bluuebunny/crossref_metadata_2025_split huggingface dataset:
Site‐directed 13C solid‐state NMR studies on membrane proteins: strategy and goals toward revealing conformation and dynamics as illustrated for bacteriorhodopsin labeled with [1‐13C]amino acid residues
Authors: Saitô Hazime , Mikami Jun , Yamaguchi Satoru , Tanio Michikazu , Kira Atushi , Arakawa Tadashi , Yamamoto Kazutoshi , Tuzi Satoru 
Published: January 2004

AbstractWe have so far demonstrated that well‐resolved and site‐specifically assigned 13C peaks as recorded by site‐directed NMR study on 13C‐labeled membrane proteins can serve as a convenient probe to reveal their local conformation and dynamics. We attempted here to clarify the extent to which 13C NMR spectra of 13C‐labeled fully hydrated bacteriorhodopsin (bR) as a typical membrane protein are visible or well resolved in the presence of inherent fluctuation motions with frequency of 102–108 Hz, especially at the membrane surfaces. Accordingly, we estimated the relative proportion of 13C NMR signals from the surface areas with and without peak suppression by the accelerated transverse relaxation effect by surface‐bound Mn2+ ions, which could be effective for residues within 8.7 Å of the membrane surface. It turned out that the experimental findings are consistent with the predicted amount of amino acid residues under consideration located within 8.7 Å of the surface for [1‐13C]Val‐ and Ile‐labeled bR and also [3‐13C]Ala‐bR. In contrast, 13C NMR peaks from such surfaces area are almost completely or partially suppressed for [1‐13C]Gly‐, Ala‐, Leu‐, Phe‐ and Trp‐labeled bR, as a result of plausible interference of the fluctuation frequency with frequency of magic angle spinning (104 Hz). We further assigned several 13C NMR signals of [1‐13C] Val‐, Trp‐ and Ile‐labeled bR on the basis of a variety of site‐directed mutants with reference to those of the wild type. Further, we recorded the 13C NMR of bR in lipid bilayers to search for the optimal conditions to be able to obtain signals with the highest peak intensities and spectral resolution. Backbone dynamics turn out to be essential for recording 13C NMR spectra so as to escape from motional frequencies of the order of 104–105 Hz, either in the direction of accelerated fluctuation or slowed motions in the direction of forming the 2D array. Copyright © 2004 John Wiley & Sons, Ltd.
The αM1 transmembrane segment of the nicotinic acetylcholine receptor interacts strongly with model membranes
Authors: de Planque Maurits R. R. , Rijkers Dirk T. S. , Liskamp Rob M. J. , Separovic Frances 
Published: January 2004

AbstractThe transmembrane domain of the nicotinic acetylcholine receptor (nAChR) plays a role in the regulation of the activity of this important ligand‐gated ion channel. The lipid composition of the host membrane affects conformational equilibria of the nAChR and several classes of inhibitors, most notably anaesthetics, interact directly or indirectly with the four transmembrane M‐segments, M1–M4, of the nAChR subunits. It has proven difficult to gain insight into structure–function relationships of the M‐segments in the context of the entire receptor and the biomembrane environment. However, model membrane systems are well suited to obtain detailed information about protein–lipid interactions. In this solid‐state NMR study, we characterized interactions between a synthetic αM1 segment of the T. californica nAChR and model membranes of different phosphatidylcholine (PC) lipids. The results indicate that αM1 interacts strongly with PC bilayers: the peptide orders the lipid acyl chains and induces the formation of small vesicles, possibly through modification of the lateral pressure profile in the bilayer. The multilamellar vesicle morphology was stabilized by the presence of cholesterol, implying that either the rigidity or the bilayer thickness is a relevant parameter for αM1–membrane interactions, which also has been suggested for the entire nAChR. Our results suggest that the model systems are to a certain extent sensitive to peptide–bilayer hydrophobic matching requirements, but that the lipid response to hydrophobic mismatch alone is not the explanation. The effect of αM1 on different PC bilayers may indicate that the peptide is conformationally flexible, which in turn would support a membrane‐mediated modulation of the conformation of transmembrane segments of the nAChR. Copyright © 2004 John Wiley & Sons, Ltd.
Effects of antidepressants on the conformation of phospholipid headgroups studied by solid‐state NMR
Authors: Santos Jose S. , Lee Dong‐Kuk , Ramamoorthy Ayyalusamy 
Published: January 2004

AbstractThe effect of tricyclic antidepressants (TCA) on phospholipid bilayer structure and dynamics was studied to provide insight into the mechanism of TCA‐induced intracellular accumulation of lipids (known as lipidosis). Specifically we asked if the lipid–TCA interaction was TCA or lipid specific and if such physical interactions could contribute to lipidosis. These interactions were probed in multilamellar vesicles and mechanically oriented bilayers of mixed phosphatidylcholine–phosphatidylglycerol (PC–PG) phospholipids using 31P and 14N solid‐state NMR techniques. Changes in bilayer architecture in the presence of TCAs were observed to be dependent on the TCA's effective charge and steric constraints. The results further show that desipramine and imipramine evoke distinguishable changes on the membrane surface, particularly on the headgroup order, conformation and dynamics of phospholipids. Desipramine increases the disorder of the choline site at the phosphatidylcholine headgroup while leaving the conformation and dynamics of the phosphate region largely unchanged. Incorporation of imipramine changes both lipid headgroup conformation and dynamics. Our results suggest that a correlation between TCA‐induced changes in bilayer architecture and the ability of these compounds to induce lipidosis is, however, not straightforward as imipramine was shown to induce more dramatic changes in bilayer conformation and dynamics than desipramine. The use of 14N as a probe was instrumental in arriving at the presented conclusions. Copyright © 2004 John Wiley & Sons, Ltd.
Precision 1H–1H distance measurement via 13C NMR signals: utilization of 1H–1H double‐quantum dipolar interactions recoupled under magic angle spinning conditions
Authors: Matsuki Yoh , Akutsu Hideo , Fujiwara Toshimichi 
Published: January 2004

AbstractWe applied the POST‐C7 DQ‐dipolar recoupling pulse sequence to the measurement of 1H–1H distances with high precision. The spectral resolution is enhanced by detecting the 1H magnetization via 13C signals. A least‐squares fitting of the build‐up curve of the transferred magnetization to the exact numerical simulations yielded a 1Hα–1Hβ distance of 248 ± 4 pm for fully 13C‐labeled L‐valine. This distance agrees with the neutron diffraction study. The negative transferred magnetization clearly indicates that the direct DQ 1H–1H dipolar couplings have the largest effect. The signal for the magnetization transfer builds up rapidly by the direct 1H–1H dipolar coupling, and decreases to zero at longer mixing time when the relayed magnetization transfer becomes significant. This large intensity change of the signal leads to the high precision in the distance measurement. We inspected factors that limit the effective bandwidth of the POST‐C7 recoupling for the 1H and 13C homonuclear spin systems. The spin interactions at times shorter than the cycle time of the C7 sequence were also evaluated to measure the distances. The carbon‐detected 2D 1H DQ mixing experiment was demonstrated for the measurement of multiple 1H–1H distances. Copyright © 2004 John Wiley & Sons, Ltd.
Multi‐dimensional pulsed field gradient magic angle spinning NMR experiments on membranes
Authors: Gaede Holly C. , Gawrisch Klaus 
Published: January 2004

AbstractThe benefits of gradient techniques in the study of lipid membranes are demonstrated on a sample of 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3 phosphocholine (POPC) liposomes embedded with ibuprofen. Most techniques from gradient NMR spectroscopy on solution samples are directly applicable to membrane samples subjected to magic angle spinning (MAS). Gradient‐enhanced homo‐ and heteronuclear chemical shift correlation techniques were used to make resonance assignments. Gradient NOESY experiments provide insight into the location and dynamics of lipids, ibuprofen and water. Application of gradients not only reduces experiment time but also the t1 noise in the multi‐dimensional spectra. Diffusion measurements with pulsed field gradients characterize lateral movements of lipid and drug molecules in membranes. The theoretical framework for data analysis of MAS diffusion experiments on randomly oriented multilamellar liposomes is presented. Published in 2004 by John Wiley & Sons, Ltd.
A solid‐state NMR study of the fast and slow dynamics of collagen fibrils at varying hydration levels
Authors: Reichert Detlef , Pascui Ovidiu , deAzevedo Eduardo R. , Bonagamba Tito J. , Arnold Klaus , Huster Daniel 
Published: January 2004

AbstractWe report solid‐state NMR investigations of the effect of temperature and hydration on the molecular mobility of collagen isolated from bovine achilles tendon. 13C cross‐polarization magic angle spinning (MAS) experiments were performed on samples at natural abundance, using NMR methods that detect motionally averaged dipolar interactions and chemical shift anisotropies and also slow reorientational processes. Fast motions with correlation times much shorter than 40 µs scale dipolar couplings and chemical shift anisotropies of the carbon sites in collagen. These motionally averaged anisotropic interactions provide a measure of the amplitudes of the segmental motions expressed by a molecular order parameter. The data reveal that increasing hydration has a much stronger effect on the amplitude of the molecular processes than increasing temperature. In particular, the Cγ carbons of the hydroxyproline residues exhibit a strong dependence of the amplitude of motion on the hydration level. This could be correlated with the effect of hydration on the hydrogen bonding structure in collagen, for which this residue is known to play a crucial role. The applicability of 1D MAS exchange experiments to investigate motions on the millisecond time‐scale is discussed and first results are presented. Slow motions with correlation times of the order of milliseconds have also been detected for hydrated collagen. Copyright © 2004 John Wiley & Sons, Ltd.
NMR methods for studying the structure and dynamics of oncogenic and antihistaminic peptides in biomembranes
Authors: Sizun Christina , Aussenac Fabien , Grelard Axelle , Dufourc Erick J. 
Published: January 2004

AbstractWe present several applications of both wide‐line and magic angle spinning (MAS) solid‐state NMR of bicelles in which are embedded fragments of a tyrosine kinase receptor or enkephalins. The magnetically orientable bicelle membranes are shown to be of particular interest for studying the functional properties of lipids and proteins in a state that is very close to their natural environment. Quadrupolar, dipolar and chemical shielding interactions can be used to determine minute alterations of internal membrane dynamics and the orientation of peptides with respect to the membrane plane. MAS of bicelles can in turn lead to high‐resolution proton spectra of hydrated membranes. Using deuterium–proton contrast methods one can then obtain pseudo‐high‐resolution proton spectra of peptides or proteins embedded in deuterated membranes and determine their atomic 3D structure using quasi‐conventional liquid‐state NMR methods. Copyright © 2004 John Wiley & Sons, Ltd.
Structural role of tyrosine in Bombyx mori silk fibroin, studied by solid‐state NMR and molecular mechanics on a model peptide prepared as silk I and II
Authors: Asakura Tetsuo , Suita Kohei , Kameda Tsunenori , Afonin Sergii , Ulrich Anne S. 
Published: January 2004

AbstractThe influence of the bulky and H‐bonding Tyr side‐chain on its Ala‐ and Gly‐rich environment in Bombyx mori silk fibroin was examined by 13C cross‐polarization magic angle spinning (CP/MAS), static 2H and 19F NMR and molecular mechanics calculations. Model peptides of the type (AG)15 were synthesized with Tyr in a number of different positions, precipitated under conditions favoring either of the two characteristic protein conformations, and the resulting structures were assigned from their 13C chemical shifts. Dialysis of native fibroin or the simple (AG)15 peptide from a 9 M LiBr solution against water produces silk I (the structure of silk before spinning), whereas drying from formic acid yields silk II (fibrous structure after spinning). We found that the introduction one or more Tyr into (AG)15 can have a dramatic effect not only on the local backbone conformation but also on the long‐range intermolecular chain packing in the samples. The antiparallel β‐sheet conformation of silk II is able readily to accommodate a single Tyr residue. Interestingly, the β‐turn conformation of silk I only remains stable when Tyr is positioned near the chain terminus in (AG)12YG(AG)2, but the conformation is driven towards silk II when Tyr is located in the central region of (AG)7YG(AG)7. The role of H‐bonding was tested by replacing Tyr with Phe or 4F‐Phe, which are no longer compatible with silk I and fully induced a silk II conformation. In the presence of several Tyr residues a mixture of distorted β‐sheet and β‐turn conformations was obtained, regardless of the precipitation conditions. Static 2H NMR of ring‐deuterated [3′,5′‐2H2]Tyr located in the central region of (AG)7YG(AG)7 showed that the side‐chain is immobilized in both silk I and II, which was also observed by static 19F NMR of the 4F‐Phe analogue. To visualize the local packing around the Tyr side‐chain, molecular mechanics calculations were performed on a mixture of (AG)4 and AGAGYGAG, starting from either the β‐turn type II or the antiparallel β‐sheet structure. The resulting structures show that the intermolecular chain arrangement is significantly affected by Tyr, thus explaining the long‐range packing effects in the semi‐crystalline regions of silk fibers compared with the crystalline regions that are devoid of Tyr. Copyright © 2004 John Wiley & Sons, Ltd.
Improved pulse sequences for pure exchange solid‐state NMR spectroscopy
Authors: Vosegaard Thomas , Nielsen Niels C. 
Published: January 2004

AbstractSpin‐exchange experiments are useful for improving the resolution and establishment of sequential assignments in solid‐state NMR spectra of uniformly 15N‐labeled proteins oriented macroscopically in phospholipid bilayers. To exploit this advantage fully, it is crucial that the diagonal peaks in the two‐dimensional exchange spectra are suppressed. This may be accomplished using the recent pure‐exchange (PUREX) experiments, which, however, suffer from up to a threefold reduction of the cross‐peak intensity relative to experiments without diagonal‐peak suppression. This loss in sensitivity may severely hamper the applicability for the study of membrane proteins. In this paper, we present a two‐dimensional exchange experiment (iPUREX) which improves the PUREX sensitivity by 50%. The performance of iPUREX is demonstrated experimentally by proton‐mediated 15N–15N spin‐exchange experiments for a 15N‐labeled N‐acetyl‐L‐valyl‐L‐leucine dipeptide. The relevance of exchange experiments with diagonal‐peak suppression for large, uniformly 15N‐labeled membrane proteins in oriented phospholipid bilayers is demonstrated numerically for the G‐protein coupled receptor rhodopsin. Copyright © 2004 John Wiley & Sons, Ltd.
Synthesis of polybutadiene‐based particles via dispersion ring‐opening metathesis polymerization
Authors: Chemtob Abraham , Héroguez Valérie , Gnanou Yves 
Published: January 2004

AbstractLatex particles based on 1,4‐polybutadiene were synthesized via dispersion ring‐opening metathesis copolymerization of 1,5‐cyclooctadiene with a α‐norbornenyl poly(ethylene oxide) macromonomer. Stable but polydisperse colloidal dispersions in the 50 nm to 10 μm size range were obtained. In this work, particular attention was paid to the effects of the kinetics of copolymerization on the structure of the graft copolymers formed and on the onset of turbidity. Strategies to prepare monodisperse polybutadiene particles were also designed through the growth of a polybutadiene shell from a well‐defined polynorbornene seed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1154–1163, 2004
Synthesis of ultra‐high‐molecular‐weight poly(α‐olefin)s by thiobis(phenoxy)titanium/modified methylaluminoxane system
Authors: Fujita Masayuki , Seki Yoshinori , Miyatake Tatsuya 
Published: January 2004

AbstractPolymerization of 1‐butene with thiobis(phenoxy)titanium dichloride (TBPTiCl2) with modified methylaluminoxane (MMAO) yielded atactic and regioirregular poly(1‐butene) with low molecular weight. However, productivity was increased and the yielded polymer structure was changed when water‐modified MMAO (WM‐MMAO) was used as a cocatalyst. The molecular weight of obtained polymer with TBPTiCl2/WM‐MMAO was dramatically increased and reached over 9 million g/mol. Poly(1‐butene) produced by this catalytic system was almost regioregular and slightly isotactic. 1‐Hexene and 1‐octene were also polymerized by this catalytic system. In all cases, the ultra‐high‐molecular‐weight polymers were obtained. These results showed that the modified structure of the MMAO cluster by water affects the productivity, molecular weight, regioregularity, and stereoregularity of yielded polyolefins. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1107–1111, 2004
The effect of compositions and combinations of the reactants upon the copolycondensation of isophthalic acid/terephthalic acid with a combination of hydroquinones and bisphenols by tosyl chloride/dimethylformamide/pyridine
Authors: Higashi Fukuji , Koyanagi Sakura 
Published: January 2004

AbstractCopolycondensation of isophthalic acid (IPA)/terephthalic acid (TPA) with various combinations of 2,2‐bis(4‐hydroxyphenyl)propane (BPA) and hydroquinones (HQs) or bisphenols (BPs) was conducted to study the effects of the compositions of IPA/TPA and of BPA/HQs or BPA/BPs upon the reaction. Different from homopolycondensation of each of diol components examined where most of the reaction was facilitated by lower contents of IPA at about 70 mol %, the copolycondensation was influenced by a combination of diol components. With chlorohydroquinone (ClHQ) or bis(4‐hydroxyphenyl)sulfone (BPS) having a polar chlorine or sulfonyl substituent, the reaction proceeded most satisfactorily at IPA/TPA = 30/70, whereas it was IPA/TPA = 50/50 for the reaction with nonpolar methyl substituted methylhydroquinone (MeHQ). The reaction with 2,2‐bis(3,5‐dichloro‐4‐hydroxyphenyl)propane (TC‐BPA), despite having polar chlorine substituents in TC‐BPA, was not affected by IPA/TPA compositions. Alternatively, from the viewpoint of the compositions of diol components, the reactions containing 30–50 mol % of HQs or BPS yielded better results except for the reaction of IPA/TPA = 70/30, in which higher contents of MeHQ was more favorable. On the basis of sequence distributions of diol components in the resultant copolymers determined by NMR, compositions of IPA/TPA or diol components and combinations of the diols producing random copolymers yielded better results in copolycondensation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1100–1106, 2004
Effects of tris(pentafluorophenyl)borane on the activation of a metal alkyl‐free Ni‐based catalyst in the polymerization of 1,3‐butadiene
Authors: Jang Youngchan , Choi Dai Seung , Han Shin 
Published: January 2004

AbstractThe activation of a metal alkyl‐free Ni‐based catalyst with B(C6F5)3 was investigated in the polymerization of 1,3‐butadiene. A catalyst of bis(1,5‐cyclooctadiene)nickel (Ni(COD)2)/B(C6F5)3 was found to have high catalytic activity and 1,4‐cis stereoregularity. The catalyst was also found to provide polybutadiene having a molecular weight (Mw) of up to 117,000, even in the absence of AlR3 and MAO. Variations in the mol ratio of B(C6F5)3 to Ni affected catalytic activity, 1,4‐cis stereoregularity, and the Mw of polybutadiene, while the molecular weight distribution (MWD) of polybutadiene showed little correlation with the mol ratio of B(C6F5)3 to Ni. The use of other borane compounds such as B(C6H5)3, BEt3, and BF3 etherate in place of B(C6F5)3 clearly showed the two main functions of B(C6F5)3 in the present catalyst. The high Lewis acidity of B(C6F5)3 enabled it to activate catalytic complexes, thus inducing the polymerization. The steric bulkiness of B(C6F5)3 suppressed chain transfer reactions, contributing to the production of polybutadiene with a high Mw. Kinetic studies showed that the catalyst had an induction period, possibly due to the time needed for the formation of catalytic complexes starting from Ni(COD)2. A plot of −ln (1−X), where X is the fractional conversion, as a function of time resulted in a linear relationship, showing that the present catalyst system followed first‐order kinetics with respect to monomer concentration. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1164–1173, 2004
Amphiphilic block copolymers of high‐molecular‐weight poly(ethylene oxide) and either ε‐caprolactone or γ‐methyl‐ε‐caprolactone: Synthesis and characterization
Authors: Vangeyte P. , Jérôme R. 
Published: January 2004

AbstractEthylene oxide (EO) has been block‐polymerized with both ε‐caprolactone (ε‐CL) and γ‐methyl‐ε‐caprolactone (MCL) through the combination of the anionic polymerization of EO and the ring‐opening polymerization (ROP) of ε‐CL and MCL. ω‐Hydroxyl poly(ethylene oxide) has been reacted with triethylaluminum (OH/Al = 1) and converted into a macroinitiator for ROP of ε‐CL and MCL. In toluene at room temperature, this polymerization leads to a bimodal molecular weight distribution as a result of monomer insertion in only some of the aluminum alkoxide bonds. However, in a more polar solvent (methylene chloride) added with 1 equiv of a Lewis base (pyridine), the expected diblock is formed selectively, and this indicates that aggregation of the active species in toluene is responsible for a macroinitiator efficiency of less than 1. A series of amphiphilic diblock copolymers with poly(ε‐caprolactone) (semicrystalline) and poly(γ‐methyl‐ε‐caprolactone) (amorphous) as the hydrophobic blocks have been prepared and characterized with size exclusion chromatography,1H NMR, IR, and wide‐angle X‐ray scattering. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1132–1142, 2004
Properties of double‐hydrophilic graft copolymers with a polyacetal backbone
Authors: Baśko Małgorzata , Kubisa Przemysław 
Published: January 2004

AbstractGraft copolymers consisting of the polyacetal backbone substituted with COO− groups and poly(oxyethylene) (POE) side chains in different proportions were synthesized, and their efficiency as modifiers of the processes occurring at the interphase between inorganic particles and the surrounding aqueous phase was studied. The series of double hydrophilic graft copolymers (DHGC) was obtained differing in the length of the main chain, the number of carboxylate groups, and the number of POE side chains in the macromolecule. Studies of the effect of DHGC on the stability of aqueous calcium carbonate (CaCO3) suspensions and on the shape and size of the CaCO3 crystals formed in the presence of DHGC allowed us to draw some conclusions concerning the effect of the structure of DHGC on their efficiency as modifiers of the processes occurring at the interphase between inorganic particles and the surrounding aqueous phase. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1189–1197, 2004