Light-induced Modulation of Protein Dynamics During the Photocycle of Bacteriorhodopsin

Advanced and In Situ Analytical Methods for Solar Fuel Materials

In situ and operando techniques can play important roles in the development of better performing photoelectrodes, photocatalysts, and electrocatalysts by helping to elucidate crucial intermediates and mechanistic steps. The development of high throughput screening methods has also accelerated the evaluation of relevant photoelectrochemical and electrochemical properties for new solar fuel materials. In this chapter, several in situ and high throughput characterization tools are discussed in detail along with their impact on our understanding of solar fuel materials.

Anharmonic transitions in nearly dry L-cysteine I

Two special dynamical transitions of universal character have recently been observed in macromolecules (lysozyme, myoglobin, bacteriorhodopsin, DNA and RNA) at T* ~100-150 K and T(D) ~180-220 K. The underlying mechanisms governing these transitions have been the subject of debate. In the present work, a survey is reported on the temperature dependence of structural, vibrational and thermodynamical properties of a nearly anhydrous amino acid (orthorhombic polymorph of the amino acid l-cysteine at a hydration level of 3.5%). The temperature dependence of x-ray powder diffraction patterns, Raman spectra and specific heat revealed these two transitions at T* = 70 K and T(D) = 230 K for this sample. The data were analyzed considering amino acid-amino acid, amino acid-water, water-water phonon-phonon interactions and molecular rotor activation. Our results indicated that the two referred temperatures define the triggering of very simple and particular events that govern all the interactions of the biomolecular: activation of CH(2) rigid rotors (T < T* ), phonon-phonon interactions between specific amino acid and water dimer vibrational modes (T* < T < T(D)), and water rotational barriers surpassing (T > T(D)).

Time-resolved quasielastic neutron scattering studies of native photosystems

The internal molecular dynamics of proteins plays an important role in a number of functional processes in native photosystems. Prominent examples include the photocycle of bacteriorhodopsin and electron transfer in the reaction center of plant photosystem II. In this regard, the recently developed technique of time-resolved quasielastic neutron scattering with laser excitation opens up new perspectives for the study of protein/membrane dynamics in specific functional states of even complex systems. The first direct observation of a functionally modulated protein dynamics has just recently been reported for the model system bacteriorhodopsin (Pieper et al., Phys. Rev. Lett. 100, 2008, 228103.), where a transient softening of the protein was observed on a timescale of approximately 1 ms along with the large-scale structural change in the M-intermediate of bacteriorhodopsin. In contrast, photosystem II membrane fragments with inhibited electron transfer show a suppression of protein dynamics approximately 160 mus after the actinic laser flash (Pieper and Renger, Biochemistry 48, 2009, 6111). This effect may reflect aggregation-like conformational changes capable of dissipation of excess excitation energy to prevent photodamage in the absence of Q(A)-->Q(B) electron transfer. These findings indicate that proteins exhibit a remarkable flexibility to accommodate different functional processes. This contribution will discuss methodical aspects, challenges, and recent applications of laser-excited, time-resolved quasielastic neutron scattering.

Flash-induced structural dynamics in photosystem II membrane fragments of green plants

Time-resolved quasielastic neutron scattering with laser excitation is a promising novel pump-probe approach, which opens up new perspectives for the study of protein-membrane dynamics in specific functional states of even complex systems. This is demonstrated here for the case of photosystem II membrane fragments with inhibited electron transfer. In contrast to the case of the model system bacteriorhodopsin, a transient reduction of the dynamics is observed approximately 160 micros after the actinic laser flash. This effect is the first observation of a modulated structural dynamics in photosystem II membrane fragments.

From protons to OXPHOS supercomplexes and Alzheimer's disease: structure-dynamics-function relationships of energy-transducing membranes

By the elucidation of high-resolution structures the view of the bioenergetic processes has become more precise. But in the face of these fundamental advances, many problems are still unresolved. We have examined a variety of aspects of energy-transducing membranes from large protein complexes down to the level of protons and functional relevant picosecond protein dynamics. Based on the central role of the ATP synthase for supplying the biological fuel ATP, one main emphasis was put on this protein complex from both chloroplast and mitochondria. In particular the stoichiometry of protons required for the synthesis of one ATP molecule and the supramolecular organisation of ATP synthases were examined. Since formation of supercomplexes also concerns other complexes of the respiratory chain, our work was directed to unravel this kind of organisation, e.g. of the OXPHOS supercomplex I(1)III(2)IV(1), in terms of structure and function. Not only the large protein complexes or supercomplexes work as key players for biological energy conversion, but also small components as quinones which facilitate the transfer of electrons and protons. Therefore, their location in the membrane profile was determined by neutron diffraction. Physico-chemical features of the path of protons from the generators of the electrochemical gradient to the ATP synthase, as well as of their interaction with the membrane surface, could be elucidated by time-resolved absorption spectroscopy in combination with optical pH indicators. Diseases such as Alzheimer's dementia (AD) are triggered by perturbation of membranes and bioenergetics as demonstrated by our neutron scattering studies.

New sources and instrumentation for neutrons in biology

Neutron radiation offers significant advantages for the study of biological molecular structure and dynamics. A broad and significant effort towards instrumental and methodological development to facilitate biology experiments at neutron sources worldwide is reviewed.