Cyclic voltammetry was employed to investigate the extent of electrostatic binding for metal complexes such as ferrocyanide, [Fe(CN)6]4-, to full generations of polyamidoamine (PAMAM) dendrimers (G1.0−G5.0) as a function of size and pH. Addition of dendrimer solutions to metal complexes caused a large net positive shift in E 1/2 and reduced peak currents that varied with the size of the dendrimer. Binding ratios, K 4 -/K 3 -, calculated from the potential shifts changed from a ratio of five with small dendrimers (G1.0−G3.0) to 10 for G4.0 and G5.0, consistent with known structural changes in PAMAM dendrimers. Conditional binding constants for [Fe(CN)6]4-, calculated from the diffusion-limited oxidation currents, also increased from G3.0 (8.8 × 104 M-1) to G4.0 (31 × 104 M-1). At pH 5, the binding ratios increased dramatically at all generations, suggesting that the interior amine sites in the dendrimer are easily accessible to metal complexes and that dendrimers show some charge selectivity.

An advanced undergraduate laboratory project is described that integrates inorganic, analytical, physical, and biochemical techniques to reveal differences in binding between cationic metal complexes and anionic DNA (herring testes). Students were guided to formulate testable hypotheses based on the title question and a list of different metal complexes. Student teams synthesized the target complexes, such as tris(1,10-phenanthroline)cobalt(III) or tris(2,2′-bipyrydyl)cobalt(III), and characterized them by voltammetry and spectroscopy. Separately, DNA stock solutions were prepared and analyzed via published spectroscopic methods. Aliquots of the DNA solutions, added into a metal-complex solution, gave decreases in the cyclic voltammetry peak currents due to the slower diffusion rate of the DNA−metal complex. A nonlinear curve fit analysis of the 1:1 binding isotherms confirmed the literature result (unknown to students) of larger binding constants for the phenanthroline complex due to intercalative binding. Student team results were shared in a group meeting and assessment was by group reports and individual portfolios. The project was an effective way to link the various laboratory techniques common to the chemical disciplines and encouraged team building in a research atmosphere.

The quenched emission intensity for *Ru(bpy)3 2+ in the presence of ferrocyanide, [Fe(CN)6 4−], was restored in aqueous solutions at pH 8 via the addition of PAMAM dendrimers due to competitive electrostatic binding of the ferrocyanide to the dendrimer. Binding equations were developed assuming a 1:1 interaction between the quencher and a dendrimer binding site size (defined as the number of terminal amine groups). Nonlinear curve fitting of titration data for emission intensity versus added dendrimer at fixed [Fe(CN)6 4−] resulted in calculated values for the binding constants, binding site sizes, and bound quenching rate constants between ferrocyanide and different sized dendrimers. When the PAMAM dendrimer size was increased, the ferrocyanide binding constant increased and the fully bound quenching rate constant decreased, but the binding site size remained the same (between 5 and 6 terminal amines). Changing the buffer from Tris to phosphate at pH 8 dramatically changes the binding parameters as predicted for primarily electrostatic binding. Direct binding studies using solvatochromic 8-anilino-1-naphthalenesulfonic acid (ANS) emission and PAMAM dendrimers indicated a large hydrophobic change going from G3 to G4 that was not seen for ferrocyanide binding. ANS binding constants increased going from pH 6 to 8, which may be explained by the unique protonation behavior and macromolecular topology of PAMAM dendrimers.

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