The key steps in photosynthetic water splitting take place in a protein complex referred to as photosystem II (PS II). It has been shown by incoherent quasielastic neutron scattering that electron transfer in PSII is intimately coupled with hydration-induced protein dynamics. Less information is available about the presence and functional role of collective hydration water dynamics. To address this point, we propose to investigate functionally intact, D2O-hydrated PSII samples at 75 and 90% relative humidity on the BRISP spectrometer.