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The Journal of Physical Chemistry Letters, 10 (21), ISSN:1948-7185

The structure of the major light harvesting chlorophyll a/b complex (LHCII) was analyzed by pulsed EPR measurements and compared with the crystal structure. Site specific spin labeling of the recombinant protein allowed the measurement of distance distributions over several intra and intermolecular distances in monomeric and trimeric LHCII yielding information on the protein structure and its local flexibility. A spin label rotamer library based on a molecular dynamics simulation was used to take the local mobility of spin labels into account. The core of LHCII in solution adopts a structure very similar or identical to the one seen in crystallized LHCII trimers with little motional freedom as indicated by narrow distance distributions along and between a helices. However distances comprising the lumenal loop domain show broader distance distributions indicating some mobility of this loop structure. Positions in the hydrophilic N terminal domain upstream of the first trans membrane a helix exhibit more and more mobility the closer they are to the N terminus. The nine amino acids at the very N terminus that have not been resolved in any of the crystal structure analyses give rise to very broad and possibly bimodal distance distributions which may represent two families of preferred conformations. © 2012 by The American Society for Biochemistry and Molecular Biology Inc.

Electron paramagnetic resonance studies on diamagnetic proteins are based on site-directed spin labelling and require relatively much effort for engineering cystein point mutations as well as expressing and labelling the protein. Therefore, it is advantageous to predict those sites and pairs of sites that can provide the most precise and most reliable information on accessibility and distances. Systematic site scans based on a rotamer library approach for spin label side groups allow for such predictions. Figures of merit are defined that can be used to rank sites according to their potential suitability in studies of structure and structural changes. Such site scans can still provide useful results if only a backbone model and the amino acid sequence of the protein are known.

ABC transporters are ubiquitous membrane proteins that translocate solutes across biological membranes at the expense of ATP. In prokaryotic ABC importers, the extracytoplasmic anchoring of the substrate-binding protein (receptor) is emerging as a key determinant for the structural rearrangements in the cytoplasmically exposed ATP-binding cassette domains and in the transmembrane gates during the nucleotide cycle. Here the molecular mechanism of such signaling events was addressed by electron paramagnetic resonance spectroscopy of spin-labeled ATP-binding cassette maltose transporter variants (MalFGK2-E). A series of doubly spin-labeled mutants in the MalF-P2 domain involving positions 92, 205, 239, 252, and 273 and one triple mutant labeled at positions 205/252 in P2 and 83 in the Q-loop of MalK were assayed. The EPR data revealed that the substrate-binding protein MalE is bound to the transporter throughout the transport cycle. Concomitantly with the three conformations of the ATP-binding cassette MalK2, three functionally relevant conformations are found also in the periplasmic MalF-P2 loop, strictly dependent on cytoplasmic nucleotide binding and periplasmic docking of liganded MalE to MalFG. The reciprocal communication across the membrane unveiled here gives first insights into the stimulatory effect of MalE on the ATPase activity, and it is suggested to be an important mechanistic feature of receptor-coupled ABC transporters.

Electron spin resonance spectroscopy (ESR) has been widely used to study properties of polymers, e.g. microviscosity or polarity, on a local scale as sensed by spin probes. The underlying principle is the very high sensitivity of rotational and lateral diffusion of a spin probe to its environment. Nitroxide free radical spin-probes have been studied most intensely for this purpose because of their rather high chemical stabilities. In general, three lines are observed in a continuous-wave (CW) ESR nitroxide spectrum due to hyperfine coupling between the electron spin and the nuclear spin of 14N, which is characterized by the splitting between the outermost extrema in the spectra (2Azz ). In addition, the shape of the ESR spectra can be strongly altered by interactions between spin probes and the surrounding medium. The film formation process from dispersions, and phase transitions, particularly glass transitions, in polymers have been studied by ESR probe or label techniques.We have recently reported aqueous dispersions of nanoscale polyethylene (PE) crystals, consisting of a single lamella of only 6 nm thickness and with a lateral pseudo-diameter of ca. 12 nm. In addition to these dispersions (I) composed of linear PE, dispersions (II) with similar overall particle size but composed of branched (50 methyl branches per 1000 carbon atoms), low crystalline PE are accessible. The uptake of fluorescence probes as guest molecules by the dispersed particles was observed by optical spectroscopy. The environment experienced by these guests can be switched by the melting/crystallization of the PE nanoparticles, and the temperature at which this occurs can be controlled by the degree of branching of the polymer. Here we present preliminary results on spin probes as a reporter for the interactions of guest molecules with the PE particles.

A comprehensive study of the photoluminescence spectra and the electrophysical parameters have been performed on GaAs epitaxial layers grown by the low temperature liquid phase epitaxy (LPE) method from solution-melts doped with Yb impurity. The characteristic variations of the relative intensity of the components of the PL spectra, electroconductivity, concentration and mobility of the free charge carriers in the grown layers where established as a function of the concentration of the Yb impurity in the initial solutionmelts. A possible mechanism of the influence of the Yb impurity on the native defects and background impurities in the epitaxially grown GaAs is proposed.

Decoherence arises from a fluctuating spin environment, captured by its noise spectrum S(ω). Dynamical decoupling (DD) with n π pulses extends the dephasing time if the associated filter function attenuates S(ω). Inversely, DD noise spectroscopy (DDNS) reconstructs S(ω) from DD data by approximating the filters pass band by a δ-function. This restricts application to qubit-like spin systems with inherently long dephasing times and/or many applicable pulses. We introduce regularized DDNS to lift this limitation and thereby infer S(ω) from DD traces of paramagnetic centers in glassy o-terphenyl and water–glycerol matrices recorded with n ≤ 5. For nitroxide radicals at low temperatures, we utilize deuteration to identify distinct matrix- and spin center-induced spectral features. The former extends up to a matrix-specific cut-off frequency and characterizes nuclear spin diffusion. We demonstrate that rotational tunneling of intramolecular methyl groups drives the latter process, whereas at elevated temperatures S(ω) reflects the classical methyl group reorientation. Ultimately, S(ω) visualizes and quantifies variations in the electron spins couplings and thus reports on the underlying spin dynamics as a powerful decoherence descriptor., Physical Chemistry Chemical Physics, 23 (38), ISSN:1463-9084, ISSN:1463-9076