Dendrimer–Fullerenol Soft-Condensed Nanoassembly

Nanoscale assembly is an area of research that has vast implications for molecular design, sensing, nanofabrication, supramolecular chemistry, catalysis, and environmental remediation. Here we show that poly­(amidoamine) (PAMAM) dendrimers of both generations 1 (G1) and 4 (G4) can host 1 fullerenol per 2 dendrimer primary amines as evidenced by isothermal titration calorimetry, dynamic light scattering, and spectrofluorometry. Thermodynamically, the interactions were similarly spontaneous between both generations of dendrimers and fullerenols, however, G4 formed stronger complexes with fullerenols resulting from their higher surface charge density and more internal voids, as demonstrated by spectrofluorometry. In addition to hydrogen bonding that existed between the dendrimer primary amines and the fullerenol oxygens, hydrophobic and electrostatic interactions also contributed to complex formation and dynamics. Such a hybrid of soft and condensed nanoassembly may have implications for environmental remediation of discharged nanomaterials and entail new applications in drug delivery

Thermodynamics of Zn²⁺ Binding to Cys₂His₂ and Cys₂HisCys Zinc Fingers and a Cys₄ Transcription Factor Site

The thermodynamics of Zn²⁺ binding to three peptides corresponding to naturally occurring Zn-binding sequences in transcription factors have been quantified with isothermal titration calorimetry (ITC). These peptides, the third zinc finger of Sp1 (Sp1-3), the second zinc finger of myelin transcription factor 1 (MyT1-2), and the second Zn-binding sequence of the DNA-binding domain of glucocorticoid receptor (GR-2), bind Zn²⁺ with Cys₂His₂, Cys₂HisCys, and Cys₄ coordination, respectively. Circular dichroism confirms that Sp1-3 and MyT1-2 have considerable and negligible Zn-stabilized secondary structure, respectively, and indicate only a small amount for GR-2. The p<i>K</i>_a’s of the Sp1-3 cysteines and histidines were determined by NMR and used to estimate the number of protons displaced by Zn²⁺ at pH 7.4. ITC was also used to determine this number, and the two methods agree. Subtraction of buffer contributions to the calorimetric data reveals that all three peptides have a similar affinity for Zn²⁺, which has equal enthalpy and entropy components for Sp1-3 but is more enthalpically disfavored and entropically favored with increasing Cys ligands. The resulting enthalpy–entropy compensation originates from the Zn-Cys coordination, as subtraction of the cysteine deprotonation enthalpy results in a similar Zn²⁺-binding enthalpy for all three peptides, and the binding entropy tracks with the number of displaced protons. Metal and protein components of the binding enthalpy and entropy have been estimated. While dominated by Zn²⁺ coordination to the cysteines and histidines, other residues in the sequence affect the protein contributions that modulate the stability of these motifs