Interação de nanotubos de carbono com segmentos de DNA

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Single- and double-stranded DNA (deoxyribonucleic acid) molecules can strongly bind to single-walled carbon nanotubes (SWNT) via non-covalent interactions. Under certain conditions, the DNA molecule spontaneously self-assembles into a helical wrapping around the tubular structure of the carbon nanotubes to form DNA/SWNT hybrids, which are both stable and soluble in water. This system has recently received extensive attention, since, besides rendering SWNTs dispersable in water as individual tubes, the DNA hybrids are very promising candidates for many applications in nanotechnology and molecular biology. All the possible applications for DNA-SWNT hybrids require, however, a fully understanding of DNA-nanotube wrapping mechanism which is still lacking in the literature. In this context, the aim of this work was to investigate the non-covalent interaction in aqueous medium between SWNTs and synthetic DNA segments having a known nucleotide sequence. Initially, the study was focused on poly d(GT)n sequences (n = 10, 30 and 45) that contain a sequence of alternating guanine and thymine bases and for which the efficiency to disperse and separate carbon nanotubes has already been demonstrated. Besides the size of GT sequences, the effects of ionic strength and pH in the interaction were also investigated. Afterwards, we studied the interaction of SWNT with DNA molecules that contain only a single type of nitrogenous base (DNA homopolymers), which has not been reported in details in the literature. We investigated homopolymers of poly dA20, poly dT20, poly dC20 and the duplex poly dA20:dT20. Most of the study was carried out with small-diameter HiPcoÒ SWNTs (with diameters between 0.7 and 1.2 nm). In some studies, SWNTs with diameter around 1.4 nm, synthesized via laser ablation and arc-discharge methods, were also investigated. The arc-discharge nanotubes used in this study were functionalized with carboxylic groups (-COOH) due to their purification using strong acid solutions. The efficiency of DNA segments to disperse SWNT was further investigated through the comparison with known dispersant agents such as sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (NaDDBS) and sodium cholate (NaC). Concerning to HiPco/d(GT)n system, it was demonstrated, for the first time, that the ionic strength plays an important role in the interaction, there being a minimum value below which the DNA wrapping does not take place. Above a certain NaCl concentration, the aggregation of biomolecules is promoted, decreasing the number of suspended nanotubes in the solution. Through optical absorption measurements, the optimal NaCl concentration at physiological pH (pH=7.4) was determined for each one of the sequences studied. By circular dichroim (CD) measurements, it was demonstrated, for the first time, that SWNTs induce a clear conformational change of d(GT)n oligomers. These sequences undergo a transition from the right-handed configuration (dextrorotatory), when they are free, to the left-handed one (laevorotatory), after the interaction with the tubes. We further demonstrate that the conformation assumed by the d(GT)n sequences is dependent on the chemical groups attached to the SWNT surfaces and the ionic strength. FT-IR, UV-Vis absorption and CD results suggest that carboxilic groups drive the poly d(GT)10 strand to assembly onto the carbon nanotube side-walls in a completely different way than do nanotubes without functionalization. For the SWNTCOOH/ d(GT)20 system, the change from right- to left-handed conformation was only observed after the addition of HgCl2. These results alert for the necessity to check the chemical properties of carbon nanotubes submitted to strong purification procedures. For homopolymeric single-stranded DNA molecules, the CD results also indicate pronounced changes on the helical organization upon interaction with SWNTs. Agarose gel electrophoresis study showed the presence of double-stranded DNA only in poly dA20:dT20 system. Through optical absorption measurements, it was also demonstrated that poly dA20:dT20 has the smallest dispersion efficiency, which provides strong evidence that DNA/SWNT interaction is more favorable for single-stranded DNAs. By using resonant Raman scattering and optical absorption in the near-red region, it was demonstrated that the interaction with DNA oligonucleotides causes shifts (DE) in the first set of optical transition energies (E11) for metallic and semiconducting SWNTs greater than those observed for the other dispersant agents. The DE values depend on the tube diameter, increasing with increasing SWNT diameter. However, any evidence for charge transfer between bases and the nanotube surface, as recently suggested in a theoretical study, could be observed.

ASSUNTO(S)

dna nanotubos nanotubes carbon carbono nanostructures quimica do estado condensado nanoestruturas dna

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