Labeling profile of probe 14, 16 and 23. (A) HEK293T lysates (10 μg total protein) were incubated with the indicated concentrations of 23, 24 (left panel), 14 (middle panel) or 16 (right panel) for 1 h at 37 °C. (B) HEK293T cells (some 5 × 105 cells) were exposed to the indicated concentrations of 23 or 24 (left panel), 14 or 25 (middle panel) or 16 (right panel) for 2 h at 37 °C. (C) Quantification of fluorescent labeling of living cells treated with 23 (left panel), 14 (middle panel) and 16 (right panel). Corresponding gels are shown in Fig. 3B. (D) Structures of reference probes 24 and 25.