pH-dependent conformational change of Fe(3+)-binding giant DNA for control of aggregation of gold nanoparticles.

The pH-dependent conformational change of Calf thymus DNA (ctDNA, a typical giant DNA) binding with Fe3+ (Fe(3+)-ctDNA) for control of aggregation of gold nanoparticles (AuNPs) hybridized with Fe(3+)-ctDNA was described, which was attributed to the interaction between ctDNA and Fe3+. In acidic conditions, significant conformational change of ctDNA from elongated coil state to folded compact state occurs by binding of Fe3+ with the phosphate groups of ctDNA. The SPR absorbance and thus the colors of AuNPs hybridized with Fe(3+)-ctDNA would vary pH-dependently within the pH range 6.0-10.6 as the folded compact ctDNA could not stabilize AuNPs as effectively as the elongated coil ctDNA. Zeta potentials and Raman spectra were studied for revealing the interaction between Fe3+ and ctDNA as well as that between AuNPs and DNA (ctDNA and Fe(3+)-ctDNA). The conformational change of ctDNA driven by Fe3+ was also investigated using CD spectroscopy. This study prompts the possibility that the spectral, and thus color, change of AuNPs can be used to follow the conformational change of giant DNA driven by metal ions and biological relevant pH conditions.