Probing the Local Dielectric Constant of Plasmid DNA in Solution and Adsorbed on Chemically Graded Aminosilane Surfaces.

Nile Red dye was used to determine the dielectric constant, ε, of nonpolar microenvironments in double stranded DNA (ds-DNA) both in aqueous buffer solution and when adsorbed on amine-modified surfaces. The value of ε within the DNA decreased with increasing buffer concentration. Values of ε ∼ 6.75 and ∼3.00 were obtained in 0.1 mM phosphate buffered saline (PBS) and in 10 mM PBS, respectively. Similar effects were observed upon adsorption to chemically graded amine-modified silica surfaces. Under 1 mM buffer, ε was measured to be ∼2.84 and ∼1.90 at the low amine (high silica) and high amine (low silica) ends of the gradient, respectively. An increase in the buffer concentration again led to a decrease in ε, but only at the low amine end. It is concluded that high buffer concentrations and binding to an amine surface cause a condensation of the DNA, forming less polar microenvironments. These results provide important knowledge of the factors governing the polarity of DNA microenvironments to which intercalators bind.