DNA Cladding with Inorganic Sulphide Nanoparticles

The formation of CdS and CoS2 nanoparticles along the DNA strands have been 
studied. The systems DNA-Cdl2 and DNA-CoCl2 have been investigated in the solid 
state in order to form DNA-MX2 complex salts used as precursors. They react with 
S2-ions giving birth to CdS and CoS2 nanoparticles. A model describing the DNA-nanosulphide 
association is proposed.


INTRODUCTION
In a recent publication Niemeyer [1] proposed to use biological macromolecules such as proteins or nucleic acids for the construction of nanostructured systems. Such a goal have been actually achieved by Mirkin et al., who rationally assembled gold nanoparticles using oligonucleotides as template [2]. Similar results have been obtained by Alivisatos et al. [3]. During  this case also, the purpose was to dictate the shape of nanoparticle assemblies by DNA [4].
Another potential application of DNA-nanocomposites could be the non-viral delivery of genes ( Fig. 1). Indeed, it is well known that solid particles can penetrate into the cell when their size is adequately small [5]. It is the case for DNA-liposome complexes [6] or DNA immobilised on calcium phosphate [7].
In order to prospect such a potentiality, in a first step, we have developed a method for assembling nanoparticle to DNA. Although the use of toxic Sas a drug is unrealistic, we have chosen, for this initial exploration, cadmium and cobalt sulphides because of the ease of synthesis of isolated quantum-confined nanoparticles.

RESULTS
DNA, from herring testes, was purchased from ALDRICH. It was solubilized in water (3,33g/1). Cdl2 (ALDRICH, 99%) or COC12, === DNA nanoparticles 1 FIGURE Schematic transfection process using a gene cladded by inorganic nanoparticles, in vitro. injection in the culture medium; 2, 3, 4 phagocytosis of the bio-hybrids; 5 opening of the phagosome; 6 separation of DNA from the inorganic vector (?); 7 internalisation of DNA in the nucleus. 6H20 (ALDRICH, 98%) were also dissolved in water (Ccd 10g/l; Cco= 5 g/l). Various amount of the previous solutions were added to the DNA solution.
The precipitation of the complex salts DNA-Cdl2 or DNA-CoC12 is observed for an atomic ratio M/P (M Cd, Co) close to 0.5. The solution is evaporated, then dried under vacuum. Colourless transparent films are obtained in the cadmium case or blue for the cobalt. The solubility limit has been determined by XRD that shows the formation of the salt that occurs for an atomic ratio M/P of about 1.25 (Fig. 2).
The sulphide nanoparticules have been obtained by soaking the previous DNA-MCI2 (M/P 1) films in a normal solution of ammonium sulphide during 5 seconds. The gels were immediately washed in water until disappearance of ammonium iodide or chloride. Then, they were dried under vacuum. The cadmium and cobalt based films are respectively pale yellow and black.
In this case of cadmium, the compound formed is identified as CdS (wurtzite type). From the width of the XRD lines and using the Scherrer formula, the size of the diffracting smallest particles have been estimated to be 3 nm. The micrographs obtained by TEM confirm this analysis. Indeed, the size of the particles ranges from 2 to 20 nm.
Conversely, in the cobalt case, the DNA-CoSx composite is amorphous, We have prepared cobalt sulphide using the same experimental conditions but without DNA. The obtained CoS2 compound is crystalline (cattierite type). It is very probable that this compound is also formed in the bio-composite. The  evidences the presence of small particles with a diameter ranging from to 15nm.

DISCUSSION
The reaction of Sions onto the cadmium and cobalt DNA complexes leads to a reorganisation of Co2+and Cd 2 + cations along the DNA strands. Varnali (Fig. 3).
In fact the proposed inter-nanoparticles distances are correlated to a supposed linear DNA molecules. Actually, it is very probable that the DNA is winded around nanoparticles like it does with proteins. This hypothesis will be investigated in a next future.