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 < 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 < T < 240 K), the total mean square displacement 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 temperature-dependent increase of the electron transport efficiency from Q(A)(-) 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 Q(A)(-) reoxidation by Q(B) 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 section.

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Dynamics-function correlations are usually inferred when molecular mobility and protein function are simultaneously impaired at characteristic temperatures or hydration levels. In this sense, excitation energy transfer in the photosynthetic light-harvesting complex II (LHC II) is an untypical example because it remains fully functional even at cryogenic temperatures relying mainly on interactions of electronic states with protein vibrations. Here, we study the vibrational and conformational protein dynamics of monomeric and trimeric LHC II from spinach using inelastic neutron scattering (INS) in the temperature range of 20-305 K. INS spectra of trimeric LHC II reveal a distinct vibrational peak at ∼2.4 meV. At temperatures above ∼160 K, however, the inelastic peak shifts toward lower energies, which is attributed to vibrational anharmonicity. A more drastic shift is observed at about 240 K, which is interpreted in terms of a "softening" of the protein matrix along with the dynamical transition. Monomeric LHC II exhibits a higher degree of conformational mobility at physiological temperatures, which can be attributed to a higher number of solvent-exposed side chains at the protein surface. The effects of the changes in protein dynamics on the spectroscopic properties of LHC II are considered in comparative model calculations. The absorption line shapes of a pigment molecule embedded into LHC II are simulated for the cases of (i) a rigid protein matrix, (ii) a protein matrix with temperature-dependent spectral density of protein vibrations, and (iii) temperature-dependent electron-phonon coupling strength. Our findings indicate that vibrational and conformational protein dynamics affect the spectroscopic (absorption) properties of LHC II at physiological temperatures.