Longer-range distances by spinning-angle-encoding solid-state NMR spectroscopy

A new spinning-angle-encoding spin-echo solid-state NMR approach is used to accurately determine the dipolar coupling corresponding to a C-C distance over 4 ? in a fully labelled dipeptide. The dipolar coupling dependent spin-echo modulation was recorded off magic angle, switching back to the magic angle for the acquisition of the free-induction decay, so as to obtain optimum sensitivity. The retention of both ideal resolution and long-range distance sensitivity was achieved by redesigning a 600 MHz HX MAS NMR probe to provide fast angle switching during the NMR experiment: for 1.8 mm rotors, angle changes of up to ～5° in ～10 ms were achieved at 12 kHz MAS. A new experimental design that combines a reference and a dipolar-modulated experiment and a master-curve approach to data interpretation is presented.     

13C/15N distance determination by CPMAS NMR in uniformly 13C labeled molecules

The REDOR and CPMAS techniques are applied for measuring 13C-15N dipolar coupling constants in glycine. It is shown that the selective CP or SPECIFIC CP technique removes the coherent evolution of the spin system under homonuclear 13C-13C J couplings. While the large coupling constant (approximately 900 Hz) is readily determined because of the presence of large oscillations in the CPMAS dynamics, their absence precludes the measurement of the small coupling constant (approximately 200 Hz). The experimental results and numerical simulations demonstrate that the determination of 13C-15N coupling constants of medium size (<1 kHz) by the CPMAS technique is mainly limited by the strength of the 1H decoupling field and the size of the 13C and 15N chemical shift anisotropies.     

Conformational constraints in solid-state NMR of uniformly labeled polypeptides from double single-quantum-filtered rotational echo double resonance

A solid-state nuclear magnetic resonance (NMR) technique is described for obtaining constraints on the backbone conformation of a protein or peptide that is prepared with uniform (15)N,(13)C labeling of consecutive pairs of amino acids or of longer segments. The technique, called double single-quantum-filtered rotational echo double resonance (DSQ-REDOR), uses frequency-selective REDOR to prepare DSQ coherences involving directly bonded backbone (13)CO and (15)NH sites, to dephase these coherences under longer-range (15)NH-(13)CO dipole-dipole couplings in a conformationally dependent manner, and to convert the remaining DSQ coherences to detectable transverse (13)C-spin polarization. The efficacy of DSQ-REDOR is demonstrated in experiments on two isotopically labeled samples, the helical peptide MB(i + 4)EK and the amyloid-forming peptide Abeta(11-25).     

Estimation of carbon-carbon bond lengths and medium-range internuclear distances by solid-state nuclear magnetic resonance.

We describe magic-angle-spinning NMR methods for the accurate determination of internuclear dipole-dipole couplings between homonuclear spins-(1/2) in the solid state. The new sequences use symmetry principles to treat the effect of magic-angle sample-rotation and resonant radio frequency fields. The pulse-sequence symmetries generate selection rules which reduce the interference of undesirable interactions and improve the robustness of the pulse sequences with respect to chemical shift anisotropies. We show that the pulse sequences may be used to estimate distances between 13C spins in organic solids, including bond lengths in systems with large chemical shift anisotropies, such as conjugated systems. For bond-length measurements, the precision of the method is +/-2 pm with a systematic overestimate of the internuclear distance by 3 +/- 1 pm. The method is expected to be a useful tool for investigating structural changes in macromolecules.     

A new method of calculation of internuclear distances in molecules and crystals

It was shown that the earlier derived equation for calculation of the lattice energy of ionic crystals contains a term that can characterize the radius of the overlap region of the electron clouds of interacting atoms. On this basis, simple and rather exact formulas were derived for the calculation of internuclear distances in crystals and molecules. The error of calculation R ₁₂ is from 1.2% (for crystalline Group IA metal halides) to 3.8–5.7% (for various gaseous molecules). The coordination of ions was shown to be essential for determining internuclear distances. The adequacy of the Stuart-Briegleb model of molecular structure was confirmed.     

Selective internuclear coupling estimation in the solid-state NMR of multiple-spin systems

A new solid-state NMR method is presented for estimating homonuclear dipole-dipole couplings for selected groups of nuclear spins in a multiple-spin coupled network. The methodology combines off-magic-angle spinning, frequency selective spin echoes, and multiple quantum filtering. The new method is insensitive to incoherent relaxation effects and may be used to estimate weak couplings. Internuclear (13)C-(13)C couplings are estimated in uniformly (13)C-labelled l-Histidine·HCl·H(2)O. Weak intermolecular couplings between (13)C nuclei separated by distances exceeding 6 ? are estimated.     

Accurate measurement of 13C-15N distances with solid-state NMR

Solid-state NMR technique for measuring distances between heteronuclei in static powder samples is described. It is based on a two-dimensional single-echo scheme enhanced with adiabatic cross polarization. As an example, the results for intramolecular distances in alpha-crystalline form of glycine are presented. The measured NMR distances (13)C(alpha)-(15)N and (13)C(')-(15)N are 1.496+/-0.002 and 2.50+/-0.02 A, respectively.     

Dynamics of p-nitroaniline molecules in micoporous aluminophosphate AlPO4-5 studied by solid-state NMR

Dynamics of deuterated p-nitroaniline (pNA-d) molecules in the micropores of AlPO4-5 has been investigated by means of solid-state NMR. The adsorbed amounts of pNA-d were 5.0 and 10.1 mass % of the total mass. We have measured 13C magic-angle-spinning (MAS) and 2H NMR spectra of the guest molecules and 31P and 27Al MAS NMR spectra of the host framework. The pNA-d molecules distribute rather inhomogeneously in the channel, and do not coordinate to Al strongly like H2O. The intermolecular hydrogen bonds are formed between a part of the guest molecules only when the loading level is high. The 2H NMR spectra are successfully analyzed, elucidating the orientation and the motion of the guest molecules. The molecular axis of pNA-d is inclined to the channel axis, and the molecular plane is perpendicular to the inner wall. The guest molecule jumps among 12 sites or 12 orientations. This motion is faster in the sample of 5.0 mass % than in the sample of 10.1 mass %, suggesting that the guest-guest interaction hinders the motion. The mean residence times of the molecules are estimated from the analysis of the 2H NMR spectra, which are affected by the size of the nanospace as well as the property of the adsorbed site.     

Shrinkages of the internuclear distances in some hexafluorides of octahedral symmetry

A brief analysis of the shrinkages or shortening of the internuclear distances in many molecular systems has been made. Analytical expressions for the linear shrinkage and nonlinear shrinkage have been derived in terms of the meansquare perpendicular amplitudes. Analytical expressions for the generalized mean-square amplitudes (mean-square parallel amplitudes, mean-square perpendicular amplitudes, and mean cross products) for an octahedralXY ₆ molecule have been given in terms of the mean-square amplitude matrices. The linear and nonlinear shrinkages for the hexafluorides of sulfur, selenium, tellurium, molybdenum, technetium, ruthenium, rhodium, tungsten, rhenium, osmium, iridium, platinum, uranium, neptunium, and plutonium have been computed at 298°K and 500°K, and the results have been briefly discussed.This paper is based on a thesis to be submitted byThomas S. Adams to the Graduate School of the Valdosta State College in partial fulfillment of the requirements for the degree of Master of Science.     

Determination of membrane protein structure and dynamics by magic-angle-spinning solid-state NMR spectroscopy

It is shown that molecular structure and dynamics of a uniformly labeled membrane protein can be studied under magic-angle-spinning conditions. For this purpose, dipolar recoupling experiments are combined with novel through-bond correlation schemes that probe mobile protein segments. These NMR schemes are demonstrated on a uniformly [13C,15N] variant of the 52-residue polypeptide phospholamban. When reconstituted in lipid bilayers, the NMR data are consistent with an alpha-helical trans-membrane segment and a cytoplasmic domain that exhibits a high degree of structural disorder.     

Relations between mean amplitudes of vibration and corresponding internuclear distances : II. Bonded carbon-carbon distances

The empirical relation u(CC_bond) = −0.071856 + 0.124162 r − 0.028974 r² was established from a statistical treatment of 57 electron diffraction measurements (1.2086 r 1.549, where r is the bond distance, and u is the mean amplitude of vibration; both in Å units). The agreement between observed and calculated u values is significantly improved when compared with the corresponding treatment of bonded and non-bonded CC distances taken together.