Electrochemical behaviour of tetraazamacrocyclic nickel(II) complexes functionalized with crown ethers

Tetraazamacrocyclic nickel(II) complexes linked by the system of conjugated double bonds with one or two benzo-15-crown-5 moieties (2Ni and 3Ni) were synthesized and characterized. The CV of these complexes in acetonitrile (ACN) solution exhibited reversible Ni^II/III redox process. At more positive potentials oxidation of the bezno-15-crown-5 ether (B15C5) moiety was found. In complexes 2Ni and 3Ni redox potential of Ni^II/III did not change after the interactions of alkali and alkaline earth metal cations with B15C5 unit. However, the presence of guest cations caused a significant change of B15C5 oxidation potential. Complexation of two Cl⁻ anions to the coordinated Ni^II in 2Ni and 3Ni resulted in an appreciable shift of both Ni^II/III and B15C5 redox processes after addition of metal cations. Behaviour of complexes 2Ni and 3Ni was compared with behaviour of similar Cu^II complexes.     

(R,R)-salen/salan-based polymer fluorescence sensors for Zn detection

(R,R)-salen-based polymer fluorescence sensor P-1 could be synthesized by the polymerization of 5,5′-(isoquinoline-5,8-diylbis(ethyne-2,1-diyl))-bis(3-tert-butyl-2-hydroxybenzaldehyde) (M-1) with (R,R)-1,2-diaminocyclohexane (M-2) via nucleophilic addition-elimination reaction, and (R,R)-salan-based polymer sensor P-2 could be obtained by the reduction reaction of P-1 with NaBH₄. The fluorescence response behaviors of two chiral polymers P-1 and P-2 on Zn²⁺ were investigated by fluorescence spectra. The fluorescence intensities of P-1 and P-2 can exhibit gradual enhancement upon addition of Zn²⁺. Compared with other cations, such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Cd²⁺, Cr³⁺ and Pb²⁺, Zn²⁺ can lead to the pronounced fluorescence enhancement as high as 22.8-fold for P-1 and 3.75-fold for P-2, respectively. The results show that P-1 and P-2 incorporating (R,R)-salen/salan moieties as receptors in the polymer main chain backbone can exhibit high sensitivity and selectivity for Zn²⁺ detection.     

A highly selective fluorescent sensor for Hg based on the water-soluble poly(p-phenyleneethynylene)

The conjugated polymer P-1 could be synthesized by the polymerization of 4,7-diethynyl-benzo[2,1,3]thiadiazole (M-1) and 1,4-bis[3′-(N,N-diethylamino)-1′-oxapropyl]-2,5-diiodobenzene (M-2) via Pd-catalyzed Sonogashira reaction. The water-soluble conjugated polyelectrolyte P-2 could be obtained by the reaction of P-1 with ethyl bromide. Both P-1 and P-2 can emit orange fluorescence. The responsive optical properties of P-1 and P-2 on Hg²⁺ were investigated by fluorescence spectra. Hg²⁺ can lead to nearly complete fluorescence quenching of P-1. On the contrary, Hg²⁺ can show the most pronounced fluorescence enhancement response of P-2 in aqueous solution without interference from those coexistent ions, such as K⁺, Mg²⁺, Pb²⁺, Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺, Fe³⁺ and Zn²⁺. The results also exhibit that this kind of water-soluble conjugated polyelectrolyte can be used as a highly sensitive and selective fluorescence sensor for Hg²⁺ detection in water.     

Crown ether bridged homo- and heterodinuclear copper(II) and nickel(II) cyclidene complexes: Interaction with anions

Homodinuclear (3CuCu, 3NiNi) and heterdionuclear (3CuNi) cyclidene complexes linked by two 1,10-diaza-18-crown-6 ethers were synthesized and their properties characterized. For the 3CuCu in the negative potential range the mixed-valence state Cu^I-Cu^II was observed and the comproportionation constants were determined. The redox potentials of M^II/III showed a good correlation with DN of the solvent. In acetonitrile solution containing 0.1 mol dm⁻³ Bu₄NPF₆ reversible M^II/III redox processes were observed. However, in the presence of stronger ion pairing anions (e.g. BF₄⁻ and ClO₄⁻) the “potential inversion” occurred. Redox potentials of dinuclear complexes were compared with mononuclear cyclidene complexes. The influence of selected anions (Cl⁻ and NO₃⁻) on the redox process M^II/III was studied. The effect of Cl⁻ anion was different for 3CuCu and 3NiNi. In 3CuCu coordination of two chloride anions took place after oxidation of copper centers. In the 3NiNi complex two Cl⁻ anions were coordinated to one of the nickel(II) centers facilitating oxidation, at different potentials, of both nickel(II) cations. The behaviour of heteronuclear complex with Cl⁻ anions was similar to 3NiNi. All dinuclear complexes interacted with NO₃⁻ anions and the observed potential shifts were larger for nickel(II) than for copper(II) cations.     

Hg detection by aniline-based conjugated copolymers with high selectivity

Conjugated polymers (P1, P1L and P2-P5) constructed by alkynyl-substituted aniline and substituted arene analogs could be synthesized through Pd-catalyzed Sonogashira coupling. The responsive optical properties of poly(2-ethynyl aniline) (P1 and P1L for different molecular weights, P1L with longer chains on average) toward various metal ions (including Ni²⁺, Ag⁺, Cu²⁺, Zn²⁺, Fe³⁺, Fe²⁺, Mn²⁺, Na⁺, Ca²⁺, Pb²⁺, K⁺, Cr³⁺, Al³⁺, Cd²⁺, Pt²⁺, Au³⁺ and Tl³⁺) were investigated. Hg²⁺ exhibited the most pronounced fluorescence response of both P1 and P1L without interference from those coexistent ions due to aniline in the polymer backbone as the metal binding ligand, while other metal ions does not cause obvious change of fluorescence. Compared with P1, P1L exhibits better sensitivity toward Hg²⁺. Introducing pyridyl, thienyl or phenyl groups into the polymer backbone would weaken the quenching responses to Hg²⁺ compared with P1. The results indicated P1 and P1L could be used as a selective fluorescence sensor toward Hg²⁺.     

A highly selective and sensitive fluorescence chemosensor based on optically active polybinaphthyls for Hg

The chiral polymer was synthesized by the polymerization of 4,7-diethynylbenzo[2,1,3]-thiadiazole (M-1) with (R)-6,6′-dibutyl-3,3′-diiodo-2,2′-bis(diethylaminoethoxy)-1,1′-binaphthyl (R-M-1) via Pd-catalyzed Sonogashira reaction. The chiral polymer has orange fluorescence due to the extended π-electronic structure between binaphthyl unit and benzo[2,1,3]thiadiazole (BT) group via ethynyl bridge. The responsive optical properties of the polymer on various metal ions were investigated by fluorescence spectra. The fluorescence of the chiral polymer can produce the pronounced enhancement as high as 1.8-fold upon addition of 1:2 molar ratio of Hg²⁺. Compared with other cations, such as K⁺, Mg²⁺, Pb²⁺, Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺, Zn²⁺, Mn²⁺ and Fe³⁺, Hg²⁺ can produce the pronounced fluorescence response of the polymer. The result indicates this kind of chiral polybinaphthyls incorporating diethylamino and benzo[2,1,3]thiadiazole (BT) moieties as receptors exhibits highly sensitive and selective behavior for Hg²⁺ detection.     

Synthesis and electrochemical characterisation of benzylmercapto and dodecylmercapto tetra substituted cobalt, iron, and zinc phthalocyanines complexes

The work reports on cyclic voltammetry (CV), square wave voltammetry and spectroelectrochemistry of the following complexes: tetrakis (benzylmercapto) phthalocyanine complexes of Zn(II) (ZnTBMPc, 4a), Co(II) (CoTBMPc, 5a), and Fe(II) (FeTBMPc 6a); tetrakis (dodecylmercapto) phthalocyanine complexes of Zn(II) (ZnTDMPc, 4b), Co(II) (CoTDMPc, 5b), and Fe(II) (FeTDMPc, 6b). More reversible CV couples were observed for complexes 4a, 5a, and 6a containing thiol phenyl ring substituents. Complexes 4b, 5b, and 6b containing long chain thiol substituents showed less reversible couples. Complexes 6a and 6b showed a relatively large number of redox processes (5 for 6a and 6 for 6b) within the potential window employed in this work. The processes for FePc derivatives (6a) are assigned to Fe^IIIPc⁻¹/Fe^IIIPc⁻², Fe^IIIPc⁻²/Fe^IIPc⁻², Fe^IIPc⁻²/Fe^IPc⁻², Fe^IPc⁻²/Fe^IPc⁻³, and Fe^IPc⁻³/Fe^IPc⁻⁴ and for the CoPc derivative (5a) to Co^IIIPc⁻¹/Co^IIIPc⁻², Co^IIIPc⁻²/Co^IIPc⁻², Co^IIPc⁻²/Co^IPc⁻², and Co^IPc⁻²/Co^IPc⁻³.     

Synthesis, electrochemical and spectroelectrochemical characterization of novel soluble phthalocyanines bearing chloro and quaternizable bulky substituents on peripheral positions

A new phthalonitrile derivative (2), bearing diethylaminophenoxy - and chloro-substituents at peripheral positions was synthesized in this work. Cyclotetramerization of (2) in hexanol gave the desired metal-free (4) and metallophthalocyanines (5-8). These new phthalocyanines (4-8) were converted into water-soluble quaternized products by the reaction with methyl iodide (9-11). The novel compounds have been characterized by using elemental analysis, UV-Vis, FT-IR, ¹H NMR and MS spectroscopic data. The aggregation behaviors of the phthalocyanine complexes were studied in different solvents and concentrations. Electrochemical and spectroelectrochemical characterization of the complexes were also performed in solution. Cobalt phthalocyanine gives both metal-based and ring-based reduction processes in comparison to the complexes having 2H⁺, Zn²⁺, Ni²⁺ and Cu²⁺ metal center which give only ring-based reduction processes. Electrochemical and spectroelectrochemical measurements exhibit that all complexes oxidatively electro-polymerize on the Pt working electrode during repetitive cyclic voltammetry measurements. An in-situ electrocolorimetric method was applied to investigate the color of the electrogenerated anionic and cationic forms of the complexes for possible electrochromatic applications.     

Novel coumarin fluorescent dyes: Synthesis, structural characterization and recognition behavior towards Cu(II) and Ni(II)

A series of novel coumarin dyes (3-12) were designed and synthesized. The structures of the dyes were characterized by IR, ¹H NMR, ¹³C NMR, ¹⁹F NMR, MS and single crystal X-ray diffraction. 3-(2-Benzoylhydrazonotrifluoroethyl)-7-(N,N-diethylamino)coumarin (11) could recognize Cu²⁺ and Ni²⁺ selectively in aqueous solution. Upon addition of Cu²⁺ or Ni²⁺ to 11 a blue shift or a bathochromatic shift of the absorption band was observed while the emission band blue-shifted with decrease in the fluorescence intensity. Upon addition of Cu²⁺ the color of the solution of 11 changed from orange to red. The results showed that 11 could be used as an optical chemosensor of Cu²⁺ and Ni²⁺.     

Electroanalytical studies on cobalt(II) selective potentiometric sensor based on bridge modified calixarene in poly(vinyl chloride)

A bridge modified 4-tert-butylthiacalix[4]arene (I) has been employed as electroactive material in the preparation of cobalt selective sensor. Polyvinyl chloride (PVC)-based membranes of (I) using sodium tetraphenylborate (NaTPB) as anion discriminator and bis(2-ethylhexyl) sebacate (DOS), chloronaphthalene (CN), tri-n-butylphosphate (TBP), o-nitrophenyl octyl ether (NPOE), tris(2-ethylhexyl)phosphate (TEHP) as plasticizers were prepared and investigated as cobalt selective sensors. A number of membranes of different compositions were prepared and investigated. The best performance was observed with the membrane having composition of 2:66:1.5:127 (mg) = I:NaTPB:PVC:NPOE. The potential response of this membrane is linear to Co²⁺ ions in the concentration range 5.3 × 10⁻⁶ to 1.0 × 10⁻¹ M with near-Nernstian slope of 30.0 mV/decade of activity and a detection limit of ∼0.3 ppm. This membrane also showed lowest response time of 10 s and works satisfactorily in partially non-aqueous medium. The selectivity studies of this sensor, evaluated with fixed interference method and matched potential method, show that the sensor under consideration possesses excellent selectivity for Co²⁺ over a large number of mono-, bi- and trivalent cations such as Na⁺, K⁺, Ag⁺, Ca²⁺, Mg²⁺, Cu²⁺, Hg²⁺, Pb²⁺, Li⁺, Ba²⁺, Zn²⁺, Sr²⁺, Cr³⁺, Ni²⁺, etc. The sensor could be used as an indicator electrode in the quantification of Co²⁺ by potentiometric titration against EDTA as well as in determination of cobalt content in wastewater and beer samples.     

Synthesis and properties of chiral helical polymers based on optically active polybinaphthyls

The chiral polymer P-1 was synthesized by the polymerization of (R)-6,6′-dibutyl-3,3′-diiodo-2,2′-bisoctoxy-1,1′-binaphthyl (R-M-1) with 5,5′-divinyl-2,2′-bipyridine (M-1)via Pd-catalyzed Heck reaction. P-2 and P-2′ were prepared by Wittig-Horner reaction of (R)-6,6′-dibutyl-2,2′-bisoctoxy-1,1′-binaphthyl-3,3′-dicarbaldehyde (R-M-2) with 5,5′-bis (diethylphosphonomethyl)-2,2′-bipyridine (M-2) in the presence of EtONa or NaH, respectively. P-3 was synthesized by Wittig-Horner reaction of (R)-6,6′-di(4-trifluoromethylphenyl)-2,2′-bisoctoxy-1,1′-binaphthyl-3,3′-dicarbaldehyde (R-M-3) with M-2 using NaH as a base. The four polymers have strong blue-green fluorescence due to the extended π-electronic structure between the chiral model compounds (R)-6,6′-dibutyl-/di(4-trifluoromethylphenyl)-2,2′-bisoctoxy-1,1′-binaphthyl (R-1 or R-2) and the conjugated linker 2,2′-bipyridyl group via vinylene bridge. Both monomers and polymers were analyzed by NMR, MS, FT-IR, UV-vis spectroscopy, DSC-TGA, fluorescence spectroscopy, GPC and circular dichroism (CD) spectroscopy. Based on the great differences of specific rotation values and CD spectra, P-1 and P-2 may adopt a zigzag chain configuration, while P-2′ and P-3 may adopt a helical configuration. The responsive optical properties of the two chiral helical polymers P-2′ and P-3 on transition metal ions were investigated by fluorescence, UV-vis and CD spectra. The results show that Ag⁺ and Ni²⁺ lead to nearly complete fluorescence quenching of P-2′ and P-3, Cu²⁺ and Fe²⁺ can cause obvious fluorescence quenching, but Zn²⁺ and Cd²⁺ can only produce slight fluorescence quenching. Ag⁺, Ni²⁺, Cu²⁺ and Fe²⁺ can also lead to the obvious changes of UV-vis spectra of P-2′ and P-3. On the contrary, Zn²⁺ and Cd²⁺ cause little changes. Most importantly, the CD intensities and wavelengths of the chiral helical polymers P-2′ and P-3 exhibit the pronounced changes upon addition of Ag⁺ and Ni²⁺.     

New polyacetylene-based chemosensory materials for the “turn-on” sensing of α-amino acids

For the first time, a new disubstituted polyacetylene (P1) was utilized to sense α-amino acids based on a “turn-on” model, through an indirect approach. Thus, first, the strong blue fluorescence of P1 was quenched by trace copper ions efficiently, then the addition of α-amino acids to the P1/Cu²⁺ complex could recover the strong fluorescence of P1 to report the presence of α-amino acids. Interestingly, the P1/Cu²⁺ complex demonstrated much higher sensitivity towards histidine, with the detection limit of ∼2 ppm (∼1.3 × 10⁻⁵ mol/L). With the aid of a normal UV lamp, histidine could be differed from other α-amino acids visually by the observation of its strong blue fluorescence, at the concentration as low as 4.0 × 10⁻⁵ mol/L. Due to the convenient synthesis of P1, and the so far reported many other conjugated fluorescent polymers, the work reported here might open up a new avenue to develop new biosensors.     

Electrocatalytic four-electron reductions of O2 to H2O with cytochrome c oxidase model compounds

Three different complexes, heme-Cu ([(⁶L)Fe^IICu^I]⁺ (1), ⁶L=a binucleating ligand having a heme and covalently tethered copper binding tris(2-pyridyl)methylamine tetradentate moiety), heme complex ((⁶L)Fe^II (4), (with “empty” tethered chelate)), and the “parent” iron-porphyrinate ((F₈TPP)Fe^II (5), F₈TPP=tetrakis(2,6-difluorophenyl)porphyrinate) were employed for the electrocatalytic reduction of O₂. Complexes 1 and 4 reduce O₂ to water (four-electron reduction) with good efficiency (74 and 59%, respectively), but complex 5 exhibited only an ∼20% efficiency, thus primarily the two-electron reduction to hydrogen peroxide occurred. The results of the present electrochemical O₂-reduction studies and the previous studies have elucidated the O₂-binding nature of these three complexes, indicating the formation of quite stable Fe^III(O₂²⁻)Cu^II (peroxo) or Fe^IIIO₂⁻ (superoxo) species. In line with the thinking of other researchers, the fact that both 1 and 4 can well stabilize Fe^IIIO₂⁻ superoxo species may suggest that the formation of the latter is a key to the pathway favoring four-electron reduction of dioxygen to water.     

Azo-hydrazone tautomerism by in situ Cu ion catalysis and complexation with the H2O2 oxidant of C.I. Disperse Yellow 79

A novel dinuclear copper(II) complex 2 bearing double μ₂-oxo bridges has been obtained by in situ Cu^II ion catalysis and complexation with the H₂O₂ oxidant of C.I. Disperse Yellow 79 (1), where a new dianionic ligand is in situ formed having an additional phenolic group and it coordinates with the central Cu^II ion as a tridentate chelating ligand. Furthermore, both of the starting material 1 and the final neutral dye-metal product 2 have been structurally and spectrally characterized and compared, where azo-hydrazone tautomerism has been observed before and after metal-ion complexation. To the best of our knowledge, this is the first structural report about the heterocyclic dyes having quinoline-2,4-dione skeleton and the corresponding azo-hydrazone tautomerism between them.     

Polymer-based fluorescence sensor incorporating triazole moieties for Hg detection via click reaction

The polymer could be obtained by the polymerization of 1,4-dibutoxy-2,5-diethynylbenzene (M-1) with 1,4-diazidobenzene (M-2)via click reaction. The polymer show blue fluorescence. The responsive optical properties of the polymer on various transition metal ions were investigated by fluorescence spectra. Compared with other cations, such as Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺ and Zn²⁺, Hg²⁺ can exhibit the most pronounced fluorescence response of the polymer Hg²⁺ can exhibit the most pronounced fluorescence response of the polymer due to triazole moiety in the polymer main chain as the metal binding ligand. The results indicate this kind of conjugated polymer with triazole moiety synthesized by click reaction can be used as a selective fluorescence sensor for Hg²⁺ detection.     

Electrochemical characterisation of tetra- and octa-substituted oxo(phthalocyaninato)titanium(IV) complexes

The synthesis and electrochemical characterisation of the following oxotitanium tetra-substituted phthalocyanines are reported: 1,(4)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (5a); 1,(4)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (5b); 2,(3)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (6a) and 2,(3)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (6b). The electrochemical characterisation of complexes octa-substituted with 4-(benzyloxy)phenoxy (9b), phenoxy (9c) and tert-butylphenoxy (9d) groups is also reported. The cyclic voltammograms of the complexes exhibit reversible couples I-III and couple IV is quasi-reversible for complexes 5a, 5b, 6a and 6b. The first two reductions are metal-based processes, confirmed by spectroelectrochemistry to be due to Ti^IVPc²⁻/Ti^IIIPc²⁻ and Ti^IIIPc²⁻/Ti^IIPc²⁻ redox processes and the last two reductions are ring-based processes due to Ti^IIPc²⁻/Ti^IIPc³⁻ and Ti^IIPc³⁻/Ti^IIPc⁴⁻. Chronocoulometry confirmed a one-electron transfer at each reduction step. The electrochemistry of the above complexes is also compared to the previously reported 5c, 5d, 6c and 6d.     

Electrochemical study of stable N-alkoxyarylaminyl radicals

The electrochemical study of N-tert-butoxy-2,4-diphenyl-6-tert-butylphenylaminyl (1a), N-tert-butoxy-2,4-bis(4-chlorophenyl)-6-tert-butylphenylaminyl (1b), N-[2-(methoxycarbonyl)-2-propyl]-2,4-diphenyl-6-tert-butylphenylaminyl (2), and N-tert-butoxy-2,4,6-tris(4-chlorophenyl)phenylaminyl radicals (3) was performed by cyclic voltammetry using acetonitrile as the solvent and Bu₄NPF₆ as the supporting electrolyte. On cathodic scan (100 mV/s), all the radicals gave chemically reversible cyclic voltammograms, and the were determined to be −1.405 V (1a), −1.310 V (2a), −1.282 V (2b), and −1.195 V (3) (versus Fc⁺/Fc), respectively. On anodic scan (100 mV/s), on the other hand, 1a, 1b and 2 showed chemically reversible cyclic voltammograms, but 3 exhibited a partially reversible couple even on a scan rate of 500 mV/s, indicating that the cation species of 3 was less stable. The determined for 1a, 1b, 2 and 3 were 0.220, 0.280, 0.318 and 0.294 V (versus Fc⁺/Fc), respectively. The electrochemical data were compared with those of thioaminyl radicals, the corresponding sulfur analogues of 1-3.     

Synthesis of 1,9-bis[glycidyloxypropyl]penta(1′H,1′H,2′H,2′H-perfluoroalkylmethylsiloxane)s and copolymerization with piperazine

A series of 1,9-bis[glycidyloxypropyl]pentasiloxanes (IV-VI) were prepared by the platinum catalyzed hydrosilylation of 1,9-dihydridodecamethylpentasiloxane (I), 1,9-dihydrido-3,5,7-tris(3′,3′,3′-trifluoropropyl)heptamethylpentasiloxane (II), and 1,9-dihydrido-3,5,7-tris(1′H,1′H,2′H,2′H-perfluorooctyl)heptamethylpentasiloxane (III) with allyl glycidyl ether. Subsequently, IV-VI were copolymerized with piperazine to form high molecular weight copoly(carbosiloxane)s (VII-IX). The structures of the 1,9-bis[glycidyloxypropyl]penta-siloxanes (IV-VI) and copoly(carbosiloxane)s (VII-IX) were determined by ¹H, ¹³C, ²⁹Si, and ¹⁹F NMR as well as IR spectroscopy. The molecular weight distributions (M_w/M_n) of VII-IX have been characterized by gel permeation chromatography and their thermal properties measured by differential scanning calorimetry and thermal gravimetric analysis.     

Synthesis and photochromic properties of a multiple responsive diarylethene and its selective binding affinity for copper(II) ion

A multiple responsive photochromic diarylethene, 1-[2,5-dimethyl-3-thienyl]-2-[2-methyl-5-(2-pyridyl)-3-thienyl]perfluorocyclopentene (1) with excellent fatigue resistance has been successfully synthesized. Easy and reversible conversion is observed between the two open- and closed-ring isomers of 1 upon irradiation with UV and visible light. Addition of trifluoroacetic acid to these two isomers gives two new protonated products featuring distinctively different color changes. Photochromic diarylethene 1b is found to have selective interaction with copper(II) ion. With increasing amount of copper(II) ion, the absorption maximum of the photostationary state at 563 nm decreases and disappears eventually, and the color of the solution changes from purple to colorless. Upon UV light irradiation, the solution color remains colorless and can no longer be restored. UV-vis absorption spectroscopy, fluorescence, and NMR spectroscopy were used to investigate the selective interaction between diarylethene 1b and copper(II) ion with results indicating that diarylethene 1b exhibits selective binding affinity for copper(II) ion.     

Two-dimensional spin-exchange solid-state NMR study of the crystal structure of cellulose II

A two-dimensional (2D) ¹³C-¹³C spin-exchange NMR experiment was applied to the uniformly ¹³C-enriched cellulose II in order to obtain interatomic distance information in the cellulose II crystal. A radio frequency-driven recoupling (RFDR) rotor-synchronized π pulse sequence was incorporated during the mixing time of the 2D experiment. The 2D spin-exchange NMR recorded with a short mixing time (0.80-2.58 ms) provided the correlations between a pair of strongly coupled ¹³C spins such as neighbor carbon nuclei. This spectrum enabled us to assign all ¹³C resonance lines of two kinds of anhydroglucose residues A and B in the structure of cellulose II. On the basis of the ¹³C resonance assignment of residues A and B, the interatomic distances from each C1 to the other carbon nuclei were compared by measuring the 2D spectra recorded with longer mixing times (5.12-20.48 ms). As a result, it was revealed that the respective residues A and B are composed of independent chains (-A-A- and -B-B- repeating units) and that there are no -A-B- repeating units in the chain. This experimental technique is expected to be applicable to conformation analysis of polysaccharides as well as the other cellulose polymorphs.