Temperature and hydration dependent protein dynamics in photosystem II of green plants studied by quasielastic neutron scattering

Protein dynamics in hydrated and vacuum dried photosystem II PS II membrane fragments from spinach has been investigated by quasielastic neutron scattering QENS in the temperature range between 5 and 300 K. Three distinct temperature ranges can be clearly distinguished by active type s of protein dynamics A At low temperatures T lt; 120 K , the protein dynamics of both dry and hydrated PS II is characterized by harmonic vibrational motions. B In the intermediate temperature range 120 lt; T lt; 240 K , the total mean square displacement u2 total slightly deviates from the predicted linear behavior. The QENS data indicate that this deviation, which is virtually independent of the extent of hydration, is due to a partial onset of diffusive protein motions. C At temperatures above 240 K, the protein flexibility drastically changes because of the onset of diffusive large amplitude protein motions. This dynamical transition is clearly hydration dependent since it is strongly suppressed in dry PS II. The thermally activated onset of protein flexibility as monitored by QENS is found to be strictly correlated with the temperaturedependent increase of the electron transport efficiency from QA to QB Garbers et al. 1998 Biochemistry 37, 11399 11404 . Analogously, the freezing of protein mobility by dehydration in dry PS II appears to be responsible for the blockage of QA reoxidation by QB at hydration values lower than 45 r.h. Kaminskaya et al. 2003 Biochemistry 42, 8119 8132 . Similar effects were observed for reactions of the water oxidizing complex as outlined in the Discussion sectio