Cross-polarization Magic Angle Spinning NMR Research Articles

Enrichment of iron(III), cobalt(II), nickel(II), and copper(II) by solid-phase extraction with 1,8-dihydroxyanthraquinone anchored to silica gel before their determination by flame atomic absorption spectrometry.

A chelating matrix prepared by immobilizing 1,8-dihydroxyanthraquinone on silica gel modified with 3-aminopropyltriethoxysilane has been characterized by use of cross-polarization magic angle spinning (CPMAS) NMR, diffuse reflectance infrared Fourier transformation (DRIFT) spectroscopy, and thermogravimetric analysis and used to preconcentrate Fe(III), Co(II), Ni(II), and Cu(II) before their determination by flame atomic absorption spectrometry. The optimum pH ranges for quantitative sorption are 6.5-8.0, 6.0-7.0, 6.0-8.0, and 7.0-8.5 for Cu, Fe, Co, and Ni, respectively. All the metal ions can be desorbed with 2 mol L(-1) HCl or HNO3. The sorption capacity ( micromol g(-1) matrix) and preconcentration factor were 226.6, 250; 365.6, 300; 101.8, 150; and 109.0, 250 for Cu, Fe, Co, and Ni, respectively. The lowest concentration for quantitative recovery was 4.0, 3.3, 6.6, and 4.0 ng mL(-1), respectively for the four metal ions. The limits up to which electrolytes NaNO3, NaCl, NaBr, Na2SO4, and Na3PO4 and cations Ca(II) and Mg(II) can coexist with the four metal ions during their sorption without adverse effect are reported. The simultaneous enrichment and determination of all the four metals is possible if the total load of metal ions is less than the sorption capacity. Flame AAS was used to determine the metal ions in underground, tap, and river water samples (RSD<or=7.1%) after their enrichment with the matrix. The cobalt content of pharmaceutical samples (multivitamin tablet) preconcentrated by use of this silica gel matrix and determined by FAAS was found to be 2.00 microg g(-1), with RSDtilde;1.7% (reported concentration level, 1.99 microg g(-1)).

Fractionation of natural organic matter in drinking water and characterization by 13C cross-polarization magic-angle spinning NMR spectroscopy and size exclusion chromatography.

Natural organic matter from drinking water sources was fractionated, and the fractions were characterized by NMR and SEC with the aim of relating NOM structure to treatability. Organic matter was isolated from two Australian surface waters (Horsham, Moorabool) by reverse osmosis and from a groundwater (Wanneroo) by anion exchange. The isolates were fractionated according to polarity and charge by resin adsorption. 13C NMR spectra of the freeze-dried fractions showed the most hydrophobic fraction to be high in aliphatic and aromatic carbon while slightly hydrophobic organics have more carbonyl and alkoxyl carbon. The Horsham and Wanneroo hydrophilic fractions show strong alkoxyl signals attributed to carbohydrate. Moorabool hydrophilics contain aromatic (phenolic) carbon; the apparent absence of carbohydrate appears to be an artifact. Size-exclusion chromatograms were recorded on the original and fractionated organics with both UV and dissolved organic carbon detection. The Horsham and Moorabool organics have similar molecular size distributions while Wanneroo is dominated by strongly absorbing species having large hydrodynamic radii. The hydrophobic and charged hydrophilic fractions also have high apparent MW, while the neutral fraction is higher in low-MW compounds of relatively low specific absorbance, suggestive of carbohydrates.

X-ray diffraction, Raman spectroscopic, and solid-state NMR studies of the group 14 metal-(tetracarbonyl)cobalt complexes Ph3MCo(CO)4(M = Si, Sn, Pb)

Single-crystal X-ray diffraction studies illustrate that the three title compounds are isomorphous, belonging to the triclinic space group P[Formula: see text], with slightly distorted trigonal bipyramidal geometry about cobalt. The solid-state29Si,119Sn, and207Pb cross-polarization magic angle spinning (CP MAS) NMR spectra are presented. The indirect spin–spin coupling constant (J), quadrupolar–dipolar shift (d), direct dipolar coupling constant (D' ), anisotropy in spin–spin coupling (ΔJ), and the chemical shift tensor were extracted. A plot of the reduced coupling constant vs. s-electron densities at the nucleus indicates that the Fermi contact term may be dominant for the tin and lead complexes; however, the large ΔJ for all complexes indicate that there are also significant anisotropic terms. Trends in the Raman scattering spectra are also discussed.Key words:29Si,119Sn, and207Pb CP MAS NMR, tetracarbonyl cobalt, spin–spin coupling, chemical shift tensor, quadrupole coupling, Fermi contact, cobalt–group 14.

Copper(II) and nickel(II) uptake from aqueous solutions by cork wastes: a NMR and potentiometric study

Due to the increasing interest in the use of biomasses from agriculture as removing agents for heavy metals in waste effluents, we present here a joint potentiometric and 13C MAS and cross polarization-magic angle spinning NMR (CP-MAS NMR) study on cork interaction with Cu(II) and Ni(II). Through potentiometry we were able to distinguish two kind of interactions, a stronger one which involves protonated sites on cork and a second that allows hydroxide precipitation of the bound metal ion into the cork structure. NMR signal analysis suggests a specific metal complexation on the carbohydrate moieties of polymeric cork matrix. The 13C MAS NMR suggests that the dynamics in the MHz range and the overall arrangement of polymeric matrix is not affected by metal binding; and the spin relaxation times T 1ρ( 1H) and T 1ρ( 13C) show that no significant difference of relaxation processes in the KHz range is introduced by metal coordination.

Elemental Quantitation of Natural Organic Matter by CPMAS 13C NMR Spectroscopy

Cross-polarized magic-angle-spinning NMR (CPMAS-NMR) techniques are assumed to be only semi-quantitative in the assessment of carbon distribution in humic substances or natural organic matter, due to a number of interferences such as spinning side bands (SSB) in spectra, paramagnetic species in samples, and low or remote protonation of aromatic carbons. Fast rotor spin rates or direct polarization NMR techniques are normally applied to improve quantitative signal detectability. Variable contact time pulse sequences were used here to obtain CPMAS-NMR spectra of organic compounds of known structure and different humic substances. Integration of spectral areas, previously subtracted of SSB, and relative stoichiometric factors were used for mathematical elaboration to calculate the elemental content in samples. These values did not significantly differ from those obtained by direct determination of elemental content with quantitative elemental analysis. Our results showed that the carbon observed CPMAS-NMR provides a quantitative representation of the whole carbon content in humic substances.

Chain Dynamics, Conformations, and Phase Transformations for Form III Polymorph of Isotactic Poly(1-butene) Investigated by High-Resolution Solid-State13C NMR Spectroscopy and Molecular Mechanics Calculations

High-resolution solid-state 13C NMR spectroscopy and MM3 molecular mechanics calculations have been applied to investigate side-chain conformations, side- and main-chain dynamics, and phase transformations of the form III polymorph of isotactic-poly(1-butene) (i-PB). MM3 calculations indicate that the t−g‘ side-chain conformation is much more stable than the g‘−g and g−t side-chain conformations in the crystal lattices of form III. The 13C cross-polarization magic-angle spinning (CPMAS) NMR spectrum at 199 K shows that the methyl group in the side chain adopts only the t−g‘ conformation. Upfield shift for the 13C methyl signal above 338 K indicates dynamic disorder in the side-chain conformations. The 13C two-dimensional exchange NMR reveals that the polymer chains in form III consist of two components. The 74% polymer chains execute the 90° helical jump motion with an activation energy of 79.3 ± 5.6 kJ/mol, while the residual chains are immobile. It is found that the 90° helical jump motion begins to occur at the temperature very close to the glass-transition temperature (Tg). Upfield shifts for the 13C main-chain signals at 338 K indicate the motional mode change from the 90° helical jump motion to “more or less” rotational or jump motion with different angle displacements. Above 338 K, two phase transformations from form III to forms II and I‘ are confirmed in the 13C CPMAS spectra. The 1H spin−lattice relaxation times in the rotating frame show that the transformation from form III to form I‘ proceeds within the form III phase via the crystal-to-crystal process, whereas the line-shape analyses of the 13C direct-polarization magic-angle spinning (DPMAS) NMR spectra indicate that the transformation from form III to form II proceeds via the quasi-melting/recrystallization process. The frozen chain conformations in the amorphous phase below Tg are also investigated on the basis of the 13C chemical shifts. It is suggested that most of the main chains adopt the helical (tg‘)n conformation, and the residual part adopts the (tt) conformation.

High-Resolution 13C CP/MAS NMR Study on Structure and Structural Transition of Antheraea pernyi Silk Fibroin Containing Poly(l-alanine) and Gly-Rich Regions

The structure and structural transition of silk fibroin from a wild silkworm Antheraea pernyi (A. pernyi), whose amino acid sequence consists of poly(l-alanine) (PLA) and a Gly-rich region, were studied with solid-state 13C cross-polarization magic angle spinning (CP/MAS) NMR. As such, there has been limited information on the inherent “conformationally flexible” Gly-rich region of A. pernyi, presumably due to lack of an appropriate analytical technique required to contribute for the understanding of unique structural properties. The effective use of conformation-dependent 13C NMR chemical shifts of isotopically labeled silk sample may overcome such limitations and prove valuable the structural analysis. Thus, the Cα and Cβ carbons of Ser residue, Cβ carbon of Tyr residue, and the CO carbon of Gly residue were 13C-isotope-labeled, and the conformation-dependent 13C chemical shifts of these residues were used for elucidating the conformation. In the silk fibroin film prepared from the silk gland 65% of Ser residues are in the α-helical state. These Ser residues with α-helix form can be assigned to those located at the N-terminal of PLA and are considered to be incorporated into the α-helix of PLA. The Tyr and other Ser residues take the random coil form. The structural transition from α-helix to β-sheet in the PLA region occurs by immersing the film in 3:2 methanol−water mixture, but 20% of Ala residues still remain as an α-helix. Eighty percent of the Ser residues take the β-sheet, including a small amount of random coil form. The Gly carbonyl carbon peaks shift to a higher field by 1.4 ppm (172.3−170.9 ppm) when the helix to β-sheet transition of PLA region occurs, indicating that the structural change from random coil to β-sheet structure occurs in the Gly residues although the distribution in the conformation is large, as judged from the broad Gly CO peak. Most of the Tyr residue remains as a random coil after the structural transition of the PLA region.

Non-selective signal loss in the 13C CPMAS NMR spectra of soil organic matter. Investigations of particle size fractions.

In 13C cross-polarisation magic angle spinning (CPMAS) spectra of soil samples, the paramagnetic soil constituents partially discriminate against the signal intensity of the 13C atoms. The aim of the present study was to check to what extent this signal depression depends on the paramagnetic ion content and if it is selective for a certain kind of carbon species. The spectra of a variety of particle size fractions of five quite different soils were recorded. A procedure was developed to compare quantitatively the relative carbon content of the different spectra. It was found that iron ions differ in their efficiency to discriminate against the carbon signals. Nevertheless a strong linear correlation between the detectability of carbon and cube root(wt.%Fe) was observed for both the total signal and the signal of the different carbon species as well. This underlines that iron ions mainly influence the depression of the 13C signal in soil spectra. Furthermore we found, that for C: Fe ratios of 1.5 to 20 non-selective signal losses dominate. Despite a high percentage of non-detectable carbon (up to 90%), the 13C CPMAS spectra of our soil samples correctly reflect the relative composition of soil organic matter.

Structural studies of 4-aryloctahydro-pyrido[1,2- c]pyrimidine derivatives

13C cross-polarisation magic angle spinning NMR data have been reported for four derivatives of 4-aryl-octahydro-pyrido[1,2- c]pyrimidine-1,3-dione and the X-ray diffraction data for two (with 2′-Me and 2′-OMe). The crystal structures show the presence of centrosymmetric cyclic dimers with intermolecular C1O⋯H–N or C3O⋯H–N hydrogen bonds, the configuration at the chiral centres (C4 and C4 a) was determined as RR (SS). The twisting of aromatic ring at C4 with respect to the pyrido[1,2- c]pyrimidine skeleton is about 68–109°.

High-resolution and cross-polarization magic angle spinning NMR studies of tricarbonylchromium complexes with polycyclic aromatic ligands

The strategy of investigation of the structures and transformations of organometallic compounds by high-resolution NMR spectroscopy in solutions and in the solid state (cross-polarization magic angle spinning NMR) was considered in relation to tricarbonylchromium complexes with polycyclic aromatic ligands.

Solid‐state 1H → 19F/19F → 1H CP/MAS NMR study of poly(vinylidene fluoride)

AbstractSolid‐state 1H → 19F and 19F → 1H cross‐polarization magic angle spinning (CP/MAS) NMR spectra have been investigated for a semicrystalline fluoropolymer, namely poly(vinylidene fluoride) (PVDF). The 1H → 19F CP/MAS spectra can be fitted by five Lorentzian functions, and the amorphous peaks were selectively observed by the DIVAM CP pulse sequences. Solid‐state spin‐lock experiments showed significant differences in T1ρF and T1ρH between the crystalline and amorphous domains, and the effective time constants, THF* and T1ρ*, which were estimated from the 1H → 19F CP curves, also clarify the difference in the strengths of dipolar interactions. Heteronuclear dipolar oscillation behaviour is observed in both standard CP and 1H → 19F inversion recovery CP (IRCP) experiments. The inverse 19F → 1H CP‐MAS and 1H → 19F CP‐drain MAS experiments gave complementary information to the standard 1H → 19F CP/MAS spectra in a manner reported in our previous papers for other fluoropolymers. The value of NF/NH (where N is a spin density) estimated from the CP‐drain curve is within experimental error equal to unity, which is consistent with the chemical structure. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Double Resonance NMR and Molecular Simulations of Hydrofluorocarbon Binding on Faujasite Zeolites NaX and NaY: the Importance of Hydrogen Bonding in Controlling Adsorption Geometries

Double resonance NMR experiments have been used to study the binding of the asymmetric hydrofluorocarbons (HFCs) CF3CFH2 (HFC-134a), CF3CF2H (HFC-125) and CF2HCFH2 (HFC-143) on zeolites NaX and NaY. By exploiting the very large differences in 19F chemical shifts for the −CF3-xHx end groups of each molecule, individual 19F → 23Na cross-polarization (CP) build-up curves involving polarization transfer from different parts of the molecule have been obtained. CP efficiencies in the order CF3≪CF2H<CFH2 were found, indicating that the hydrogen-containing groups are bound more strongly to the zeolite framework. This effect is most pronounced for the lowest-sodium content zeolite studied (NaY: Si/Al = 7.6), and increases with HFC loading. Both the 1H NMR resonances and 1H → 27Al and 1H → 17O CP MAS NMR experiments are consistent with H-bonding interactions with the zeolite framework. Molecular dynamics and docking calculations for HFC-134 and 134a on model NaY and NaX zeolites revealed the importance of both H-bonding and Na−F interactions in determining the low-energy sorption sites. Stable binding sites for HFC-134a in NaY were found by docking to involve only CFH2−Na(SII) contacts, in qualitative agreement with the CP results. The simulations indicate that Na-binding to the groups with more H atoms is favored by the possibility of achieving shorter Na−F distances and maximizing the number of H-bonds with the framework. The higher sodium content zeolite containing both SII and SIII cations gives minimum energy binding sites involving multiple Na−F and H-bonding contacts. For HFC-134 (gauche conformer), the global energy minimum actually involves several H-bonds but no short Na−F contacts.

Optically Active Ortho-Metalated Half-Sandwich Ruthenium Complexes: Solid-State NMR as a Convenient Tool To Analyze Mixtures of Diastereomers

Bulk solid samples of various ratios of the cyclometalated arene ruthenium diastereomers (S)Ru- and (R)Ru-[(eta(6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)PMe2Ph](+)PF6(-) (3a/3b), of which the configurational stability at the metal center has been established by classical solution techniques, have been analyzed by the 13C cross-polarization magic angle spinning (CP-MAS) and 31P MAS NMR. The spectra obtained allowed us to detect both isomers and to estimate their respective proportions by 31P spectra. This technique was applied to a bulk solid sample of the diastereomers (S)Ru- and (R)Ru-[(eta(6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)NCMe](+)PF6(-) (1a/1b), which were shown to be configurationally labile by classical solution experiments. Detection of isomer 1a only in the resulting 13C CP-MAS NMR spectrum demonstrated that there has been epimerization of 1b to 1a during crystallization, thus confirming the configurational lability at the metal center.

29Si nuclear-magnetic-resonance and vibrational spectroscopy studies of SiO2–TiO2 powders prepared by the sol-gel process

The sol-gel synthesis and structural characterization of SiO2–TiO2 powders are reported. Samples with titania concentrations in the range of 0 to 30 mol%, calcined at temperatures between 120 and 1300 °C, were characterized by 29Si magic-angle-spinning nuclear-magnetic-resonance (MAS NMR) and cross polarization MAS NMR, Raman and Fourier transform infrared spectroscopies, and powder x-ray diffraction. Tetrahedrally coordinated Ti (IV) ions were found in the titanosilicate matrix. When the titania content increases beyond approximately 10 mol% and upon calcination at temperatures over approximately 700 °C, the segregation of an amorphous silica-rich phase occurs, together with the precipitation of anatase nanocrystallites with average sizes between 3 and 17 nm, depending on the temperature. An increase in the heat treatment temperature, or the titania content, leads to an increase in the degree of homocondensation (Si–O–Si and Ti–O–Ti bonds) and a simultaneous reduction in the degree of heterocondensation (Si–O–Ti bonds).

A model of reversible inhibitors in the gastric H+/K+-ATPase binding site determined by rotational echo double resonance NMR.

Several close analogues of the noncovalent H(+)/K(+)-ATPase inhibitor SCH28080 (2-methyl-3-cyanomethyl-8-(phenylmethoxy)imidazo[1,2-a]pyridine) have been screened for activity and examined in the pharmacological site of action by solid-state NMR spectroscopy. TMPIP, the 1,2,3-trimethyl analogue of SCH28080, and variants of TMPIP containing fluorine in the phenylmethoxy ring exhibited IC(50) values for porcine H(+)/K(+)-ATPase inhibition falling in the sub-10 microm range. Deuterium NMR spectra of a (2)H-labeled inhibitor titrated into H(+)/K(+)-ATPase membranes revealed that 80-100% of inhibitor was bound to the protein, and K(+)-competition (2)H NMR experiments confirmed that the inhibitor lay within the active site. The active binding conformation of the pentafluorophenylmethoxy analogue of TMPIP was determined from (13)C-(19)F dipolar coupling measurements using the cross-polarization magic angle spinning NMR method, REDOR. It was found that the inhibitor adopts an energetically favorable extended conformation falling between fully planar and partially bowed extremes. These findings allowed a model to be proposed for the binding of this inhibitor to H(+)/K(+)-ATPase based on the results of independent site-directed mutagenesis studies. In the model, the partially bowed inhibitor interacts with Phe(126) close to the N-terminal membrane spanning helix M1 and residues in the extracellular loop bridging membrane helices M5 and M6 and is flanked by residues in M4.

Side-chain intercrossing of poly(alkyl glutamate): LB and CP-MAS studies

The side-chain intercrossing of the substituted polyglutamate has been suspected as the main reason for the inhomogeneous monolayer at the air/water interface as well as the multi-domains in bulk. Before the side-chain melting at 60°C, the substituted chain intercrossing was confused by several experimental approaches. Here, we investigated the side-chain intercrossing of ethyl, hexyl, and stearyl substituted polyglutamate by surface pressure–surface area (π–A) isotherms and CP-MAS (cross-polarization magic angle spinning) NMR. We verify that the solvent drying process may be the main cause of the intercrossing of the side-chains in the glutamate polymers and provide the quantitative information about the intercrossing ratios for the side-chain polyglutamates. At room temperature, 60–90% of the total length of the side-chain intercrossed and the intercrossing ratios decreased at higher temperatures.

Determination of the degree of acetylation of chitin materials by 13C CP/MAS NMR spectroscopy

13C CP/MAS NMR spectroscopy has been shown to be a powerful tool to quantify the degree of acetylation of chitin and chitosan. In order to optimise the parameters which afford quantitative 13C cross-polarisation magic-angle spinning NMR spectra, a detailed relaxation study has been carried out on selected chitin and deacetylated chitin samples. A relaxation delay of 5 s and a contact time of 1 ms have been found to yield quantitative NMR spectra of samples with deacetylation degree values of 0.68 and 0.16. The measured spin-lattice relaxation times in the rotating frame, T 1ρH, are in the range 6.4–8.9 ms for chitin and 4.3–7.3 ms for deacetylated chitin, while T CH values for both samples are very similar and range from 0.03 to 0.19 ms. Spin-counting experiments indicate that, within experimental error, all carbon is detected by NMR indicating that the samples studied contain no (or very few) paramagnetic centres.

Cytoplasmic surface structure of bacteriorhodopsin consisting of interhelical loops and C-terminal alpha helix, modified by a variety of environmental factors as studied by (13)C-NMR.

We have examined the (13)C-NMR spectra of [3-(13)C] Ala-labeled bacteriorhodopsin and its mutants by varying a variety of environmental or intrinsic factors such as ionic strength, temperature, pH, truncation of the C-terminal alpha helix, and site-directed mutation at cytoplasmic loops, in order to gain insight into a plausible surface structure arising from the C-terminal alpha helix and loops. It is found that the surface structure can be characterized as a complex stabilized by salt bridges or metal-mediated linkages among charged side chains. The surface complex in bacteriorhodopsin is most pronounced under the conditions of 10 mM NaCl at neutral pH but is destabilized to yield relaxed states when environmental factors are changed to high ionic strength, low pH and higher temperature. These two states were readily distinguished by associated spectral changes, including suppressed (cross polarization-magic angle spinning NMR) or displaced (upfield) (13)C signals from the C-terminal alpha helix, or modified spectral features in the loop region. It is also noteworthy that such spectral changes, when going from the complexed to relaxed states, occur either when the C-terminal alpha helix is deleted or site-directed mutations were introduced at a cytoplasmic loop. These observations clearly emphasize that organization of the cytoplasmic surface complex is important in the stabilization of the three-dimensional structure at ambient temperature, and subsequently plays an essential role in biological functions.

Cocrystallization and phase segregation of blends of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The competition between cocrystallization and phase segregation in blends of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB–HV) containing 9 and 15% HV has been studied by 13C cross-polarization magic-angle sample spinning (CPMAS) NMR and DSC techniques. The PHB homopolyester samples with selectively 13C-enriched methylene carbon (PHB∗) were used. Assuming a simple two-phase model, the 13C resonances were resolved into two peaks arising from the crystalline and amorphous phases. Owing to the 13C-enrichment of PHB∗, the relative areas of the crystalline peaks for the methylene and the methine carbon resonances on 13C CPMAS NMR spectra change depending on the PHB∗ content in the crystalline phase. By comparing the ratio of these areas for the blends with those for pure PHB∗ and PHB–HV, the composition in the crystalline phase of the blends was determined. For PHB∗/PHB–9%HV blends, the composition in the crystalline phase is very similar to that in the blend. This means that almost perfect cocrystallization of PHB and PHB–9%HV occurs in these blends. For PHB∗/PHB–15%HV blends, the PHB content in the crystalline phase is larger than that in the whole blend. The phase segregation precedes the crystallization in these blends. Therefore, the degree of phase segregation (or the percentage of PHB–HV that segregates from the growth front of crystals) changes depending on the HV content of PHB–HV. As the HV content increases, the copolymer content in the crystalline phase decreases.

Surface dynamics of bacteriorhodopsin as revealed by (13)C NMR studies on [(13)C]Ala-labeled proteins: detection of millisecond or microsecond motions in interhelical loops and C-terminal alpha-helix.

We have recorded (13)C NMR spectra of [2-(13)C]-, [1-(13)C]-, [3-(13)C],- and [1,2,3-(13)C(3)]Ala-labeled bacteriorhodopsin (bR), and its mutants, A196G, A160G, and A103C, by means of cross polarization-magic angle spinning (CP-MAS) and dipolar decoupled-magic angle spinning (DD-MAS) techniques, to reveal the conformation and dynamics of bR, with emphasis on the loop and C-terminus structures. The (13)C NMR signals of the loop (C-D, E-F, and F-G) regions were almost completely suppressed from [2-(13)C]-, [1-(13)C]Ala-, and [1-(13)C]Gly-labeled bR, due to the presence of conformational fluctuation with correlation times of 10(-4) s that interfered with the peak-narrowing by magic angle spinning. The observation of such suppressed peaks for specific residues provides a unique means of detecting intermediate frequency motions on the time scale of ms or micros in the surface loops of membrane proteins. Instead, the three well-resolved (13)C CP-MAS NMR signals of [2-(13)C]Ala-bR, at 50.38, 49.90, and 47.96 ppm, were ascribed to the C-terminal alpha-helix previously proposed from the data for [3-(13)C]Ala-bR: the former two peaks were assigned to Ala 232 and 238, in view of the results of successive proteolysis experiments, while the highest-field peak was ascribed to Ala 235 prior to Pro 236. Even such (13)C NMR signals were substantially broadened when (13)C NMR spectra of fully labeled [1,2,3-(13)C]Ala-bR were recorded, because the broadening and splitting of peaks due to the accelerated transverse relaxation rate caused by the increased number of relaxation pathways through a number of (13)C-(13)C homo-nuclear dipolar interactions and scalar J couplings, respectively, are dominant among (13)C-labeled nuclei. In addition, approximate correlation times for local conformational fluctuations of different domains, including the C-terminal tail, C-terminal alpha-helix, loops, and transmembrane alpha-helices, were estimated by measurement of the spin-lattice relaxation times in the laboratory frame and spin-spin relaxation times under the conditions of cross-polarization-magic angle spinning, and comparative study of suppressed specific peaks between the CP-MAS and DD-MAS experiments.