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/DQ2 up to Q = 0.091 nm−1. The hydrodynamic radius RH determined from XPCS using Stokes–Einstein diffusion D = kT/(6πηRH) is 43% larger than the geometric radius R0 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)2SO4, a possible effect of aggregation as well as slight variation of the structures of the capsid resulting from the salt–protein interactions. Stokes–Einstein diffusion of active Cowpea mosaic virus was measured up to a 69 nm length‐scale with 19 µs‐resolved X‐ray photon correlation spectroscopy.