New insights into differential aggregation of enantiomerically pure and racemic aβ40 systems

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 beta (A beta) 42 peptides is an effective strategy to induce an oligomer-to-fibril conversion, which puts A beta 42 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 A beta 40 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 A beta 40 fibrils were found by transmission electron microscopy to be distinct in morphology, exhibiting a similar to 2-fold narrowing as compared with their enantiopure counterparts.