Estimating the relative populations of 3(10)-helix and alpha-helix in Ala-rich peptides: A hydrogen exchange and high field NMR study

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Copper(II) binding to PBT2 differs from that of other 8-hydroxyquinoline chelators: implications for the treatment of neurodegenerative protein misfolding diseases

PBT2 (5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline) is a small Cu(II)-binding drug that has been investigated in the treatment of neurodegenerative diseases, namely, Alzheimer's disease (AD). PBT2 is thought to be highly effective at crossing the blood-brain barrier and has been proposed to exert anti-Alzheimer's effects through the modulation of metal ion concentrations in the brain, specifically the sequestration of Cu(II) from amyloid plaques. However, despite promising initial results in animal models and in clinical trials where PBT2 was shown to improve cognitive function, larger-scale clinical trials did not find PBT2 to have a significant effect on the amyloid plaque burden compared with controls. We propose that the results of these clinical trials likely point to a more complex mechanism of action for PBT2 other than simple Cu(II) sequestration. To this end, herein we have investigated the solution chemistry of Cu(II) coordination by PBT2 primarily using X-ray absorption spectroscopy (XAS), high-energy-resolution fluorescence-detected XAS, and electron paramagnetic resonance. We propose that a novel bis-PBT2 Cu(II) complex with asymmetric coordination may coexist in solution with a symmetric four-coordinate Cu(II)-bis-PBT2 complex distorted from coplanarity. Additionally, PBT2 is a more flexible ligand than other 8HQs because it can act as both a bidentate and a tridentate ligand as well as coordinate Cu(II) in both 1:1 and 2:1 PBT2/Cu(II) complexes.Kelly L. Summers, Graham P. Roseman, George J. Sopasis, Glenn L. Millhauser, Hugh H. Harris, Ingrid J. Pickering and Graham N. Georg

Alzheimer’s Drug PBT2 Interacts with the Amyloid β 1–42 Peptide Differently than Other 8-Hydroxyquinoline Chelating Drugs

Although Alzheimer’s disease (AD) was first described over a century ago, it remains the leading cause of age-related dementia. Innumerable changes have been linked to the pathology of AD; however, there remains much discord regarding which might be the initial cause of the disease. The “amyloid cascade hypothesis” proposes that the amyloid β (Aβ) peptide is central to disease pathology, which is supported by elevated Aβ levels in the brain before the development of symptoms and correlations of amyloid burden with cognitive impairment. The “metals hypothesis” proposes a role for metal ions such as iron, copper, and zinc in the pathology of AD, which is supported by the accumulation of these metals within amyloid plaques in the brain. Metals have been shown to induce aggregation of Aβ, and metal ion chelators have been shown to reverse this reaction in vitro. 8-Hydroxyquinoline-based chelators showed early promise as anti-Alzheimer’s drugs. Both 5-chloro-7-iodo-8-hydroxyquinoline (CQ) and 5,7-dichloro-2- [(dimethylamino)methyl]-8-hydroxyquinoline (PBT2) underwent unsuccessful clinical trials for the treatment of AD. To gain insight into the mechanism of action of 8HQs, we have investigated the potential interaction of CQ, PBT2, and 5,7-dibromo-8- hydroxyquinoline (B2Q) with Cu(II)-bound Aβ(1−42) using X-ray absorption spectroscopy (XAS), high energy resolution fluorescence detected (HERFD) XAS, and electron paramagnetic resonance (EPR). By XAS, we found CQ and B2Q sequestered ∼83% of the Cu(II) from Aβ(1−42), whereas PBT2 sequestered only ∼59% of the Cu(II) from Aβ(1−42), suggesting that CQ and B2Q have a higher relative Cu(II) affinity than PBT2. From our EPR, it became clear that PBT2 sequestered Cu(II) from a heterogeneous mixture of Cu(II)Aβ(1−42) species in solution, leaving a single Cu(II)Aβ(1−42) species. It follows that the Cu(II) site in this Cu(II)Aβ(1−42) species is inaccessible to PBT2 and may be less solvent-exposed than in other Cu(II)Aβ(1−42) species. We found no evidence to suggest that these 8HQs form ternary complexes with Cu(II)Aβ(1−42).Kelly L. Summers, Graham Roseman, Kevin M. Schilling, Natalia V. Dolgova, M. Jake Pushie, Dimosthenis Sokaras, Thomas Kroll, Hugh H. Harris, Glenn L. Millhauser, Ingrid J. Pickering, and Graham N. Georg