Phospholipid head group dynamics have been studied by pulsed phosphorus-31 nuclear magnetic resonance (31P-NMR) of unoriented and macroscopically aligned dimyristoylphosphatidylcholine model membranes in the temperature range, 203-343 K. Lineshapes and echo intensities have been recorded as a function of interpulse delay times, temperature and macroscopic orientation of the bilayer normal with respect to the magnetic field. The dipolar proton-phosphorus (1H-31P) contribution to the transverse relaxation time, T2E, and to lineshapes was eliminated by means of a proton spin-lock sequence. In case of longitudinal spin relaxation, T1Z, the amount of dipolar coupling was evaluated by measuring the maximum nuclear Overhauser enhancement. Hence, the results could be analyzed by considering chemical shift anisotropy as the only relaxation mechanism. The presence of various minima both in T1Z and T2E temperature plots as well as the angular dependence of these relaxation times allowed description of the dynamics of the phosphate head group in the 31P-NMR time window, by three different motional classes, i.e., intramolecular, intermolecular and collective motions. The intramolecular motions consist of two hindered rotations and one free rotation around the bonds linking the phosphate head group to the glycerol backbone. These motions are the fastest in the hierarchy of time with correlation times varying from less than 10(-12) to 10(-6) s in the temperature range investigated. The intermolecular motions are assigned to phospholipid long axis rotation and fluctuation. They have correlation times ranging from 10(-11) s at high temperatures to 10(-3) s at low temperatures. The slowest motion affecting the 31P-NMR observables is assigned to viscoelastic modes, i.e., so called order director fluctuations and is only detected at high temperatures, above the main transition in pulse frequency dependent T2ECP experiments. Comprehensive analysis of the phosphate head group dynamics is achieved by a dynamic NMR model based on the stochastic Liouville equation. In addition to correlation times, this analysis provides activation energies and order parameters for the various motions, and a value for the bilayer elastic constant.

The structural investigation of large RNP complexes by X-ray crystallography can be a difficult task due to the flexibility of the RNA and of the protein-RNA interfaces, which may hinder crystallization. In these cases, NMR spectroscopy is an attractive alternative to crystallography, although the large size of typical RNP complexes may limit the applicability of solution NMR. Solid-state NMR spectroscopy, however, is not subject to any intrinsic limitations with respect to the size of the object under investigation, with restrictions imposed solely by the sensitivity of the instrumentation. In addition, it does not require large, well-ordered crystals and can therefore be applied to flexible, partially disordered complexes. Here we show for the first time that solid-state NMR spectroscopy can be used to probe intermolecular interactions at the protein-RNA interface in RNP complexes. Distances between the (15)N nuclei of the protein backbone and the (31)P nuclei of the RNA backbone can be measured in TEDOR experiments and used as restraints in structure calculations. The distance measurement is accurate, as proven for the test case of the L7Ae-box C/D RNA complex, for which a crystal structure is available. The results presented here reveal the as yet unexplored potential of solid-state NMR spectroscopy in the investigation of large RNP complexes.

Transverse nuclear spin relaxation measurements employing Carr−Purcell (CP) pulse sequences can provide detailed information on the slow-motional dynamics in biomembranes. In this paper, a comprehensive relaxation model is developed for the analysis of such experiments performed on unilamellar quasi-spherical vesicles. The basis of the model is the stochastic Liouville equation in which two different relaxation processes are considered (i.e., vesicle shape fluctuations and molecular translational diffusion). It is shown that for vesicle radii R 0 ≥ 200 nm, translational diffusion of the lipid molecules along the vesicle shell is too slow to contribute significantly to transverse spin relaxation in the kHz range, whereas vesicle shape fluctuations constitute the dominant transverse relaxation process. The theory is employed in model calculations for pulse frequency-dependent transverse 31P nuclear spin relaxation rates, (ω), from CP sequences. The analysis reveals that (ω), induced by vesicle fluctuations, depends linearly on ω-1 over a wide frequency range in the kHz regime. Notably, within this linear dispersion regime, the bending elastic modulus κ is the only relevant parameter because the magnitude of (ω) does not depend on the size of the vesicle R 0, the effective lateral tension σ, or the viscosity of the surrounding fluid η. On the other hand, R 0, σ, η, and κ determine the frequency at which (ω) levels off to a constant “plateau” value independent of ω. Thus, analysis of the (ω) dispersion profiles is a direct way to determine the bending elastic modulus and other viscoelastic parameters of membrane vesicles.

Contrary to common belief a single C-H … F-C contact in [(g 5 -C 5 H 5 )Pd(C 6 F 5 )(PPh 3 )] is strong enough to pair two independent molecules and render them crystallographically different as suggested by its strongest 1 H-19 F dipole-dipole coupling in a 2-D CP/MAS PILGRIM NMR experiment together with solid state 1-D 19 F{ 1 H} and 2-D 19 F RFDR NMR spectroscopy -thereby proving the power of 2-D solid-state NMR for assessing the strength of supramolecular contacts.Organometallic complexes of palladium containing cyclopentadienyl (Cp) ligands coordinated to the metal in a g 5 -fashion are rare. 1,2 Consequently, the chemistry of g 5 -CpPd is not as developed as that of earlier transition metals. We have found that the complex [(g 5 -Cp)Pd(C 6 F 5 )(PPh 3 )] 1 can be easily prepared in excellent yields by reacting the chloro-bridging organopalladium dimer [{(Pd(C 6 F 5 )(PPh 3 )} 2 (m-Cl) 2 ] with cyclopentadienylthallium:The complex is air-stable, both in the solid state and in solution.The new complex has been characterized by partial elemental analyses and spectroscopic (IR and 1 H, 19 F, 13 C and 31 P NMR) methods. The 19 F NMR spectrum in solution of complex 1 reveals the presence of a freely rotating pentafluorophenyl ring which gives three resonances (in the ratio 2 : 2 : 1) for the o-, m -, and p -fluorine atoms.Red crystals suitable for X-ray diffraction studies were obtained by slow diffusion of hexane into a dichloromethane solution of 1. Crystallographic data sets were collected at 100 and 293 K.{ The structural features for both the low and room temperature structure are the same and do not necessitate a distinction in the following discussion. Fig. 1 shows the asymmetric unit in the

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