Effects of X-ray radiation on structure and dynamics of egg white protein gels

The investigation of proteins is highly relevant due to their various functions in living organisms mand their importance in nutrition. A technique that can access the collective dynamics of proteins in solution is X-ray photon correlation spectroscopy (XPCS). Using the high coherent fluxes provided by modern X-ray facilities, XPCS can resolve dynamics down to the molecular length scale. However, the proteins' sensitivity to X-ray radiation poses a serious challenge. This thesis uses XPCS at a synchrotron source to systematically investigate the effects of the X-ray dose and the X-ray dose rate on structure and dynamics of protein gels. The protein gels are hen egg whites cooked at thirteen temperatures between 50 and 85 °C. At these temperatures, the egg white proteins unfold and interconnect, leading to the formation of a gel network at temperatures ≥ 60 °C. Among these egg white gel networks, we observe differences in the susceptibility to radiation damage. The gels prepared between 63 ◦C and 70 ◦C are more sensitive to radiation effects, and the gel structure is broken up by X-ray doses of 2 kGy to 8 kGy causing an acceleration of the sample dynamics. The egg white gel networks prepared above 73 °C are strengthened by the denaturated ovalbumin, which increases the dose thresholds for structure and dynamics by one order of magnitude.
Like other gels, the cooked egg white gels display ballistic motion where single relaxation events in the network cause directional shifts in the surrounding sites. From the XPCS results, we derive the velocity of this ballistic motion as a function of ten different fluences, revealing a linear dependency of this velocity on the fluence. From this we calculate fluence thresholds above which radiation-induced effects dominate the observed dynamics and find Φ∗ = (3±2)×10^−3phs^−1 nm^−2 for the radiation-sensitive gels prepared at ≤ 70 ◦C and Φ∗ = (0.9 ± 0.3) ph s^−1 nm^−2 for those prepared above 70 °C. A comparison to other sample systems suggests a connection between the samples' viscoelasticity and their sensitivity to X-ray radiation effects.
This thesis demonstrates how to determine a window of opportunity in terms of dose and dose rate where intrinsic dynamical and structural properties can be measured with XPCS. These insights can be used to evaluate new measurement schemes and make use of the improved coherent flux at the next generation of X-ray facilities, which will perspectively also enable XPCS measurements on medically highly relevant protein systems.