Also, aggregation of protein leads to high molecular size species, which diffuses out slowly from the confocal volume, giving low diffusivity. 

Thus, environment in which protein molecules are having colloidal instability, will tend to give lower diffusivity values. This means that such protein formulation will not have a longer shelf life. 

If you are measuring D values at single molecule concentration (very very dilute protein solution), then decrease in diffusivity would mean conformational instability. Which means partial or fully unfolded protein molecule.

Thank You all for the answers.

I searched to know about Diffusion, but could not find any good article. Please suggest any book or article to know about diffusion, 2D diffusion, 3D diffusion in bio-molecules/proteins.

Thanking You                                                                                                             Bikash

For theory- http://www.biophysics.org/portals/1/pdfs/education/schwille.pdf

for application- http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2955502/

see if this is useful.

Thank You

Mr. Swain,

By mass spectrometry, using TOF-analyzer you can obtain this information at a high level of accuracy, comparing with a Fs analysis

Thank You Mam, I do not know about mass spectroscopy. I will read it.

What is the difference between translational diffusion and rotational diffusion ?

Sorry Sir,

I know the meaning of rotation and translation motion. I was little confused if the meaning is same in diffusion or not.  I was reading FCS measurement of Gold nanoparticles, where translational and rotational diffusion is mentioned . I was little confused and could not convinced myself , so asked about it.