New insights into differential aggregation of enantiomerically pure and racemic Aβ40 systems

Abstract

Racemic mixtures frequently display properties that are different from those associated with their enantiopure counterparts, and are often characterized by higher propensity to form aggregates. Our previous research established that mixing of the enantiomers of Alzheimer amyloid β (Aβ) 42 peptides is an effective strategy to induce an oligomer‐to‐fibril conversion, which puts Aβ42 into a substantially less toxic state. Here, new insights into this chiral inactivation effect are presented. In addition to the commonly used Thioflavin T fibril formation assays, the use of the less aggregation‐prone Aβ40 system allowed us to monitor peptide aggregation by NMR. Whereas enantiopure peptide was well soluble under the chosen experimental conditions and showed no sign of precipitation, addition of one equivalent of the mirror‐image peptide triggered an instant and rapid aggregation, observable through the attenuation of the NMR signal. The racemic Aβ40 fibrils were found by transmission electron microscopy to be distinct in morphology, exhibiting a ~2‐fold narrowing as compared with their enantiopure counterparts.

A DFT study of structure and stability of pleated and rippled cross‐β sheets with hydrophobic sidechains

“…[ 10,11 ] The Raskatov lab developed Aβ Chiral Inactivation (Aβ‐CI) as an approach to induce an oligomer‐to‐fibril transition to suppress peptide toxicity, using mirror‐image Aβ42 as the molecular mirror‐image chaperone of the natural all‐L Aβ42 peptide. [ 12,13 ] Consistent trends were observed for the Aβ40 system, [ 14 ] and subsequent DFT‐based work found the rippled parallel LVFFA dimer to be favored over three distinct, experimentally observed pleated frameworks. [ 15 ] The rippled cross‐β topography has thus burgeoned as a valuable design principle for materials and in biomedical applications.…”

“…Cross‐β dimers were constructed from Pauling‐Corey coordinates, as described previously. [ 14 ] The topographies used to construct DFT input structures are shown in Figure 1, and have been deposited as part of the Supporting Information. The pentapeptidic strings were N‐terminally acetylated and C‐terminally amidated and t Bu sidechains added geometrically at all C α positions using Molden.…”

Conformational Selection as the Driving Force of Amyloid β Chiral Inactivation

“…Previous research by Dutta et al reported a low-energy DFToptimized rippled LVFFA:lvffa dimer structure. [9] This published geometry was used as initial input structure here, but it was possible to locate a lower-energy rippled cross-β dimer structure (see Computational Details). This dimeric system was fully re-optimized and determined to be energetically favored by 4.2 kcal/mol over the most stable of the pleated structures considered in this study, that is, (LVFFA) 2 Aβ42 5oqv , rot.A .…”

“…[8] Our more recent work showed that racemic Aβ40 fibrils were distinct from enantiopure ones. [9] Morphologically, fibrils formed from enantiopure Aβ40 display a twist, whereas those made from racemic Aβ40 did not. Also, there was a notable difference in width of the mature fibrils, which were approximately two times as wide in the enantiopure case.…”

“…The rationale behind the choice of the parallel (as opposed to antiparallel) cross-β topology as well as the omission of the terminal charged residues in the computational model system has been discussed elsewhere. [9]…”