The best way to preserve Abeta is to store lyophilized powder. You can dissolve you Abeta vial in an organic solvent (like HFIP) and aliquot it in a way that gives you minimum Abeta per aliquot required for you experiments. Then evaporate the solvent in each aliquot using Nitorgen evap or lyophilizer. The dried aliquots can then be stored for a longer time in -20. When you need Abeta for your experiments, you can take the necessary number of aliquots and make your preparation.

I would store the Abeta peptide in the organic solvent hexafluoroisopropanol (HFIP). That way the peptide will remain monomeric and more or less unfolded.

When you want to do your experiments you simply evaporate the solvent, add the buffer of choice, and sonicate mildly to bring the peptide into solution. Then you can do fibrillization reactions, spectroscopy, or whatever you like.

HFIP, acidic treatment, DMSO are often used. However, you will get a lot of irreproducibility. If you want to study the assembly of the petide, you have to purify the monomeric form or at least to check this state. The more straightforward approach is Size exclusion Chromatography. We could get highly reproducible results by flash-freezing the monomeric form eluted from the column. With HFIP, the so-called monomeric form was not eluted in a single peak corresponding to the monomer.... at least in our hands.

I agree with the other answers, HFIP is a good organic solvent to store Abeta peptide, but be careful, HFIP is very unstable solution (very "volatile"), working in ice because HFIP evaporate very easily  and when you prepare the aliquots the concentration of Abeta peptide could change. However if you looking for a buffer for biophysical experiments of aggregation I suggest you to use Potassium Phosphate (KPP) 10 mM K2HPO4, 10 mM KH2PO4 at pH 7.4. Good luck

The group of Sarah Linse has done quite some work on keeping Abeta in a "handable" way.

For instance, you can have a look at the procedure used by my former collegue Axel Abelein which was inspired by her work:

Article Formation of Dynamic Soluble Surfactant-Induced Amyloid Beta...

Thanks a lot for your kind attention and suggestion.