Centerband-only Detection Of Exchange Research Articles

Analytical solutions to several magic-angle spinning NMR experiments

Using the Anderson–Weiss (AW) formalism, analytical expressions of the NMR signal are obtained for the following magic-angle spinning (MAS) experiments: total suppression of sidebands (TOSS); phase adjusted spinning sidebands (PASS); rotational-echo double-resonance (REDOR); rotor-encoded REDOR (REREDOR); cross-polarization magic-angle spinning (CPMAS); exchange induced sidebands (EIS); one-dimensional exchange spectroscopy by sideband alternation (ODESSA); time-reverse ODESSA (trODESSA); centerband-only detection of exchange (CODEX). In order to test the validity of the AW approach, the Gaussian powder approximation is compared with exact powder calculations. A quantitative study of the effect of molecular dynamics on the efficiency of the TOSS and REDOR pulse sequences is then presented.

Slow motion in [ring‐fluoro]polycarbonate by CODEX

AbstractChain dynamics in [ring‐fluoro]polycarbonate (an A‐B alternating copolymer that has a single fluorine substituent on every fourth main chain ring) have been characterized by centerband only detection of exchange (CODEX) and rotating‐frame 13C spin‐lattice relaxation. The slow motions detected by CODEX are facilitated by a mechanically active lattice reorganization that permits a flip of the fluorinated ring about its C2 axis. Nonfluorinated rings undergo small‐amplitude reorientations and C2 flips, both of which are fast and not CODEX active. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1062–1066, 2008

Twist glass transition in regioregulated poly(3-alkylthiophene)

The molecular structure and dynamics of regioregulated poly(3-butylthiophene) (P3BT), poly(3-hexylthiophene) (P3HT), and poly(3-dodecylthiophene) (P3DDT) were investigated using Fourier-transform infrared absorption (FTIR), solid-state $^{13}\mathrm{C}$ nuclear-magnetic resonance (NMR), and differential-scanning calorimetry (DSC) measurements. In the DSC measurements, the endothermic peak was obtained around $340\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in P3BT, and assigned to enthalpy relaxation that originated from the glass transition of the thiophene-ring twist in crystalline phase from results of FTIR, $^{13}\mathrm{C}$ cross-polarization and magic-angle spinning (CPMAS) NMR, $^{13}\mathrm{C}$ spin-lattice relaxation-time measurements, and centerband-only detection of exchange (CODEX) measurements. We defined this transition as a twist-glass transition, which is analogous to the plastic crystal--glassy crystal transition.

Determination of the Oligomeric Number and Intermolecular Distances of Membrane Protein Assemblies by Anisotropic 1H-Driven Spin Diffusion NMR Spectroscopy

Determination of the high-resolution quaternary structure of oligomeric membrane proteins requires knowledge of both the oligomeric number and intermolecular distances. The centerband-only detection of exchange (CODEX) technique has been shown to enable the extraction of the oligomeric number through the equilibrium exchange intensity at long mixing times. To obtain quantitative distances, we now provide an analysis of the mixing-time-dependent CODEX intensities using the 1H-driven spin diffusion theory. The exchange curve is fit to a rate equation, where the rate constants are proportional to the square of the dipolar coupling and the spectral overlap integral between the exchanging spins. Using a number of 13C- and 19F-labeled crystalline model compounds with known intermolecular distances, we empirically determined the overlap integrals of 13C and 19F CODEX for specific spinning speeds and chemical shift anisotropies. These consensus overlap integral values can be applied to structurally unknown systems to determine distances. Applying the 19F CODEX experiment and analysis, we studied the transmembrane peptide of the M2 protein (M2TMP) of influenza A virus bound to 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine bilayers. The experiment proved for the first time that M2TMP associates as tetramers in lipid bilayers, similar to its oligomeric state in detergent micelles. Moreover, the nearest-neighbor interhelical F-F distance between (4-19F)Phe30 is 7.9-9.5 angstroms. This distance constrains the orientation and the packing of the helices in the tetrameric bundle and supports the structural model derived from previous solid-state NMR 15N orientational data. Thus, the CODEX technique presents a general method for determining the oligomeric number and intermolecular distances in the approximately 10 angstroms range in membrane proteins and other complex biological assemblies.

Solid-state exchange NMR characterization of segmental dynamics in glassy poly(alkyl methacrylate)s

Slow side-group and main-chain β-relaxation dynamics in a series of glassy poly(alkyl methacrylate)s (PRMAs) with varying side-group sizes were studied by one-dimensional (1-D) centerband-only detection of exchange (CODEX) and static pure-exchange (PUREX) nuclear magnetic resonance (NMR) techniques. The13COO signal in 1-D PUREX experiments was analyzed to characterize the flips and small-angle motions of the side groups quantitatively. By comparing the13COO exchange intensities obtained from 1-D PUREX and CODEX experiments, it was possible to show that the small-angle reorientations vary between backbone segments. Analysis of the backbone13CH2 CODEX data supplied direct and quantitative information about the amplitudes of the main-chain motion, which is also reflected in the side-group reorientation due to the coupling between side group and backbone. The data obtained from both NMR exchange techniques were correlated, providing an accurate, consistent description of the backbone and side-group motion. The effects of side-group size and temperature on the fraction of flipping side groups were studied systematically by 1-D PUREX NMR. The flipping fraction at ambient temperature decreases with increasing the side-group size. It increases gradually with temperature for two PRMAs with conformationally flexible side groups, while it is constant over wide temperature ranges for poly(methyl methacrylate) and poly(cyclohexyl methacrylate).

Slow Chain Dynamics in Isotactic-poly(4-methyl-1-pentene) Crystallites near the Glass Transition Temperature Characterized by Solid-State 13C MAS Exchange NMR

The chain dynamics for isotactic-poly(4-methyl-1-pentene) (iP4M1P) crystallites near the glass transition temperature (Tg = 304 K) is characterized by solid-state 13C MAS NMR methods at natural abundance. The 13C line width under high-power proton decoupling and the 13C spin−lattice relaxation time in the rotating frame (T1ρc) detect the segmental motions in the amorphous and crystalline regions with correlation times of about 0.2 × 10-5 s at 360−382 K and about 448 K, respectively. Centerband-only detection of exchange (CODEX) with an additional T1ρc filter is applied to investigate the motional geometry and kinetic parameters for the main- and side-chain dynamics in iP4M1P crystallites in a sample. The CODEX evolution-time dependence of the resolved signals indicates a large-angle reorientational process: the simulation of the experimental data of the main-chain CH2 signal reveals that iP4M1P crystallite performs the helical jump motions with jump angles of 72−145° in the disordered 72 helix. The CODEX...

Conformational dynamics of phenylene rings in poly(p-phenylene vinylene) as revealed by 13C magic-angle-spinning exchange nuclear magnetic resonance experiments

Poly (p -phenylene vinylene) (PPV) has shown a great potential for electro-optical applications due to its electroluminescent and semiconducting properties. Such properties are directly related with the polymer chain conformation and dynamics. Then, it is important to understand in detail the local chain motions. In this work, three 13C solid-state magic-angle-spinning (MAS) exchange NMR techniques were used to study conformational dynamics of phenylene rings in PPV. The standard 2D MAS exchange experiment was used to identify exchange processes between equivalent and nonequivalent sites. Centerband-only detection of exchange (CODEX) experiments were applied to determine the amplitude of the phenylene ring flips and small-angle oscillations. Additionally, a new version of the CODEX technique, which allows for the selective observation of segments executing exchange between non-equivalent sites, is demonstrated and applied to determine the flipping fractions and the activation energies of the phenylene ring rotations. It was found that, at −15 °C, (26±3)% of the rings undergo 180° flips in the millisecond time scale, with average imprecision of (30±5)° and activation energies of (23±3) kJ/mol. Other (31±10)% of the rings perform only small-angle oscillations with an average amplitude of (9±2)°. These results corroborate previous experimental data and agree with recent ab initio calculations of potential energies barriers in phenylenevinylene oligomers.

Principles of centerband-only detection of exchange in solid-state nuclear magnetic resonance, and extension to four-time centerband-only detection of exchange

Theoretical principles and experimental details of the centerband-only detection of exchange (CODEX) nuclear magnetic resonance (NMR) experiment for characterizing slow segmental dynamics in solids are described. The experiment, which is performed under magic-angle spinning, employs recoupling of the chemical-shift anisotropy before and after a long mixing time during which molecular reorientations may occur. By an analysis in terms of the difference tensor of the chemical shifts before and after the mixing time, the dependence on the reorientation angle is obtained analytically for uniaxial interactions, and a relation to two-dimensional exchange NMR patterns is established; the same theory can also be applied for analyzing stimulated-echo and pure-exchange NMR data. A favorable linear dependence is derived generally for small rotations, which makes the experiment suitable for detecting small-amplitude motions. Quantification is excellent because the peaks are narrow and intense, unlike the broad powder or sideband spectra that are characteristic of all previous NMR experiments for probing slow segmental rotations. We also introduce and demonstrate a four-time CODEX experiment that yields information previously obtained only in 3D (three-dimensional) and reduced 4D (four-dimensional) exchange NMR experiments, such as the number of orientational sites accessible to the mobile groups. Chemical-shift anisotropies required in the CODEX analysis of motional amplitudes can be estimated using a closely related chemical-shift recoupling experiment. The implementation of total suppression of sidebands before detection is also explained. The experiments are demonstrated on dimethylsulfone, isotactic polypropylene, and poly(methyl methacrylate), PMMA. In isotactic poly(1-butene), the signals of the amorphous and interfacial regions have been observed selectively by using pure-exchange CODEX near the glass transition. The four-time CODEX experiment confirms that in the β-relaxation process of glassy PMMA, fewer than half of the sidegroups perform jumps between two orientations.

Centerband-Only Detection of Exchange: Efficient Analysis of Dynamics in Solids by NMR

Centerband-Only Detection of Exchange: Efficient Analysis of Dynamics in Solids by NMR