The Palladium(II) Complex of Aβ4−16 as Suitable Model for Structural Studies of Biorelevant Copper(II) Complexes of N-Truncated Beta-Amyloids

Mariusz Mital,Kosma Szutkowski,Wojciech Bal,Tomasz Frączyk,Jarosław Poznański,Piotr Skrobecki,Simon C Drew,Karolina Bossak-Ahmad,Igor Zhukov

doi:10.3390/ijms21239200

Abstract

The A peptide is a major beta-amyloid species in the human brain, forming toxic aggregates related to Alzheimer’s Disease. It also strongly chelates Cu(II) at the N-terminal Phe-Arg-His ATCUN motif, as demonstrated in A and A model peptides. The resulting complex resists ROS generation and exchange processes and may help protect synapses from copper-related oxidative damage. Structural characterization of Cu(II)A complexes by NMR would help elucidate their biological function, but is precluded by Cu(II) paramagneticism. Instead we used an isostructural diamagnetic Pd(II)-A complex as a model. To avoid a kinetic trapping of Pd(II) in an inappropriate transient structure, we designed an appropriate pH-dependent synthetic procedure for ATCUN Pd(II)A, controlled by CD, fluorescence and ESI-MS. Its assignments and structure at pH 6.5 were obtained by TOCSY, NOESY, ROESY, H-C HSQC and H-N HSQC NMR experiments, for natural abundance C and N isotopes, aided by corresponding experiments for Pd(II)-Phe-Arg-His. The square-planar Pd(II)-ATCUN coordination was confirmed, with the rest of the peptide mostly unstructured. The diffusion rates of A, Pd(II)-A and their mixture determined using PGSE-NMR experiment suggested that the Pd(II) complex forms a supramolecular assembly with the apopeptide. These results confirm that Pd(II) substitution enables NMR studies of structural aspects of Cu(II)-A complexes.

Highlights

The Aβ4−42 peptide is a major beta-amyloid species in the human brain
In order to avoid the kinetic entrapment of Pd(II) in such hydroxides en route to the desired Cu(II)-Aβ4−16 mimic, we used K2PdCl4 as the Pd(II) source, which can resist hydrolysis and polymerization up to pH 5.5 at appropriate temperatures and concentrations [36,37,38]
The peptide was incubated at room temperature with substoichiometric (0.85 mol eq.) K2PdCl4 at pH 4.0, where the formation process of Pd(OH)2 and its polymerization to [Pd(OH)2-xClx]n [39] was slower than the anchoring of Pd(II) to the peptide

Summary

Introduction

The Aβ4−42 peptide is a major beta-amyloid species in the human brain. It was first co-discovered in 1985 as a major component of amyloid plaques of Alzheimer’s Disease (AD) brains, and was found to be more abundant than the commonly studied Aβ1−42 and Aβ1−40 peptides [1,2]. This discovery was soon disregarded, and the research was focused on the latter two peptides as forming neurotoxic aggregates. The logK for Cu(II) binding to Aβ1−16 and Aβ1−40 peptides at a physiological pH of 7.4 was determined as 10.0 and 10.1, respectively [11]

Methods

Results

Discussion

Conclusion