A number of bioactive marine natural products have been isolated so far, but it is still difficult to disclose their modes of action. In this study, we prepared fluorescently labeled chemical probes from the cytotoxic marine cyclic peptides kapakahines A (1) and F (2) to visualize their localization as the first step of the study of their modes of action. We used fluorescent dyes 3a or 3a/b (a 1:1 mixture of 3a and 3b) whose terminal N-hydroxysuccinimide (NHS) group can react with the free amino groups of kapakahines. The fluorescently labeled kapakahine A (Kap A-5-FL, 5a) stained P388 murine leukemia cells and HeLa human cervical cancer cells, while cells treated with fluorescently labeled kapakahine F (Kap F-5-FL, 6a) only weakly stained them. Further analysis of the confocal images of the stained cells with higher magnification (×100) indicated the localization of Kap A-5-FL (5a) in the cells. In this paper, we report the small-scale preparation and a new delivery method of fluorescent probes, as well as the application of these procedures to cell staining.
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