Non-covalent binding of nucleic acids with gold nanoparticles provides their stability and effective desorption in environment mimicking biological media.

The ability of gold nanoparticles to bind different substances has resulted in the high interest of researchers determining their usage as a promising carrier of various biological substances including nucleic acids (NAs) for therapeutic applications. Most publications report covalent binding (conjugation) of an NA to spherical AuNPs via the Au-S bond. In this work, we obtained non-covalent associates of different ssDNA, ssRNA and siRNAs with spherical gold nanoparticles (AuNPs) and examined their physico-chemical properties and stability in media mimicking intracellular space (bacterial 'cytosol') and cell culture media (10% FBS in DMEM). The 'cytosol' was obtained from E. coli and possessed nuclease activity. For the first time, we used the phosphoryl guanidine (dimethylimidazolidin-2-imine, Dmi) group for modification of 3'-ends to enhance the stability of ssRNAs and siRNAs against nuclease destruction. Trying to evaluate the material balance, we analyzed the whole nucleotide species obtained after incubation of NA-AuNPs associates in 'cytosol' and FBS and evaluated the degree of NAs destruction, a share of full-size NAs remained on the surface of the AuNPs and in the solution. Native ss- and siRNAs, both free and in composition of non-covalent associates with AuNPs, were less resistant to degrading factors than ssDNA. The introduction of two Dmi-groups into the ssDNA increased its stability in 'cytosol' three times within 2.5 h. Dmi-modified siRNAs in non-covalent associates with AuNPs were two times more stable than unmodified siRNA within 4 h. We showed that non-covalent siRNA-AuNPs associates serve as a kind of storage for full-size NAs and thereby prolong their presence in nuclease-active media. Our study showed that non-covalent binding of siRNAs with a surface of AuNPs provides desorption of both strands, which is necessary for siRNA functioning in living cells, and could be considered as an important way to construct siRNA and ssDNA delivery systems based on AuNPs.