench by forming Tra-Alexa680-biotin and Av-QSY21 (A), nAv-QSY21 (B) or sAv-QSY21 (C) complexes in vitro. a; Tra-Alexa680-biotin + PBS, b; Tra-Alexa680-biotin + QSY21 conjugated avidin or derivatives, c; Tra-Alexa680 + PBS, d; Tra-Alexa680 + QSY21 conjugated avidin or derivatives. The fluorescence was quenched by forming Tra-Alexa680-biotin and Av-QSY21, nAv-QSY21 or sAv-QSY21 complexes. 150×166mm (300 × 300 DPI)