The three‐dimensional structure of the holo form of recombinant cellular bovine heart fatty‐acid‐binding protein (H‐FABPc), a polypeptide of 133 amino acid residues with a molecular mass of 15 kDa, has been determined by multidimensional homonuclear and heteronuclear NMR spectroscopy applied to uniformly 15N‐labeled and unlabeled protein. A nearly complete set of 1H and 15N chemical shift assignments was obtained. A total of 2329 intramolecular distance constraints and 42 side‐chain χi dihedral‐angle constraints were derived from cross‐relaxation and J coupling information. 3D nuclear Overhauser enhancement and exchange spectroscopy combined with heteronuclear multiple‐quantum coherence (NOESY‐HMQC) experiments, performed on a sample of uniformly 13C‐labeled palmitic acid bound to unlabeled cellular heart fatty‐acid‐binding protein revealed 10 intermolecular contacts that determine the orientation of the bound fatty acid. An ensemble of protein conformations was calculated with the distance‐geometry algorithm for NMR applications (DIANA) using the redundant dihedral‐angle constraint (REDAC) strategy. After docking the fatty acid into the protein, the protein‐ligand arrangement was subject to distance‐restrained energy minimization. The overall conformation of the protein is a β‐barrel consisting of 10 antiparallel β‐strands which form two nearly orthogonal β‐sheets of five strands each. Two short helices form a helix‐turn‐helix motif in the N‐terminal region of the polypeptide chain. The palmitic acid is bound within the protein in a U‐shaped conformation close to the two helices. The obtained solution structure of the protein is consistent with a number of fatty‐acid‐binding‐protein crystal structures.

The NMR relaxation times T'2, T2, and T1 were measured in isolated rat lungs as functions of external magnetic field B0, temperature, and lung inflation. The observed linear dependence on B0 of the tissue-induced free induction decay rate (T'2)-1 provides independent confirmation of the air/water interface model of the lung. Furthermore, measurements of the Larmor frequency dependence of T1 are consistent with a spin-lattice relaxation rate of the form 1/T1 = A omega -1/2 + B as expected for the case in which the relaxation arises from water-biopolymer cross-relaxation, which should be proportional to the surface area of the lung. This prediction was verified by observations of an approximately linear dependence of 1/T1 on transpulmonary pressure and thus on the lung surface area.

We present a generalized nuclear spin bath model for embedded electron spin decoherence in organic solids at low temperatures, which takes the crucial influence from hindered methyl group rotation tunneling into account. This new, quantum many body model, after resolved using the cluster correlation expansion method, predicts the decoherence profiles directly from the proton relative position and methyl group tunneling splitting inputs. Decoherence profiles from this model explain adequately the influence from both strongly and weakly hindered methyl groups to embedded electron spin decoherence: The former accelerates decoherence by increasing the nearest neighbor nuclear spin coupling, while the latter enhances coherence through a novel confinement like’ mechanism, in which the very strong nuclear spin coupling from the tunneling splitting term suppresses those protons on the methyl rotors from participating in the bath dynamics. Both types of influences are successfully proven experimentally in representative organic polycrystalline matrices: methyl malonic acid for strongly hindered and acetamide for weakly hindered methyl groups, respectively.

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In this article, we present the novel application of the nuclear spin bath model and the cluster correlation expansion method on studying the matrix material structure via embedded electron spin decoherence. Profiles of embedded electron spin decoherence under the Carr-Purcell-Meiboom-Gill dynamical decoupling pulse series in a model system for organic solids (malonic acid) are calculated for different structures. Resulting decay profiles exhibit a strong correlation to the variations of an adjacent proton environment among them. In addition, the decoherence behavior of embedded spin in proton spin bath(s) of organic solids is found to be significantly different from bath models with other nuclei through the violation of the even-odd pulse parity, which characterizes the influence of large dipolar coupling between protons at the quantum level. Theoretical predictions of decoherence profiles in polycrystalline, the relative distribution of Hahn echo signal decay time scales among single crystal orientations, and the reduction in Hahn echo signal decay time scale by disorder are positively verified by experiments.

In photosynthesis, final electron transfer from ferredoxin to NADP+ is accomplished by the flavo enzyme ferredoxin:NADP+ oxidoreductase (FNR). FNR is recruited to thylakoid membranes via integral membrane thylakoid rhodanase-like protein TROL. We address the fate of electrons downstream of photosystem I when TROL is absent. We have employed electron paramagnetic resonance (EPR) spectroscopy to study free radical formation and electron partitioning in TROL-depleted chloroplasts. DMPO was used to detect superoxide anion (O2.−) formation, while the generation of other free radicals was monitored by Tiron. Chloroplasts from trol plants pre-acclimated to different light conditions consistently exhibited diminished O2.− accumulation. Generation of other radical forms was elevated in trol chloroplasts in all tested conditions, except for the plants pre-acclimated to high-light. Remarkably, dark- and growth light-acclimated trol chloroplasts were resilient to O2.− generation induced by methyl-viologen. We propose that the dynamic binding and release of FNR from TROL can control the flow of photosynthetic electrons prior to activation of the pseudo-cyclic electron transfer pathway.

The electron spin-lattice relaxation of 2,2,6,6-tetramethyl-1-piperidine-1-oxyl and 4-oxo-2,2,6,6-tetramethyl-1-piperidine-1-oxyl was measured at temperatures between 5 and 80 K in crystalline and glassy ethanol using X-band electron paramagnetic resonance spectroscopy. The experimental data at the lowest temperatures studied were explained in terms of electron-nuclear dipolar interaction between the paramagnetic center and the localized excitations, whereas at higher temperatures low-frequency vibrational modes from the host matrix and Raman processes should be considered. The strong impact of hydrogen bonding between the dopant molecule and ethanol host on the spin relaxation was observed in ethanol glass whereas in crystalline ethanol both paramagnetic guest molecules behaved similarly. DOI: 10.1103/PhysRevB.80.052201 PACS number͑s͒: 61.43.Fs, 76.30.Ϫv, 63.50.Ϫx, 65.60.ϩa Coupling of the electron spin to disorder modes of various doped matrices has been extensively studied due to the sensitivity of the approach toward dynamical properties of the observed systems.1-3 Research on disordered solids has shown that nitroxyl radicals can contribute toward the characterization of glass-forming materials, 4-6 providing experimental data for the development of self-consistent theories of molecular dynamics in glasses in general.7-10 The work presented here has been in part motivated by the lack of nitroxyl spin-lattice relaxation-time data measured below 20 K in disordered solids 11,12 and, to the best of the authors' knowledge, by the very few examples comparing paramagnetic relaxation rate data in glassy and crystalline states of the same compound. 13 Solid ethanol has been found to be a very convenient model system for the investigation of molecular solids, as it can be easily prepared in phases characterized by different types of disorder. 14,15 In our previous studies we have shown how, within the course of an X-band electron paramagnetic resonance ͑EPR͒ experiment, glassy and crystalline ethanol can be studied on the very same sample using incorporated nitroxyl radicals.5 Since nitroxyl radicals can be purposely tailored, in the context of this study we have chosen two almost identical paramagnetic probes, which differ only in one carbonyl group. We focused on the influence of hydrogen bonding between the incorporated paramagnetic guest molecule and the host matrix on the microscopic nature of probe/matrix dynamics. The central point is the comparative analysis of spin relaxation in crystalline and glassy states of the same host material. The experiments were performed in the temperature range 5-80 K, which is well below the ethanol glass transition ͑95 K͒. 14 The liquid ethanol ͓anhydrous, min. 99.8% ͑GC͒, p.a. from Kemika, Zagreb͔ and hexadeuteroethanol ͑deuteration degree Ͼ99.5% from Uvasol, Merck͒ were doped with the nitroxide paramagnetic spin-probe 2,2,6,6-tetramethyl-1-piperidine-1-oxyl ͑TEMPO͒ or 4-oxo-2,2,6,6-tetramethyl-1-piperidine-1-oxyl ͑TEMPONE͒ from Aldrich, at a concentration of 0.7 mM. Glass...