My protein has a hydrodynamic radius of 1nm (at 0.3mg/ml) at 15 degree according to DLS data but when the temperature is increased to 25 degree the size is increased to 3.8 nm.How can I calculate the number of molecules present when the radius is 1nm (15degree) and 3.8 nm (25degree)?Can anyone help me?
If you're using the Zetasizer there's a calculator in it that allows you to estimate molecular mass from size based on some assumption of shape (rods, coils etc). See, for example, threads by Ulf Nobbmann on this forum and on Materials Talks:
http://www.materials-talks.com/blog/2015/06/30/key-protein-features-in-the-zetasizer-software/
http://www.materials-talks.com/blog/2017/11/02/guide-to-molecular-weight-with-the-zetasizer/
Obviously you'll need to know the molecular mass of your monomer protein to get an estimate of the number of units within your entities.
What is causing the increase in size? Aggregation or swelling?
You really cannot, for the reasons explained above by Rawle. The protein might be aggregating, or it might be expanding/denaturing. Static light scattering might help here (Zimm plot, though you sound to be in the small-q limit.)
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