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 Mg2+ 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 Mg2+ 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 Mg2+ ion bound. The specific and nonspecific contributions to the interaction energy of Mg2+ with oligonucleotide single strands is found to be additive, which suggests that within the uncertainty of these surface-specific experiments, the Mg2+ 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 Mg2+ ions per 21-mer oligonucleotide, while the bases cytosine and guanine are found to bind eleven Mg2+ ions per 21-mer oligonucleotide.