Residue-specific pK(a) measurements of the beta-peptide and mechanism of pH-induced amyloid formation

The aggregation of the beta-peptide into amyloid is a key pathological even t in Alzheimer's disease. This process (beta-amyloidosis) involves the conv ersion of soluble random coil, alpha-helical or beta-sheet conformations in to insoluble, aggregated beta-pleated sheet structures. The pH is a signifi cant extrinsic factor that influences beta-amyloidosis, which must be relat ed to the presence of ionizable groups in the beta-peptide. To further eval uate this effect, we determined the dissociation constants (pK(a)) of the s ide chains for the aspartic acid (Asp), glutamic acid (Glu), histidine (His ), and tyrosine (Tyr) amino acid residues using NMR spectroscopy. The measu rements were performed under different solution conditions, where the predo minant conformation is either random coil or alpha-helix. We have used a pe ptide fragment that comprises residues 1-28 [beta-(1-28)] of the natural be ta-(1-40) or beta-(1-42) peptides, which is an appropriate model since the remaining 29-40 or 29-42 regions are devoid of polar and charged amino acid residues. The results demonstrate that the Glu and I-iis residues have lar ger pK(a) values in sodium dodecyl sulfate solution, suggesting that electr ostatic interactions are important in stabilizing the alpha-helix and preve nting an alpha-helix --> beta-sheet rearrangement. A mechanism involving un favorable interactions of the charged groups with the alpha-helix macrodipo le is proposed fur the pi-I-induced alpha-helix --> beta-sheet transformati on in water-trifluoroethanol solution.