Reversible random coil to beta-sheet transition and the early stage of aggregation of the A beta(12-28) fragment from the Alzheimer peptide

The Alzheimer peptide fragment A beta(12-28) was studied at millimolar conc entration by parallel experiments with high-resolution nuclear magnetic res onance (NMR) and circular dichroism (CD) in solution at a pH close to the i soelectric point of the peptide. A preparation procedure using low temperat ure and low ionic strength buffer gave a sample with stable and reproducibl e properties. Reversible changes in secondary structure and state of aggreg ation were studied by variation of temperature. High-temperature promotes a ggregation and beta-sheet induction, whereas low-temperature shifts the equ ilibrium toward low molecular weight fractions and less beta-sheet like str ucture. NMR diffusion experiments show that the dominating, most low molecu lar weight fraction is monomeric. With increasing temperature, residues F(2 0)A(21)E(22), overlapping with the so-called central hydrophobic segment of the A beta peptide, exhibit the most pronounced alpha-proton NMR secondary chemical shift changes from random coil toward more beta-sheet like struct ure. High ionic strength also promotes aggregation and beta-sheet induction . The combined spectroscopic results, including also molecular weight estim ations by cutoff filters, are summarized in a scheme in which monomeric mos tly random coil and heterogeneous aggregated partly beta-sheet forms of the peptide are in a temperature-dependent equilibrium, a situation which corr esponds to an early stage of the fibrillogenesis.