Materials and Methods
Peptide synthesis and preparation . Aβ40, Aβ42, single D-amino
acid substituted peptides, and di-D-amino acid substituted peptides were
synthesized, purified, and characterized in the Biopolymer Lab at UCLA
30 . Briefly, peptide synthesis was performed on
an automated peptide synthesizer (Model 433A, Applied Biosystems, Foster
City, CA) using 9-fluorenylmethoxycarbonyl-based methods. Peptides were
purified using reverse phase high-performance liquid chromatography
(RP-HPLC). The purity of the peptides were >95%.
Quantitative amino acid analysis and mass spectrometry yielded the
expected compositions and molecular weights, respectively, for each
peptide. Purified peptides were stored as lyophilizates at –20°C.
Peptide lyophilizates were solvated initially in 10% (v/v) 60 mM NaOH,
45% (v/v) H2O (prepared using a Synthesis A10 water
purification system (Millipore, Bedford, MA), and 45% (v/v) 22.2 mM
sodium phosphate, pH 7.4, on ice. The solutions were sonicated for 1 min
in an ultrasonic water bath (Model 1510, Branson Ultrasonics Corp.,
Danbury, CT) and then they were filtered using a Microcon centrifugal
filter (30 kDa molecular mass cut-off, Millipore, Bedford, MA) at 14,000
× g for 10 min at room temperature (RT; ≈22 °C) The
concentrations of the resulting filtrates were determined by UV
absorbance using an extinction coefficient ε270=1276 L
mol-1 cm-1. The peptide
concentration then was adjusted to 40 μM using 10 mM sodium phosphate,
pH 7.4.