Rigid versus Flexible Protein Matrix: Light-Harvesting Complex II Exhibits a Temperature-Dependent Phonon Spectral Density

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 light-harvesting complex II (LHC II) from spinach using inelastic neutron scattering (INS) in the temperature range between 20 and 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 towards 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 ...