CONFORMATION OF SINGLE-STRANDED POLY(DA) AND ITS INTERACTION WITH 4',6-DIAMIDINO-2-PHENYLINDOLE

We studied the interaction of 4',6-diamidino-2-phenylindole (DAPI) wit h single stranded poly(dA) using optical spectroscopic methods, includ ing absorption, circular dichroism (CD), and fluorescence spectroscopy . The temperature-dependent conformation of poly(dA) was also investig ated. The conformation of poly(dA) varied with temperature, which is e xplained by the stacking-destacking process of the adenine bases, resu lting from the sugar conformation. The hypochromicity and red-shift in the absorption spectroscopy, the lack of CD change in the drug absorp tion region, and the fluorescence behavior, especially a great accessi bility of the I, quencher to the poly(dA)-bound DAPI, suggest that DAP I binds to the outside of poly(dA). The Job plot for the DAPI-poly(dA) mixture demonstrated that a stoichiometry of one DAPI molecule binds to the one phosphate of poly(dA).