Direct measurement of Stokes–Einstein diffusion of Cowpea mosaic virus with 19 µs-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 µs time resolution. It was found that the decorrelation time τ(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/(6πηR (H)) is 43% larger than the geometric radius R (0) determined from SAXS in the 0.007 M K(3)PO(4) buffer solution, whereas it is 80% larger for CPMV in 0.5 M NaCl and 104% larger in 0.5 M (NH(4))(2)SO(4), a possible effect of aggregation as well as slight variation of the structures of the capsid resulting from the salt–protein interactions.