B.-J. van Rossum

MULTIDIMENSIONAL CP-MAS 13C NMR OF UNIFORMLY ENRICHED CHLOROPHYLL

Abstract The progress toward structure refinement of solid-type uniformly 13C enriched ([U-13C]) chlorophyll-containing biological preparations is summarised. Solid state carbon chemical shifts of aggregated [U-13C] bacteriochlorophyll (BChl) c in intact chlorosomes of Chlorobium tepidum and in [U-13C] BChl c aggregates were determined by the application of homonuclear (13C13C) magic angle spinning (MAS) NMR dipolar correlation spectroscopy. It was found that the arrangement of BChl c molecules in the chlorosomes and in the aggregates is highly similar, which provides convincing evidence that self-organisation of the BChl c is the main mechanism to support the structure of the chlorosomes. Additionally, high field 2-D (1H13C) and 3-D (1H13C13C) dipolar correlation spectroscopy was applied to determine solid state proton chemical shifts of aggregated [U-13C] BChl c in intact chlorosomes. From the high-field assignments, evidence is found for the existence of at least two well-defined interstack arrangements.

Multiple-spin effects in fast magic angle spinning Lee–Goldburg cross-polarization experiments in uniformly labeled compounds

The proton–carbon polarization exchange in Lee–Goldburg cross-polarization magic angle spinning (LG-CP MAS) nuclear magnetic resonance experiments on uniformly 13C-labeled compounds at high spinning frequency is studied. It is shown that the multiple carbon labels in the samples greatly influence the spin dynamics during the LG-CP mixing times. The zeroth order effective LG-CP MAS spin Hamiltonian is a sum of zero quantum dipolar interaction terms. These pairwise dipolar terms generally do not commute with each other, making it impossible to factorize the evolution operator. Consequently, the frequencies of the dipolar oscillations as well as the polarization transfer amplitudes become strongly dependent on the configuration of the spins involved in the multiple heteronuclear couplings. The strong carbon–proton couplings usually attenuate polarization transfers between weakly coupled spins. In practice, this implies that except for strongly coupled or isolated heteronuclear 13C–1H spin pairs, it is diffic...

Antiferromagnetism and crystal‐field effects in CeCuX3 (X=Al,Ga) compounds

We have studied two new tetragonal compounds, CeCuAl3 and CeCuGa3, which both display very large specific‐heat, c(T), values at low temperature. CeCuAl3 orders antiferromagnetically below TN=3.0 K, while CeCuGa3 remains paramagnetic down to 0.4 K. The large values of c(T) in these and other cerium intermetallics are interpreted as resulting from an unusually small crystal‐field splitting and the possible presence of atomic disorder.

Resistivity anomalies in heavy-fermion CeCu2Sb2 and CeNi2Sb2

Abstract Temperature dependences of the electrical resistivity, magnetic susceptibility and specific heat of the tetragonal compounds CeCu2Sb2 and CeNi2Sb2 have been studied. The resistivity of both compounds displays a sharp maximum followed by a shallow minimum at higher temperature, indicative of heavy-fermion behavior. The possibility of anomalous low-energy crystalline electric field states is discussed by comparison with other cerium-based 1-2-2 compounds.

13C MAS NMR evidence for structural similarity of L162YL mutant and Rhodobacter sphaeroides R26 RC, despite widely different cytochrome c2-mediated re-reduction kinetics of the oxidized primary donor

CPMAS NMR data collected from L162YL mutant [4′-13C]Tyr-enriched Rhodobacter sphaeroides RCs reveal that Tyr L162 is in a slightly heterogeneous and probably rigid section of the protein complex. The differences in chemical shifts of the individual components relative to those of the [4′-13C]Tyr Rhodobacter sphaeroides R26 response are 0.2 ppm or less. This is small compared to the total dispersion of [4′-13C] isotropic shifts, ∼ 5 ppm, which measures the shift range due to variations in the microscopic environment between the various tyrosines in the protein complex. The structural changes in the mutant with respect to Rhodobacter sphaeroides R26, as probed by the labels, are thus minimal on the scale of the NMR. This suggests that the dramatic decrease of re-reduction rate of the oxidized primary donor P upon mutation (Farchaus et al., Biochemistry 32 (1993) 10885–10893) cannot be attributed to significant structural changes in the protein. Hence the NMR is in line with the current view that the decrease of the re-reduction rate in the mutant originates from slow reorientation of the docked cytochrome.

13C−1H heteronuclear dipolar correlation studies of the hydrogen bonding of the quinones inRhodobacter sphaeroides R26 reaction centers

The photosynthetic reaction center (RC) of the photosynthetic bacteriumRhodobacter sphaeroides R26 contains two quinones, QA and QB. Solid-state heteronuclear (1H−13C) dipolar correlation spectroscopy has been used to study the binding of the quinones in the ground state for RCs reconstituted with l-13C ubiquinone-10. Lee-Goldburg cross-polarization buildup curves are recorded to determine distancesrCH between the l-13C carbon labels and the protons involved in the polarization transfer. The l-13C of both QA and QB have intermolecular correlations with protons that resonate downfield, in the region of hydrogen-bonding protons. The distances between the carbon labels and the correlated protons are short, 0.21±0.01 nm. Hence the nuclear magnetic resonance provides evidence for strong hydrogen-bonding interactions at the l-C=O of both QA and QB for RCs in the ground state. The environment of the l-13C of the QB is structurally heterogeneous compared to that of the QA. The data can be reconciled with a strong H-bonding interaction of the l-C=O of QA with Ala M260 NH, and with complex hydrogen bonding involving NH of Ile-L224 and of Gly-L225, and possibly the Ser-L223 hydroxyl group of the l-C=O of the QB, in the proximal site.

Evidence from Solid State NMR Correlation Spectroscopy for two Interstack Arrangements in the Chlorosome Antenna System

Solid state cross-polarization (CP) MAS NMR dipolar correlation spectroscopy is a rapidly growing technique that can provide structural information of systems that are inaccessible for X-ray diffraction techniques [1]. In particular, we have implemented correlation spectroscopy to provide a concept for structure determination and to study the chromophore arrangement in antenna systems. Homonuclear (13C-13C) dipolar correlation spectroscopy was used to study the stacking of bacteriochlorophyll c (Bchl c) in uniformly 13C enriched [U-13C] intact chlorosomes from Chlorobium tepidum [2]. It was possible to arrive at a full assignment of the solid state NMR carbon chemical shifts, from which a model for the stack and interstack of Bchl c in the chlorosomes could be resolved [1-3].

MAS NMR Dipolar Correlation Spectroscopy of Partially Deuterated13C- Labelled Chlorophyll a

Chlorophyll (Fig. 1) is a major constituent of photosynthetic complexes. 1H MAS NMR can provide important information about the electronic and spatial structure of chlorophyll in photosynthetic studies. However, the observation of the 1H response in solid-type samples is difficult. The proton resolution in solid state NMR is usually insufficient for direct observation, due the combination of strong homonuclear couplings between protons and a small chemical shift dispersion. To circumvent this problem, 2D and 3D heteronuclear spectroscopy has been applied, and the signals from up to ~ 50 protons in a moderately sized multispin cluster have been assigned [1, 21. In these assignment studies, a high field wideline separation (WISE) and a combination of high field and homonuclear frequency-switched Lee-Goldburg (FSLG) 1H decoupling were applied to resolve the proton response. Here we investigate if additional resolution improvement can be obtained by a suppression of 1H homonuclear dipolar couplings by dilution of the protons with deuterons. It can be stated a priori that dilution has the disadvantage that the overall 1H response is weakened, which will inevitably affect the range of the 1H MAS NMR. The dilution increases the transverse relaxation time and blocks the spin diffusion. Despite of the reduction of the sensitivity, it is important to asses the effect of 2H dilution in high magnetic field, with and without FSLG decoupling. We have chosen to use a moderate dilution level of 75% 2H, reducing the 1H signal strength by a factor of four.