The characteristics of internal molecular motions of bacteriorhodopsin in the purple membrane have been studied by quasielastic incoherent neutron scattering. Because of the quasihomogeneous distribution of hydrogen atoms in biological molecules, this technique enables one to study a wide variety of intramolecular motions, especially those occurring in the picosecond to nanosecond time scale. We performed measurements at different energy resolutions with samples at various hydration levels within a temperature range of 10-300 K. The analysis of the data revealed a dynamical transition at temperatures Td between 180 K and 220 K for all motions resolved at time scales ranging from 0.1 to a few hundred picoseconds. Whereas below Td the motions are purely vibrational, they are predominantly diffusive above Td, characterized by an enormously broad distribution of correlation times. The variation of the hydration level, on the other hand, mainly affects motions slower than a few picoseconds.

PACS. 75.30.Et -Exchange and superexchange interactions. PACS. 75.50.Xx -Molecular magnets. PACS. 78.70.Nx -Neutron inelastic scattering. Abstract. -We report an Inelastic Neutron Scattering (INS) study of the fully deuterated molecular compound K6[V IV15 As6O42]·9D2O (V15). Due to geometrical frustration, the essential physics at low temperatures of the V15 cluster containing 15 coupled V 4+ (S=1/2) is determined by three weakly coupled spin-1/2 on a triangle. The INS spectra at low-energy allow us to directly determine the effective exchange coupling J0 = 0.211(2) meV within the triangle and the gap 2∆ = 0.035(2) meV between the two spin-1/2 doublets of the ground state. Results are discussed in terms of deviations from trigonal symmetry and Dzyaloshinskii-Moriya (DM) interactions.

We have extended the exploration of microscopic dynamics of supercooled liquids to small wave numbers Q corresponding to the scale of intermediate range order, by developing a new experimental approach for precise data correction for multiple scattering noise in inelastic coherent neutron scattering. Our results in supercooled Ca0.4K0.6(NO3)(1.4) reveal the first direct experimental evidence, after a decade of controversy, that the so-called picosecond process around the glass transition corresponds to a predicted first, faster stage of the structural relaxation. In addition, they show that this process takes the spatial form of fast heterogeneous collective flow of correlated groups of atoms.

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 ∆E ) 0.920 meV and ∆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 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.

The dynamic susceptibility of propylene carbonate in the moderately viscous regime above Tc is reinvestigated by incoherent neutron and depolarised light scattering, and compared to dielectric loss and solvation response. Depending on the strength of α relaxation, a more or less extended β scaling regime is found. Mode-coupling fits yield consistently λ = 0.72 and Tc = 182 K, although different positions of the susceptibility minimum indicate that not all observables have reached the universal asymptotics.