Density of Vibrational States of the Light-Harvesting Complex II of Green Plants Studied by Inelastic Neutron Scattering

Results of inelastic neutron scattering (INS) experiments are reported for the solubilized trimeric light-harvesting complex of photosystem II (LHC II) in the temperature range from 5 to 100 K. Two incident neutron wavelengths of 2.0 (∼20 meV) and 5.1 Å (∼3.2 meV) corresponding to elastic energy resolutions of Δ E = 0.920 meV and Δ E = 0.093 meV, respectively, are employed to study INS spectra of LHC II for both neutron energy loss and gain. Solubilized LHC II and D 2 O-containing buffer solution are investigated separately in order to properly subtract the contribution of the solvent. The inelastic part of the scattering function S ( Q,ω) derived for the LHC II protein resembles the well-known “Boson-peak” and is characterized by a maximum at about 2.5 meV and a strongly asymmetric line shape with a slight tailing toward higher energy transfers. Analysis of the momentum transfer dependence of S ( Q,ω) reveals that both the elastic and inelastic contributions to S ( Q,ω) exhibit the characteristics of vibrational protein motions. Furthermore, the effective density of vibrational states is derived from the experimental data. Finally, the data are discussed in comparison to recent results of line-narrowing optical spectroscopies (Pieper, J.; et al. J. Phys. Chem. B 2001, 105, 7115). The wide distribution of vibrational frequencies found for LHC II is interpreted in terms of structurally inequivalent protein domains within the LHC II trimer leading to a partial localization of protein phonons.