Chelation-induced diradical formation as an approach to modulation of the amyloid-β aggregation pathway

Current approaches toward modulation of metal-induced A beta aggregation pathways involve the development of small molecules that bind metal ions, such as Cu(II) and Zn(II), and interact with Ab. For this effort, we present the enediyne-containing ligand (Z)-N, N'-bis[1-pyridin-2-yl-meth(E)-ylidene]oct-4-ene- 2,6-diyne-1,8-diamine (PyED), which upon chelation of Cu(II) and Zn(II) undergoes Bergmancyclization to yield diradical formation. The ability of this chelation-triggered diradical to modulate Ab aggregation is evaluated relative to the non-radical generating control pyridine-2-ylmethyl(2-{[(pyridine-2-ylmethylene)-amino]-methyl}-benzyl)-amine (PyBD). Variable-pH, ligand UV-vis titrations reveal pK(a) = 3.81(2) for PyBD, indicating it exists mainly in the neutral form at experimental pH. Lipinski's rule parameters and evaluation of blood-brain barrier (BBB) penetration potential by the PAMPA-BBB assay suggest that PyED may be CNS+ and penetrate the BBB. Both PyED and PyBD bind Zn(II) and Cu(II) as illustrated by bathochromic shifts of their UV-vis features. Speciation diagrams indicate that Cu(II)PyBD is the major species at pH 6.6 with a nanomolar K-d, suggesting the ligand may be capable of interacting with Cu(II)-A beta species. In the presence of A beta(40/42) under hyperthermic conditions (43 degrees C), the radical-generating PyED demonstrates markedly enhanced activity (2-24 h) toward the modulation of A beta species as determined by gel electrophoresis. Correspondingly, transmission electron microscopy images of these samples show distinct morphological changes to the fibril structure that are most prominent for Cu(II)-A beta cases. The loss of CO2 from the metal binding region of A beta in MALDI-TOF mass spectra further suggests that metal-ligand-A beta interaction with subsequent radical formation may play a role in the aggregation pathway modulation.