The invention provides monomeric and oligomeric amyloid beta peptide isomers that are resistant towards fibrillogenesis and their use as screening reagents or antigens in methods and pharmaceutical preparations for the treatment of Alzheimer's disease and other conditions related to protein misfolding. The alanines at positions 21 and 30, in wild type amyloid beta peptide amino acid sequence, are according to the invention replaced by cysteins, which results in an intra molecular disulphide bond. The invention further provides transgenic animals expressing modified amyloid precursor proteins or amyloid beta peptides.
