It may be difficult to go back to monomeric form unless you use strong detergents (Triton may do) or even stronger stuff like urea or guanidine... but I do not think it may be the best choice if you then want to use it in "clean" experiments. 

What I did before was to filter the protein solution with a vivaspin with a cutoff of 100kDa: this way you should be quite sure that most of the big oligomers are gone, while the solution that passes through contains the monomers that you want. After that you may need to remeasure the protein concentration, because you will probably loose a lot of material.

I hope this may be helpful! :-)

Hi Nicoletta

Thanks for your answer. But again if the protein concentration goes down, it may be difficult to get oligomers from that monomers so i dont know if its ok to sonicate or to increase the pH of the protein solution? 

It is true that if the concentration decreases it may be more difficult to get oligomers in an aggregation assays. However, changing pH or sonicating may strongly affect the behaviour of the protein and I am not quite sure that this may help to have a "clean experiment" in the end. Moreover, increasing the pH leads to fibrils dissociation to oligomers/amorphus aggregates (I think it was in Nath et al. 2010 Biophysical J), so alkalin pH may be even worse. If you can do a quick test you may try, but I would not suggest to do it, if it takes you a long time to measure what is going on.

I imagine that if you get commercial protein it may be not in high quantity, but starting from a bigger amount of protein and filterig may help you to get a reasonable protein concentration. I was wondering if you may be able to produce the protein in E. Coli. The purification protocol is quite easy (you may have a look at my publications, but there are also other different protocols from other labs), but I do not if it is fleasible in your lab. The plasmid for alpha-synuclein expression in E. Coli is available from Addgene (https://www.addgene.org/51486/).