Specific and nonspecific metal ion-nucleotide interactions at aqueous/solid interfaces functionalized with adenine, thymine, guanine, and cytosine oligomers.

This article reports nonlinear optical measurements that quantify, for the first time directly and without labels, how many Mg(2+) cations are bound to DNA 21-mers covalently linked to fused silica/water interfaces maintained at pH 7 and 10 mM NaCl, and what the thermodynamics are of these interactions. The overall interaction of Mg(2+) with adenine, thymine, guanine, and cytosine is found to involve -10.0 ± 0.3, -11.2 ± 0.3, -14.0 ± 0.4, and -14.9 ± 0.4 kJ/mol, and nonspecific interactions with the phosphate and sugar backbone are found to contribute -21.0 ± 0.6 kJ/mol for each Mg(2+) ion bound. The specific and nonspecific contributions to the interaction energy of Mg(2+) with oligonucleotide single strands is found to be additive, which suggests that within the uncertainty of these surface-specific experiments, the Mg(2+) ions are evenly distributed over the oligomers and not isolated to the most strongly binding nucleobase. The nucleobases adenine and thymine are found to bind only three Mg(2+) ions per 21-mer oligonucleotide, while the bases cytosine and guanine are found to bind eleven Mg(2+) ions per 21-mer oligonucleotide.

Interactions of Al(III), La(III), Gd(III), and Lu(III) with the fused silica/water interface studied by second harmonic generation.

The interactions of the trivalent metal cations Al(III), La(III), Gd(III), and Lu(III) with the silica/water interface were studied using the nonlinear optical technique of second harmonic generation (SHG). Specifically, the Eisenthal chi(3) technique was used to quantify the thermodynamics of trivalent ion adsorption to the bare fused silica surface. SHG adsorption isotherms were measured and fit with the triple layer surface complexation model to obtain adsorption free energies, binding constants, and interfacial charge densities. The adsorption free energy for Al(III) was found to be -37.2(5) kJ/mol, while the adsorption free energies for the three trivalent lanthanide cations ranged from -29.9(9) to -32.2(7) kJ/mol. Despite identical ionic charges, the metals under investigation displayed different affinities for the fused silica/water interface, and this finding is analyzed and interpreted in the context of size-dependent metal cation properties and metal ion speciation. The thermodynamic results from this work are valuable benchmarks for computer simulations of trivalent metal transport in the environment.

Uranyl adsorption and speciation at the fused silica/water interface studied by resonantly enhanced second harmonic generation and the chi(3) method.

We report the first use of resonantly enhanced second harmonic generation (SHG) to study uranyl adsorption at a buried mineral oxide/water interface. Uranyl adsorption is studied in real-time, under flow conditions, and in the presence of environmentally relevant screening electrolyte concentrations. The in situ SHG spectrum of surface-bound uranyl reveals a well-defined resonance at 306 nm. By monitoring the SHG response at 306 nm, adsorption isotherms were collected for uranyl species at the fused silica/water interface at pH 7, and in the presence of aqueous carbonate. The measured adsorption free energies determined by the Langmuir isotherm are consistent with physisorption via hydrogen bonding. The speciation of the surface-active uranyl species at pH 7 was elucidated via a free energy versus interfacial potential analysis, which reveals that the uranyl adsorbates are either neutral or univalent cationic species. Complementary surface charge density data, obtained using the Eisenthal chi(3) technique, reveal that the charge on the ionic uranyl species adsorbing under the experimental conditions are positive. It is proposed that a mixture of neutral and univalent, cationic uranyl species is surface active at pH 7 and in the presence of carbonate ions. Insofar as the experimental conditions model those found in natural groundwater systems, the results of this work are valuable to the prediction and assessment of uranium pollution transport in groundwater and soils. Our thermodynamic results can also serve as important benchmarks for computer simulations of U(VI) transport in heterogeneous geochemical environments.

Interaction of nitrate, barium, strontium and cadmium ions with fused quartz/water interfaces studied by second harmonic generation.

Inorganic anions and cations are ubiquitous in environmental chemistry. Here, we use second harmonic generation to track the interaction of the environmentally important metal cations barium, strontium, and cadmium and the nitrate anion with fused quartz/water interfaces at pH 7. Using a dynamic flow system, we assess the extent of reversibility in the binding process and report the absolute number density of adsorbed cations, their charge densities, and their free energies of adsorption. We also present resonantly enhanced second harmonic generation experiments that show that nitrate is surface active and report the free energies and binding constants for the adsorption process. The second harmonic generation spectrum of surface-bound nitrate shows a new adsorption band that cuts further into the solar spectrum than nitrate in the aqueous or solid state. The results that we obtain for all four inorganic ions and the implications for tropospheric and aquatic chemistry as well as geochemistry are discussed in the context of fundamental science as well as pollutant transport models.