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, 25 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 uncertainly 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.