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