ABSTRACT Amyloid beta (Aβ)42 is an aggregation-prone peptide and a believed seminal etiological agent of Alzheimer’s Disease. A common post-translational modification of Aβ42 is its oxidation at the methionine (Met) 35 residue. Met35 oxidation is physiologically relevant, with a large portion of Alzheimer’s amyloid plaques made up of Aβ42-Met35SO. There is a remarkable level of disagreement in the Alzheimer’s field with regard to both biophysical and biological consequences of Aβ42 Met35 oxidation. As such, it has been claimed to both promote and suppress aggregation and it has also been claimed to both enhance and reduce toxicity. Oxidation of Aβ42 Met35 renders the sulfur chiral. This means that two and not one Aβ42-Met35SO peptides are produced. The interconversion barrier between Aβ42-Met35-(R)-SO and Aβ42-Met35-(S)-SO is extremely high (we calculated it as 45.2 kcal/mol using quantum chemical methods), and the two peptides have to be considered distinct species under physiological conditions. The formation of two stereochemically stable chiral sulfur epimers in consequence Aβ42 Met35 oxidation has never been considered in the Alzheimer’s field. We have recently reported a supercritical CO2-based separation protocol that now allows obtaining Met-(R)- SO and Met-(S)-SO in gram quantities with purities exceeding 99.5%. Using these building blocks, we were able to make the Ab42-Met35-(R)-SO and Ab42-Met35-(S)-SO chiral sulfur epimers with no loss of stereochemical information. Here we propose to determine the degree to which sulfur chirality influences Aβ42-Met35-(R)-SO and Aβ42-Met35-(S)-SO aggregation, neuronal uptake and toxicity. We also propose to develop assays that use a combination of LC/MS and NMR to measure kinetics of Aβ42-Met35-(R)-SO and Aβ42-Met35-(S)-SO reduction by methionine sulfoxide reductases (MSR) A and B. Research proposed here is expected to resolve a long-standing controversy with regard to the functional consequences of Aβ42 Met35 oxidation and will set the stage for future R01-funded experiments to study toxic actions of Aβ42-Met35-(R)-SO and Aβ42-Met35-(S)-SO and their reduction to Aβ42 through MSRA and MSRB in hippocampal and cortical brain tissue, as well as animal models.