Direct measurement of Stokes-Einstein diffusion of Cowpea mosaic virus with 19 mu s-resolved XPCS

Brownian motion of Cowpea mosaic virus (CPMV) in water was measured using small-angle X-ray photon correlation spectroscopy (SA-XPCS) at 19.2 mu s time resolution. It was found that the decorrelation time tau(Q) = 1/DQ(2) up to Q = 0.091 nm(-1). The hydrodynamic radius R-H determined from XPCS using Stokes-Einstein diffusion D = kT/(6 pi eta R-H) is 43% larger than the geometric radius R-0 determined from SAXS in the 0.007 M K3PO4 buffer solution, whereas it is 80% larger for CPMV in 0.5 M NaCl and 104% larger in 0.5 M (NH4)(2)SO4, a possible effect of aggregation as well as slight variation of the structures of the capsid resulting from the salt-protein interactions.