The amyloid beta (A beta) peptide of Alzheimer's disease binds copper(II), and the peptide-bound metal may be a source of reactive oxygen species and neurotoxicity. To circumvent peptide aggregation and reduce redox activity, there is growing interest in using metal chelates as drug therapeutics for AD, whose design requires accurate data on the affinity of A beta peptides for copper(II). Reports on Cu2+ binding to A beta range from approximately 10(5) to approximately 10(9); these values' being obtained for different peptide lengths (1-16, 1-28, 1-40, 1-42) at varying pH. Herein, we report that Cu2+'s binding to A beta(1-40) at 37 degrees C occurs in a 1:1 stoichiometry with a pH-dependent binding constant: 1.1 (+/-0.2) x 10 (9) M (-1) and 2.4 (+/-0.2) x 10 (9) M(-1) at pH 7.2 and 7.4, respectively. Under identical conditions, A beta(1-16) reveals a comparable binding constant, confirming that this portion of the peptide is the binding region. Several previously reported values can be reconciled with the current measurement by careful consideration of thermodynamics associated with the presence of competing ligands used to solubilize copper.

SUMMARY There is no consensus on the coordinating ligands for Cu2+ by A β. Yet the differences in peptide sequence between human and rat have been hypothesized to alter metal ion binding in a manner that alters Cu2+-induced aggregation of A β. Herein, we employ isothermal titration calorimetry (ITC), circular dichroism (CD) and electron paramagnetic resonance (EPR) spectroscopy to examine the Cu2+ coordination spheres to human and rat A β and an extensive set of A β(16) mutants. EPR of the mutant peptides is consistent with a 3N1O binding geometry, like the native human peptide at pH 7.4. The thermodynamic data reveal an equilibrium between three coordination spheres, {NH2, O-, NIm His6, amide N−}, {NH2, O-, NIm His6, NIm His13} and {NH2, O-, NIm His6, NIm His14} for human A β(16) but only one for rat A β(16), { NH2, O-, NIm His6, N−}, at pH 7.4 -6.5. ITC and CD data establish that the mutation R5G is sufficient for reproducing this difference in Cu2+ binding properties at pH 7.4. The substitution of bulky and positively charged Arg by Gly is proposed to stabilize the coordination {NH2, O-, NIm His6, amide N−} that then results in one dominating coordination sphere for the case of the rat peptide. The differences in the coordination geometries for Cu2+ by the human and rat A β are proposed to contribute to the variation in the ability of Cu2+ to induce aggregation of A β peptides.

This work examines the temperature dependence of electron transfer (ET) kinetics in solid-state films of mixed-valent states of monodisperse, small (<2 nm) Au monolayer protected clusters (MPCs). The mixed valent MPC films, coated on interdigitated array electrodes, are Au25(SR)18(0/1-), Au25(SR)18(1+/0), and Au144(SR)60(1+/0), where SR = hexanethiolate for Au144 and phenylethanethiolate for Au25. Near room temperature and for ca. 1:1 mol:mol mixed valencies, the bimolecular ET rate constants (assuming a cubic lattice model) are ~2 × 10(6) M(-1) s(-1) for Au25(SR)18(0/1-), ~3 × 10(5) M(-1) s(-1) for Au25(SR)18(1+/0), and ~1 × 10(8) M(-1) s(-1) for Au144(SR)60(1+/0). Their activation energy ET barriers are 0.38, 0.34, and 0.17 eV, respectively. At lowered temperatures (down to ca. 77 K), the thermally activated (Arrhenius) ET process dissipates revealing a tunneling mechanism in which the ET rates are independent of temperature but, among the different MPCs, fall in the same order of ET rate: Au144(+1/0) > Au25(0/1-) > Au25(1+/0).

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