  Hydrated proteins undergo a transition in the deeply supercooled regime,
which is attributed to rapid changes in hydration water and protein structural
dynamics. Here, we investigate the nanoscale stress relaxation in hydrated
lysozyme proteins stimulated and probed by X-ray Photon Correlation
Spectroscopy (XPCS). This approach allows us to access the nanoscale dynamic
response in the deeply supercooled regime (T = 180 K) which is typically not
accessible through equilibrium methods. The relaxation time constants exhibit
Arrhenius temperature dependence upon cooling with a minimum in the
Kohlrausch-Williams-Watts exponent at T = 227 K. The observed minimum is
attributed to an increase in dynamical heterogeneity, which coincides with
enhanced fluctuations observed in the two-time correlation functions and a
maximum in the dynamic susceptibility quantified by the normalised variance
$\chi_T$. Our study provides new insights into X-ray stimulated stress
relaxation and the underlying mechanisms behind spatio-temporal fluctuations in
biological granular materials.
