  This study explores the possibility to measure dynamics of proteins in
solution using X-ray photon correlation spectroscopy (XPCS) at nearly
diffraction limited storage rings (DLSR). We calculate the signal to noise
ratio (SNR) of XPCS experiments from a concentrated lysozyme solution at the
length scale of the hydrodynamic radius of the protein molecule. We take
limitations given by the critical X-ray dose into account and find expressions
for the SNR as a function of beam size, sample-detector distance and photon
energy. Specifically, we show that the combined increase in coherent flux and
coherence lengths at the DLSR PETRA IV will yield an increase in SNR of more
than one order of magnitude. The resulting SNR values indicate that XPCS
experiments of biological macromolecules on nm length scales will become
feasible with the advent of a new generation of synchrotron sources. Our
findings provide valuable input for the design and construction of future XPCS
beamlines at DLSRs.
