X-ray driven and intrinsic dynamics in protein gels

Abstract

Abstract We use X-ray photon correlation spectroscopy to investigate how structure and dynamics of egg white protein gels are affected by X-ray dose and dose rate. We find that both, changes in structure and beam-induced dynamics, depend on the viscoelastic properties of the gels with soft gels prepared at low temperatures being more sensitive to beam-induced effects. Soft gels can be fluidized by X-ray doses of a few kGy with a crossover from stress relaxation dynamics (Kohlrausch–Williams–Watts exponents $$k \approx 1.5$$ k ≈ 1.5 to 2) to typical dynamical heterogeneous behavior ( $$k<$$ k < 1) while the high temperature egg white gels are radiation-stable up to doses of 15 kGy with $$k\ge 1.5$$ k ≥ 1.5 . For all gel samples we observe a crossover from equilibrium dynamics to beam induced motion upon increasing X-ray fluence and determine the resulting fluence threshold values $$\Phi _D$$ Φ D . Surprisingly small threshold values of $$\Phi _D=(3 \pm 2)\times 10^{-3} \,\textrm{ph}\,$$ Φ D = ( 3 ± 2 ) × 10 - 3 ph s $$^{-1}$$ - 1 nm $$^{-2}$$ - 2 can drive the dynamics in the soft gels while for stronger gels this threshold is increased to $$\Phi _D=(0.9 \pm 0.3) \,\textrm{ph}$$ Φ D = ( 0.9 ± 0.3 ) ph s $$^{-1}$$ - 1 nm $$^{-2}$$ - 2 . We explain our observations with the viscoelastic properties of the materials and can connect the threshold dose for structural beam damage with the dynamic properties of beam-induced motion. Our results suggest that soft viscoelastic materials can display pronounced X-ray driven motion even for low X-ray fluences. This induced motion is not detectable by static scattering as it appears at dose values well below the static damage threshold. We show that intrinsic sample dynamics can be separated from X-ray driven motion by measuring the fluence dependence of the dynamical properties.