The structure of DNA and RNA in the water-mercury interface

Abstract

The interaction of nucleic acids with a charged mercury surface has been investigated. The investigation was carried out by measuring the effect of the nucleic acid on the differential capacity of the electrical double layer between a polarized mercury surface and an 0·d1 N -NaCl solution containing various concen- trations of nucleic acid. The lowering of the capacity by adsorbed nucleic acid gives an indication of the adherence to the surface of the different molecular residues (sugar, purine, ionized groups) of the nucleic acid. The kinetics of adsorption were used to determine the surface concentration. This was done after discussing evidence for a diffusion controlled mechanism of adsorption. The lowering of the differential capacity is proportional to the surface concentration for partly covered surfaces and reaches a constant value after the surface becomes fully covered. Additional adsorption by building further layers does not affect the differential capacity. This makes it possible to determine limiting values of areas per nucleotide, when full surface coverage is reached. The areas determined in this way are: 96 A 2 /nucIeotide for E. coli RNA, and 93 A 2 /nucleotide for denatured herring sperm DNA, irrespective of the polarization of the mercury surface. At positively charged mercury surfaces, the area per nucleotide for native salmon sperm DNA is of the same order (86 A 2 ), but at negatively charged surfaces the area is only 35 A 2 /nucleotide. The results indicate that native DNA preserves its double helical structure when adsorbed on a nega- tively charged surface, whereas unfolding occurs on a positively charged mercury surface. An unfolding mechanism of the native DNA in the surface is discussed. According to the proposed model, the unfolding of the helix starts at that point in the chain which reaches the surface first and proceeds by rotation of the two double helical cylinders which protrude into the solution about their axis. The torque for rotating the cylinders is created by the binding of the nucleotides at the unravelling point which pass into an adsorbed state on the surface.