Alzheimer's Ab(1-40)-peptide aggregation in TFE caused by ultrasonication

• The translational diffusion of Alzheimer's Aβ(1-40)-peptide in sonicated trifluoroethanol (TFE) solutions was studied by the 1H NMR-diffusomery technique. Sonication was performed at the ultrasound frequency of 50 kHz and with the output of 80 W. In freshly-prepared solutions (without any preaggregates of the peptide) diffusion of Aβ(1-40) can be described by an unique diffusion coefficient (~1×10-10 m2/s), which corresponds to the calculated diffusion coefficient of Aβ(1-40) monomers using the Stokes-Einstein hard-sphere approximation. For untreated samples both the diffusion coefficient and the NMR signal amplitude were not changing with time that suggests no aggregation of the peptide in TFE. In contrary, the diffusion decay of the stimulated spin-echo in sonicated samples was more complex suggesting a presence of a few components with different diffusion coefficients. The diffusion decay was separated into spectral components using the CORE method for global analysis of correlated spectral data. This analysis revealed a new diffusion component, which is characterized by a very small diffusion coefficient (<1×10-13 m2/s), which may be attributed to large oligomers of Aβ(1-40) formed upon sonication. A fraction of this component from the total integral intensity of the 1H NMR signal does depend on the sonication time but it does not change during NMR measurements and the following storage of the sample. In all samples a large fraction of the peptide adopts the a-helical secondary structure as revealed by circular dichroism measurements. Upon ultrasonication this secondary structure changes only insignificantly. Therefore, ultrasonication leads to aggregation of Aβ(1-40)-peptide in TFE, without a detectable (by CD) disruption of its a-helical secondary structure.  An increase in the integral intensity of the 1H NMR signal in sonicated samples of Aβ(1-40) in TFE may be interpreted in terms of the transverse relaxation times, T2, of the solvent and Aβ(1-40) monomers: these times do increase as the fraction of Aβ(1-40) aggregates increases in the sample upon sonication. This effect can be further explained by either a concentration dependence of T2 (that reflects a changing ratio of "free" and "bound" solvent to the peptide molecules) or/and by a conformational change in Aβ(1-40) monomers as a result of sonication. Additional spin-echo T2-relaxation time measurements and 1H 2D NOESY NMR may assist in understanding of these interesting effects in sonicated solutions of Aβ(1-40) in TFE. Putative aggregation mechanisms and structures of monomers and oligomers in Aβ(1‑40)/TFE solutions at different duration of sonication are also discussed.

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NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

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Chemistry of Interfaces

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