Fractal behavior of functionalized fullerene aggregates. I. Aggregation of two-handed tetraaminofullerene with DNA

In tris-buffered saline (TBS) with a trace of dimethylformamide (DMF), the homoaggregation process of a functionalized fullerene, the two-handed tetraaminofullerene (TH), and the heteroaggregation process (complex formation) of TH with DNA (pGL3-control plasmid) were studied dynamically by using a combination of static and dynamic laser light scattering measurements. Fractal behavior was investigated in the aggregation process of both TH homoaggregates and TH-DNA heteroaggregates. The stability of aggregates in solution depends on the molar concentration ratio R _M, defined as the molar ratio of moles of TH to moles of the DNA base pair. Higher R _M values resulted in lower aggregate stability. The transition of the fractal dimension (D _f) in TH homoaggregation by rapidly mixing 3.78 μM TH with an equal volume of the blank buffer was found to vary from a value of 1.46 to 2.02. Dynamic light scattering results revealed that, in the aggregation process, the change in the size distribution of aggregates with time could be related to a D _f transition. In the D _f transition region, the size distribution of homoaggregates displayed a drastic change from a single-mode distribution to a bimodal distribution, which clearly suggested a restructuring process with the formation of large aggregates. When the aggregation process finally reached equilibrium, D _f = 2.02, the size of the homoaggregates had a single mode but a broad distribution. However, TH-DNA heteroaggregation showed a D _f transition from 1.58 to 1.7, but over a shorter time range of less than 5 min. Then, the D _f value fluctuated in the range of 1.7 and finally reached an equilibrium value of D _f ≈ 1.78, which was independent of molar concentration. There are two main action forces involved in the heteroaggregation process: van der Waals forces and attractive electrostatic forces, with the latter one being stronger and faster than that of the former. Therefore, a two-step action could occur in the heteroaggregation process. In the beginning of mixing, the attractive electrostatic forces dictated the aggregation process, and then van der Waals forces also got involved in the entire aggregation process. By using an initial concentration of 3.78 μM each and R _M = 1, TH-DNA heteroaggregates showed more stable solution behavior than the homoaggregates. The lower D _f value of the heteroaggregates could be related to a looser compact structure. Results from scanning electron microscopy (SEM) also disclosed the different textures between TH homoaggregates and TH-DNA heteroaggregates; the former had a more dense packing than the latter one.