13C-13C and (15)N-(13)C correlation spectroscopy of membrane-associated and uniformly labeled human immunodeficiency virus and influenza fusion peptides: amino acid-type assignments and evidence for multiple conformations

Many viruses which cause disease including human immunodeficiency virus (HIV) and influenza are "enveloped" by a membrane and infection of a host cell begins with joining or "fusion" of the viral and target cell membranes. Fusion is catalyzed by viral proteins in the viral membrane. For HIV and for the influenza virus, these fusion proteins contain an approximately 20-residue apolar "fusion peptide" that binds to target cell membranes and plays a critical role in fusion. For this study, the HIV fusion peptide (HFP) and influenza virus fusion peptide (IFP) were chemically synthesized with uniform (13)C, (15)N labeling over large contiguous regions of amino acids. Two-dimensional (13)C-(13)C and (15)N-(13)C spectra were obtained for the membrane-bound fusion peptides and an amino acid-type (13)C assignment was obtained for the labeled residues in HFP and IFP. The membrane used for the HFP sample had a lipid headgroup and cholesterol composition comparable to that of host cells of the virus, and the (13)C chemical shifts were more consistent with beta strand conformation than with helical conformation. The membrane used for the IFP sample did not contain cholesterol, and the chemical shifts of the dominant peaks were more consistent with helical conformation than with beta strand conformation. There were additional peaks in the IFP spectrum whose shifts were not consistent with helical conformation. An unambiguous (13)C and (15)N assignment was obtained in an HFP sample with more selective labeling, and two shifts were identified for the Leu-9 CO, Gly-10 N, and Gly-10 Calpha nuclei. These sets of two shifts may indicate two beta strand registries such as parallel and antiparallel. Although most spectra were obtained on a 9.4 T instrument, one (13)C-(13)C correlation spectrum was obtained on a 16.4 T instrument and was better resolved than the comparable 9.4 T spectrum. More selective labeling and higher field may, therefore, be approaches to obtaining unambiguous assignments for membrane-associated fusion peptides.     

Synthesis of [Gly-1]RA-VII, [Gly-2]RA-VII, and [Gly-4]RA-VII. Glycine-containing analogues of RA-VII, an antitumor bicyclic hexapeptide from Rubia plants

Three analogues of RA-VII (1), an antitumor bicyclic hexapeptide from Rubia plants, were synthesized. Three analogues, [Gly-1]RA-VII (4), [Gly-2]RA-VII (5), and [Gly-4]RA-VII (6), in which one of the three alanine residues in 1 was replaced by a glycine residue, were prepared by linking of the cycloisodityrosine unit, obtained by degradation of 1, to three different glycine-containing tetrapeptides followed by macrocyclization. Of these three analogues, analogue 4 showed the highest cytotoxic activity. The NMR study revealed that in solution the conformer structures and their ratios of analogue 4 were very similar to those of natural peptide 1, suggesting that the methyl groups at Ala-2 and Ala-4 should be essential for producing the bioactive conformation, whereas that at D-Ala-1 is not essential.     

Lamellar structure in poly(ala-gly) determined by solid-state NMR and statistical mechanical calculations

Lamellar structure of poly(Ala-Gly) or (AG)n in the solid was examined using 13C solid-state NMR and statistical mechanical approaches. Two doubly labeled versions, [1-13C]Gly14[1-13C]Ala15- and [1-13C]Gly18[1-13C]Ala19 of (AG)15 were examined by two-dimensional (2D) 13C spin diffusion NMR in the solid state. In addition five doubly labeled [15N,13C]-versions of the same peptide, (AG) 15 and 15 versions labeled [3-13C] in each of the successive Ala residues were utilized for REDOR and 13C CP/MAS NMR measurements, respectively. The observed spin diffusion NMR spectra were consistent with a structure containing a combination of distorted beta-turns with a large distribution of the torsion angles and antiparallel beta-sheets. The relative proportion of the distorted beta-turn form was evaluated by examination of 13C CP/MAS NMR spectra of [3-13C]Ala-(AG)15. In addition, REDOR determinations showed five kinds of atomic distances between doubly labeled 13C and 15N nuclei which were also interpreted in terms of a combination of beta-sheets and beta-turns. Our statistical mechanical analysis is in excellent agreement with our Ala Cbeta 13C CP/MAS NMR data strongly suggesting that (AG)15 has a lamellar structure.     

Temperature dependence and resonance assignment of 13C NMR spectra of selectively and uniformly labeled fusion peptides associated with membranes

HIV-1 and influenza viral fusion peptides are biologically relevant model fusion systems and, in this study, their membrane-associated structures were probed by solid-state NMR (13)C chemical shift measurements. The influenza peptide IFP-L2CF3N contained a (13)C carbonyl label at Leu-2 and a (15)N label at Phe-3 while the HIV-1 peptide HFP-UF8L9G10 was uniformly (13)C and (15)N labeled at Phe-8, Leu-9 and Gly-10. The membrane composition of the IFP-L2CF3N sample was POPC-POPG (4:1) and the membrane composition of the HFP-UF8L9G10 sample was a mixture of lipids and cholesterol which approximately reflects the lipid headgroup and cholesterol composition of host cells of the HIV-1 virus. In one-dimensional magic angle spinning spectra, labeled backbone (13)C were selectively observed using a REDOR filter of the (13)C-(15)N dipolar coupling. Backbone chemical shifts were very similar at -50 and 20 degrees C, which suggests that low temperature does not appreciably change the peptide structure. Relative to -50 degrees C, the 20 degrees C spectra had narrower signals with lower integrated intensity, which is consistent with greater motion at the higher temperature. The Leu-2 chemical shift in the IFP-L2CF3N sample correlates with a helical structure at this residue and is consistent with detection of helical structure by other biophysical techniques. Two-dimensional (13)C-(13)C correlation spectra were obtained for the HFP-UF8L9G10 sample and were used to assign the chemical shifts of all of the (13)C labels in the peptide. Secondary shift analysis was consistent with a beta-strand structure over these three residues. The high signal-to-noise ratio of the 2D spectra suggests that membrane-associated fusion peptides with longer sequences of labeled amino acids can also be assigned with 2D and 3D methods.     

15N Chemical shielding in glycyl tripeptides: measurement by solid-state NMR and correlation with X-ray structure

15N chemical shielding parameters are reported for central glycyl residues in crystallographically characterized tripeptides with alpha-helix, beta-strand, polyglycine II (3(1)-helix), and extended structures. Accurate values of the shielding components (2-5 ppm) are determined from MAS and stationary spectra of peptides containing [2-(13)C,(15)N]Gly. Two dipolar couplings, (1)H-(15)N and (13)C(alpha)-(15)N, are used to examine (15)N shielding tensor orientations in the molecular frame and the results indicate that the delta(11), delta(33) plane of the shielding tensor is not coincident with the peptide plane. The observed isotropic shifts, which vary over a range of 13 ppm, depend on hydrogen bonding (direct and indirect) and local conformation. Tensor spans, delta(span) = delta(11) - delta(33), and their deviations from axial symmetry, delta(dev) = delta(22) - delta(33), vary over a larger range and are grouped according to 2 degrees structure. Augmented by previously reported (13)C(alpha) shielding parameters, a prediction scheme for the 2 degrees structure of glycyl residues in proteins based on shielding parameters is proposed.     

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

We have so far demonstrated that well-resolved and site-specifically assigned (13)C peaks as recorded by site-directed NMR study on (13)C-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 (13)C NMR spectra of (13)C-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 10(2)-10(8) Hz, especially at the membrane surfaces. Accordingly, we estimated the relative proportion of (13)C NMR signals from the surface areas with and without peak suppression by the accelerated transverse relaxation effect by surface-bound Mn(2+) ions, which could be effective for residues within 8.7 angstroms 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 angstroms of the surface for [1-(13)C]Val- and Ile-labeled bR and also [3-(13)C]Ala-bR. In contrast, (13)C NMR peaks from such surfaces area are almost completely or partially suppressed for [1-(13)C]Gly-, Ala-, Leu-, Phe- and Trp-labeled bR, as a result of plausible interference of the fluctuation frequency with frequency of magic angle spinning (10(4) Hz). We further assigned several (13)C NMR signals of [1-(13)C] 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 (13)C 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 (13)C NMR spectra so as to escape from motional frequencies of the order of 10(4)-10(5) Hz, either in the direction of accelerated fluctuation or slowed motions in the direction of forming the 2D array.     

Glycyl C(alpha) chemical shielding in tripeptides: measurement by solid-state NMR and correlation with X-ray structure and theory

We report here (13)C(alpha) chemical shielding parameters for central Gly residues in tripeptides adopting alpha-helix, beta-strand, polyglycine II, and fully extended 2 degrees structures. To assess experimental uncertainties in the shielding parameters and the effects of (14)N-(13)C(alpha) or (15)N-(13)C(alpha) dipolar coupling, stationary and magic angle spinning (MAS) spectra with and without (15)N decoupling were obtained from natural abundance and double-labeled samples containing [2-(13)C, (15)N]Gly. We find that accurate (<1 ppm uncertainty) shielding parameters are measured with good sensitivity and resolution in (15)N decoupled 1D or 2D MAS spectra of double-labeled samples. Compared to variations of isotropic shifts with peptide angles, those of (13)C(alpha) shielding anisotropy and asymmetry are greater. Trends relating shielding parameters to the 2 degrees structure are apparent, and the correlation of the experimental values with unscaled ab initio shielding calculations has an rms error of 3 ppm. Using the experimental data and the ab initio shielding values, the empirical trends relating the 2 degrees structure to shielding are extended to the larger range of torsion angles found in proteins.     

Stable conformations of tripeptides in aqueous solution studied by UV circular dichroism spectroscopy

Determination of the precise solution structure of peptides is of utmost importance to the understanding of protein folding and peptide drugs. Herein, we have measured the UV circular dichroism (UVCD) spectra of tri-alanine dissolved in D(2)O, H(2)O, and glycerol. The results clearly show the coexistence of a polyproline II or 3(1)-helix and a somewhat disordered flat beta-strand conformation, in complete agreement with recent predictions from spectroscopic data (Eker et al. J. Am. Chem. Soc. 2002, 124, 14 330-14 341). A thermodynamic analysis revealed that enthalpic contributions of about 11 and 17 kJ/mol stabilize polyproline II in D(2)O and H(2)O, respectively, but at room temperature they are counterbalanced by entropic contributions, which clearly favor the more disordered beta-strand conformation. It is hypothesized that this delicate balance is the reason for the variety of structural propensities of amino acid residues in the absence of nonlocal interactions. The isotope effect yielding a higher occupation of polyproline II in H(2)O with respect to D(2)O strongly suggests that a hydrogen-bonding network involving the peptide and water molecules in the hydration shell plays a major role in stabilizing this conformation. The equilibrium between polyproline II and beta-strand is practically maintained in glycerol, which suggests that glycerol can substitute water as stabilizing solvent for the polyproline II conformation. We also measured the UVCD spectra of tri-valine and tri-lysine (both at acidic pD) in D(2)O and found them to adopt a flat beta-strand and left-handed turn structure, respectively, in accordance with recent analyses of vibrational spectroscopy data. Generally, the present study adds substantial evidence to the notion that the so-called random coil state of peptides is much more structured than generally assumed.     

Grafting of commercially available amines bearing aromatic rings onto poly(vinylidene‐co‐hexafluoropropene) copolymers

The grafting of poly(VDF‐co‐HFP) copolymers with different amines containing aromatic rings, such as aniline, benzylamine, and phenylpropylamine, is presented. ¹⁹F NMR characterization enabled us to show that the sites of grafting of aromatic‐containing amines were first difluoromethylene of vinylidene fluoride (VDF) in the hexafluoropropene (HFP)/VDF/HFP triad and then that of VDF adjacent to HFP. The kinetics of grafting of benzylamine, monitored by ¹H NMR spectroscopy, confirmed those sites of grafting and showed that all VDF units located between two HFPs were grafted in the first 150 min, whereas those adjacent to one HFP unit were grafted in the remaining 3000 min. Parameters such as the temperature or the molar percentage of HFP in the copolymer had an influence on the maximum rate of grafted benzylamine. The higher the temperature, the higher the molar percentage of grafted benzylamine. Furthermore, the higher the molar percentage of HFP in the copolymer, the higher the molar percentage of VDF in the HFP/VDF/HFP triad, and the higher the molar percentage of grafted benzylamine. The spacer length between the aromatic ring and the amino group had an influence on the kinetics of grafting: aniline (pK_a = 4.5) could not add onto the polymeric backbone, whereas phenylpropylamine was grafted in the first 150 min, and benzylamine required 3000 min to reach the maximum amount of grafting. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1855–1868, 2006     

Phosphate-mediated arginine insertion into lipid membranes and pore formation by a cationic membrane peptide from solid-state NMR

The insertion of charged amino acid residues into the hydrophobic part of lipid bilayers is energetically unfavorable yet found in many cationic membrane peptides and protein domains. To understand the mechanism of this translocation, we measured the (13)C-(31)P distances for an Arg-rich beta-hairpin antimicrobial peptide, PG-1, in the lipid membrane using solid-state NMR. Four residues, including two Arg's, scattered through the peptide were chosen for the distance measurements. Surprisingly, all residues show short distances to the lipid (31)P: 4.0-6.5 A in anionic POPE/POPG membranes and 6.5-8.0 A in zwitterionic POPC membranes. The shortest distance of 4.0 A, found for a guanidinium Czeta at the beta-turn, suggests N-H...O-P hydrogen bond formation. Torsion angle measurements of the two Arg's quantitatively confirm that the peptide adopts a beta-hairpin conformation in the lipid bilayer, and gel-phase 1H spin diffusion from water to the peptide indicates that PG-1 remains transmembrane in the gel phase of the membrane. For this transmembrane beta-hairpin peptide to have short (13)C-(31)P distances for multiple residues in the molecule, some phosphate groups must be embedded in the hydrophobic part of the membrane, with the local (31)P plane parallel to the beta-strand. This provides direct evidence for toroidal pores, where some lipid molecules change their orientation to merge the two monolayers. We propose that the driving force for this toroidal pore formation is guanidinium-phosphate complexation, where the cationic Arg residues drag the anionic phosphate groups along as they insert into the hydrophobic part of the membrane. This phosphate-mediated translocation of guanidinium ions may underlie the activity of other Arg-rich antimocrobial peptides and may be common among cationic membrane proteins.     

Efficient synthesis of hydroxyl functioned polyesters from natural polyols and sebacic acid

Amphiphilic hydroxyl functioned polyester（HFP） can be used as compatibilizers for blends of starch and resins.We developed a synthetic method for effective preparation of HFPs.Water was removed by high flow rate N₂ rather than high vacuum during polycondensation of sebacic acid with xylitol,sorbitol,or mannitol in the presence of dehydrative condensation catalyst and the product is with[η]of 27.2 mL/g,M_n of 1903,M_w of 167,693,T_g of -30.5℃,T_m of 44.0℃.Weight loss is 1.73%under 200℃. The integral distributions of molecular weight are 43.6 wt%and 63.8 wt%over 10,000 and 3000,respectively.The results indicated that higher molecular weight HFP was economically synthesized.     

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

The 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 (13)C cross-polarization magic angle spinning (CP/MAS), static (2)H and (19)F 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 (13)C 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 beta-sheet conformation of silk II is able readily to accommodate a single Tyr residue. Interestingly, the beta-turn conformation of silk I only remains stable when Tyr is positioned near the chain terminus in (AG)(12)YG(AG)(2), but the conformation is driven towards silk II when Tyr is located in the central region of (AG)(7)YG(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 beta-sheet and beta-turn conformations was obtained, regardless of the precipitation conditions. Static (2)H NMR of ring-deuterated [3',5'-(2)H(2)]Tyr located in the central region of (AG)(7)YG(AG)(7) showed that the side-chain is immobilized in both silk I and II, which was also observed by static (19)F 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 beta-turn type II or the antiparallel beta-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.     

Simultaneous NMR study of protein structure and dynamics using conservative mutagenesis

A novel iterative procedure is described that allows both the orientation and dynamics of internuclear bond vectors to be determined from direct interpretation of NMR dipolar couplings, measured under at least three orthogonal alignment conditions. If five orthogonal alignments are available, the approach also yields information on the degree of motional anisotropy and the direction in which the largest amplitude internal motion of each bond vector takes place. The method is demonstrated for the backbone (15)N-(1)H, (13)C(alpha)-(1)H(alpha), and (13)C(alpha)-13C' interactions in the previously well-studied protein domain GB3, dissolved in a liquid crystalline suspension of filamentous phage Pf1. Alignment variation is achieved by using conservative mutations of charged surface residues. Results indicate remarkably uniform backbone dynamics, with amplitudes that agree well with those of previous (15)N relaxation studies for most residues involved in elements of secondary structure, but larger amplitude dynamics than those found by (15)N relaxation for residues in loop and turn regions. In agreement with a previous analysis of dipolar couplings, the N-H bonds in the second beta-strand, which is involved in antibody recognition, show elevated dynamics with largest amplitudes orthogonal to the chain direction.     

Synthesis of [L-Ala-1]RA-VII, [D-Ala-2]RA-VII,and [D-Ala-4]RA-VII by epimerization of RA-VII,an antitumor bicyclic hexapeptide from Rubia plants,through oxazoles

Three epimers of a natural cyclic hexapeptide RA-VII were prepared via formation of oxazoles from thioamides or thioimidates of RA-VII followed by hydrolysis. They are the epimers at l-Ala-1, d-Ala-2, and d-Ala-4, respectively. The one having l-Ala-1 adopted trans-cis-trans-trans-trans-trans (t-c-t-t-t-t) amide configurations in the crystal, a type-VI beta-turn for residues 1-4 stabilized by one intramolecular hydrogen bond between Ala-4 NH and l-Ala-1 C = O, and in CDCl(3) existed as a mixture of six conformers, of which the major conformer was very similar to that in the crystal, but quite different from that of RA-VII in solution. The second epimer, having d-Ala-2 had in the crystalline state t-t-t-t-c-t amide configurations, a gamma-turn at Tyr-3 stabilized by two intramolecular hydrogen bonds between d-Ala-2 NH and Ala-4 C = O and between Ala-4 NH and d-Ala-2 C = O, and existed in CDCl(3) as a single conformer, the structure of which was very similar to its crystal structure, and to the crystal structure of peptide 25 except for the backbone and the side chains at residues 1 and 2. The third epimer, having d-Ala-4 had t-c-t-t-c-t amide configurations in the crystal, a type-VI beta-turn for residues 1-4 as observed in the first epimer, and in CDCl(3) existed in three conformers, of which the major one was similar to that in the crystal but different from that of RA-VII in solution. The three epimers showed very weak cytotoxicity on P-388 leukemia cells, which may be because of their conformational differences from the active conformation of RA-VII.     

Study of Carbamate‐Modified Disiloxane in Porous PVDF‐HFP Membranes: New Electrolytes/Separators for Lithium‐Ion Batteries

A gel electrolyte membrane is obtained through the absorption of a carbamate‐modified liquid disiloxane‐containing lithium bis(trifluoromethane)sulfonimide (LiTFSI) by using macroporous poly(vinylidene fluoride–hexafluoropropylene) (PVDF‐HFP) membranes. The porous membranes are prepared by means of a phase inversion technique. The resulting gel electrolyte membrane is studied by using differential scanning calorimetry, Fourier‐transform infrared (FTIR) spectroscopy, and microscope mapping through coherent anti‐Stokes Raman scattering (CARS) confocal microscopy and impedance spectroscopy. The ionic conductivity of the gel electrolyte is 10^?4 S cm^?1 at 20 °C. FTIR spectroscopy reveals interactions between LiTFSI and the carbonyl moiety of the disiloxane. No interactions between LiTFSI and PVDF‐HFP or between disiloxane and PVDF‐HFP are detected by FTIR spectroscopy. Furthermore, the distribution of the α and β/γ phases of PVDF‐HFP and the homogeneous distribution of disiloxane/LiTFSI in the gel electrolyte membranes are examined by FTIR mapping. CARS confocal microscopy is used to image the three‐dimensional interconnectivity, which reveals a reticulated structure of macrovoids in the porous PVDF‐HFP framework. Owing to properties such as electrochemical and thermal stability of the disiloxane‐based liquid electrolyte and the mechanical stability of the porous PVDF‐HFP membrane, the gel electrolyte membranes presented herein are promising candidates for applications as electrolytes/separators in lithium‐ion batteries.     

CO2 in supramolecular chemistry: preparation of switchable supramolecular polymers

CO2 gas was used to construct novel types of supramolecular polymers. Self-assembling nanostructures 11 and 13 were prepared, which employ both hydrogen bonding and dynamic, thermally reversible carbamate bonds. As precursors, calixarene ureas 1 and 2 were synthesized, which strongly aggregate/dimerize (K(D)>/=10(6) M(-1) per capsule) in apolar solution with the formation of self-assembling capsules 7 and linear polymeric chains 8, respectively, and also possess "CO2-philic" primary amino groups on the periphery. CO2 effectively reacts with molecules 7 and 8 in apolar solvents and cross-links them with the formation of multiple carbamate salt bridges. Oligomeric aggregate 11 and three-dimensional polymeric network 13 were prepared and characterized by 1H and 13C NMR spectroscopy. The morphology of supramolecular gel 13 was studied by scanning electron microscopy. Addition of a competitive solvent destroyed the hydrogen bonding in assembling structures 11 and 13, but did not influence the carbamate linkers; carbamate salts 12 and 14, respectively, were obtained. On the other hand, thermal release of CO2 from 11 and 13 was easily accomplished (1 h, 100 degrees C) while retaining the hydrogen-bonding capsules. Thus, three-dimensional polymeric network 13 was transformed back to linear polymeric chain 8 without breaking up. Encapsulation and storage of solvent molecules by 11 and 13 was demonstrated. This opens the way for switchable materials, which reversibly trap, store, and then release guest molecules. A two-parameter switch and control over hydrogen bonding and CO2-amine adducts was established.     

2,2,7,7-四甲基-5,6-二氧代-9-(2-氯苯基)-10-(4-甲基苯基)-9,10-二氢-2H-吖啶的合成和晶体结构

标题化合物2,2,7,7-四甲基-5,6-二氧代-9-(2-氯苯基)-10-(4-甲基苯基)-9,10-二氢-2H-吖啶(C30H32ClNO2,Mr=474.02)是由邻氯苯甲醛和5,5-二甲基-1,3-环己二酮和对甲基苯胺以乙二醇作溶剂在微波辐射下而得到的。结构通过单晶X-射线衍射分析确定,其晶体属于单斜晶系,空间群P21/c,a=12.048(3),b=11.044(2),c=19.989(5)?b=101.42(2),Z=4,V=2607(1)3,Dc=1.208g/cm3,m(MoKa)=0.173mm-1,F(000)=1008,R=0.0450,wR=0.0869。X-射线衍射分析表明:原子C(8),C(13),C(14),C(15),C(20)和N形成1个六员环,采用船式构象,六员环C(8)~C(13)和C(15)~C(20)采用半椅式构象。     

Copolymerization of hexafluoropropylene and tetrafluoroethylene: Effect on chain conformation and crystal structure

A series of new copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) containing up to 50 mol % of the hexafluoropropylene comonomer have been investigated with respect to chain conformation and crystal structure using wide-angle X-ray diffraction (WAXD). Increasing HFP content leads to significant departures from the highly ordered crystalline structure of the homopolymer PTFE; the helical conformation of the chain relaxes and untwists to accommodate the larger  CF₃ pendant group in the HFP unit. Simultaneously the lateral hexagonal packing of the helices becomes less ordered and the a-dimension of the hexagonal cell increases. The above effects are progressive with increasing HFP content. At 50 mol % HFP incorporation the structure is a disordered crystalline phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2811–2819, 1998