Ruep E. Lechner

Picosecond molecular motions in bacteriorhodopsin from neutron scattering

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.

Function and picosecond dynamics of bacteriorhodopsin in purple membrane at different lipidation and hydration

© 1998 Federation of European Biochemical Societies.

Low-energy spin excitations in the molecular magnetic cluster V15

We report an Inelastic Neutron Scattering (INS) study of the fully deuterated molecular compound K6[V15IVAs6O42] · 9D2O (V15). Due to geometrical frustration, the essential physics at low temperatures of the V15 cluster containing 15 coupled V4+ (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 2Δ = 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.

Reaction pattern of photosystem II: oxidative water cleavage and protein flexibility.

This short communication addresses three topics of photosynthetic water cleavage in Photosystem II (PS II): (a) effect of protonation in the acidic range on the extent of the ‘fast’ ns kinetics of P680+· reduction by YZ, (b) mechanism of O–O bond formation and (c) role of protein flexibility in the functional integrity of PS II. Based on measurements of light-induced absorption changes and quasielastic neutron scattering in combination with mechanistic considerations, evidence is presented for the protein acting as a functionally active constituent of the water cleavage machinery, in particular, for directed local proton transfer. A specific flexibility emerging above a threshold of about 230 K is an indispensable prerequisite for oxygen evolution and plastoquinol formation.

Dynamical properties of α-amylase in the folded and unfolded state: the role of thermal equilibrium fluctuations for conformational entropy and protein stabilisation

Abstract A comparative analysis of thermal equilibrium fluctuations occurring in a mesophilic and in a thermophilic α-amylase was performed to study the effect of structural fluctuations on thermostability. The thermal fluctuations determining the conformational entropy of both enzymes have been characterised for the folded (at 30°C and 60°C) and for the unfolded state by applying neutron spectroscopy (at 30°C). The folded state shows a higher structural flexibility for the thermophilic protein as compared to the mesophilic homologue. In contrast, the unfolded state of both enzymes is rather similar with respect to the structural fluctuations. On the basis of this result, a mechanism characterised by entropic stabilisation (i.e., smaller ΔS for the unfolding transition of thermophilic α-amylase) can be assumed to be responsible for the higher thermostability of the thermophilic enzyme.

Temperature dependence of molecular motions in the membrane protein bacteriorhodopsin from QINS

Abstract The internal molecular motions of the membrane protein bacteriorhodopsin (BR) have been investigated by means of quasielastic incoherent neutron scattering (QINS). Measurements using a time-of-flight spectrometer were performed in order to study the quasielastic scattering component in the temperature range 100–297 K. Within an energy range of ± 1.5 meV, it is possible to fit the data by a single Lorentzian with a line width of 100 μeV (HWHM). The line width of the quasi-elastic component is roughly T- and Q-independent. The intensity of the quasielastic scattering is negligible below 200 K and shows a pronounced increase above this temperature, which is caused by an appearance of thermal fluctuations leading to transitions between different conformational substates of BR.

Bacteriorhodopsin and rhodopsin studied by incoherent neutron scattering: dynamical properties of ground states and light activated intermediates

Abstract Quasielastic incoherent neutron scattering as a tool to study picosecond equilibrium fluctuations was used to investigate the dynamical properties of two retinal proteins, rhodopsin and bacteriorhodopsin. Upon absorption of a photon by the retinal chromophore both proteins undergo a sequence of different intermediate states. As known from numerous studies the intermediate states (Metarhodopsin II and M 2 , respectively) differ from the ground state with respect to the protein structure. Therefore, this work was focussed on the relationship between the light-induced structural changes and possible alterations of much faster picosecond fluctuations of the protein structure. In a time range from a few picoseconds to a few femtoseconds no changes of the fast internal motions have been observed between the ground states and the corresponding intermediate states.

Quasielastic small-angle neutron scattering from heavy water solutions of cyclodextrins.

We present a model for quasielastic neutron scattering (QENS) by an aqueous solution of compact and inflexible molecules. This model accounts for time-dependent spatial pair correlations between the atoms of the same as well as of distinct molecules and includes all coherent and incoherent neutron scattering contributions. The extension of the static theory of the excluded volume effect [A. K. Soper, J. Phys.: Condens. Matter 9, 2399 (1997)] to the time-dependent (dynamic) case allows us to obtain simplified model expressions for QENS spectra in the low Q region in the uniform fluid approximation. The resulting expressions describe the quasielastic small-angle neutron scattering (QESANS) spectra of D(2)O solutions of native and methylated cyclodextrins well, yielding in particular translational and rotational diffusion coefficients of these compounds in aqueous solution. Finally, we discuss the full potential of the QESANS analysis (that is, beyond the uniform fluid approximation), in particular, the information on solute-solvent interactions (e.g., hydration shell properties) that such an analysis can provide, in principle.