4',6-diamidino-2-phenylindole (DAPI) interacts with rare structures of GC polymers

The binding of 4',6-diamidino-2-phenylindole (DAPI) to double-stranded GC polymers either in the alternating or in homopolymer sequence was investigated using fluorescence techniques. We employed fluctuation correlation spectroscopy, which measures the diffusion coefficient of fluorescent particles, to demonstrate that the fluorescence was originating from relatively slowly diffusing entities. These entities display a very large heterogeneity of diffusing coefficients, indicating that molecular aggregation is extensive in our samples. We used frequency domain fluorometry to characterize the fluorescence lifetime of the species, while varying the GC polymer-dye coverage systematically. At very low coverage we observed a relatively bright fluorescent component with a lifetime value of approximately 4 ns. The stoichiometry of binding of this bright species was such that it can only arise from rare molecular structures, either unusual loops or large molecular aggregates. The amount and characteristics of this bright fluorescent component were different between the homo and the alternating polymer, indicating that the difference in sequence of the two polymers is responsible for the different aggregates which are then detected in the fluorescence experiment. At large GC polymer coverage we observed a relatively wide distribution of fluorescent species with short lifetime values, in the range between 0.12 and 0.2 ns. Given the stoichiometry of binding of this fluorescent component, we concluded that it could arise either from intercalative and/or non-specific binding to the DNA double-stranded molecules. We comment on the origin of the rare but brightly fluorescent binding sites and discuss the potential to detect such unusual DNA structures.