Concentration enrichment of urea at cellulose surfaces: results from molecular dynamics simulations and NMR spectroscopy

A combined solid-state NMR and Molecular Dynamics simulation study of cellulose in urea aqueous solution and in pure water was conducted. It was found that the local concentration of urea is significantly enhanced at the cellulose/solution interface. There, urea molecules interact directly with the cellulose through both hydrogen bonds and favorable dispersion interactions, which seem to be the driving force behind the aggregation. The CP/MAS ¹³C spectra was affected by the presence of urea at high concentrations, most notably the signal at 83.4 ppm, which has previously been assigned to C4 atoms in cellulose chains located at surfaces parallel to the (110) crystallographic plane of the cellulose Iβ crystal. Also dynamic properties of the cellulose surfaces, probed by spin-lattice relaxation time ¹³CT ₁ measurements of C4 atoms, are affected by the addition of urea. Molecular Dynamics simulations reproduce the trends of the T ₁ measurements and lends new support to the assignment of signals from individual surfaces. That urea in solution is interacting directly with cellulose may have implications on our understanding of the mechanisms behind cellulose dissolution in alkali/urea aqueous solutions.     

Quantum mechanical calculations on cellulose–water interactions: structures,energetics, vibrational frequencies and NMR chemical shifts for surfaces of Iα and Iβ cellulose

Periodic and molecular cluster density functional theory calculations were performed on the Iα (001), Iα (021), Iβ (100), and Iβ (110) surfaces of cellulose with and without explicit H₂O molecules of hydration. The energy-minimized H-bonding structures, water adsorption energies, vibrational spectra, and ¹³C NMR chemical shifts are discussed. The H-bonded structures and water adsorption energies (ΔE_ads) are used to distinguish hydrophobic and hydrophilic cellulose–water interactions. O–H stretching vibrational modes are assigned for hydrated and dry cellulose surfaces. Calculations of the ¹³C NMR chemical shifts for the C4 and C6 surface atoms demonstrate that these δ¹³C4 and δ¹³C6 values can be upfield shifted from the bulk values as observed without rotation of the hydroxymethyl groups from the bulk tg conformation to the gt conformation as previously assumed.     

Computational studies on the behavior of sodium dodecyl sulfate (SDS) at TiO2(rutile)/water interfaces

Molecular dynamics simulations to study the behavior of an anionic surfactant close to TiO(2) surfaces were carried out where each surface was modeled using three different crystallographic orientations of TiO(2) (rutile), (001), (100) and (110). Even though all three surfaces were made with the same atoms the orientation was a key to determine adsorption since surfactant molecules aggregated in different ways. For instance, simulations on the surface (100) showed that the surfactant molecules formed a hemicylinder structure whereas the molecules on the surface (110) were attached to the solid by forming a hemisphere-like structure. Structure of the aggregated molecules and surfactant adsorption on the surfaces were studied in terms of tails and headgroups density profiles as well as surface coverage. From density profiles and angular distributions of the hydrocarbon chains it was possible to determine the influence of the solid surface. For instance, on surfaces (100) and (001) the surfactant molecules formed molecular layers parallel to the surface. Finally, it was found that in the solids (100) and (110) where there are oxygen atoms exposed on the surface the surfactant molecules were attached to the surfaces along the sites between the lines of these oxygen atoms.     

A solid-state NMR study of the dynamics and interactions of phenylalanine rings in a statherin fragment bound to hydroxyapatite crystals

Extracellular matrix proteins regulate hard tissue growth by acting as adhesion sites for cells, by triggering cell signaling pathways, and by directly regulating the primary and/or secondary crystallization of hydroxyapatite, the mineral component of bone and teeth. Despite the key role that these proteins play in the regulation of hard tissue growth in humans, the exact mechanism used by these proteins to recognize mineral surfaces is poorly understood. Interactions between mineral surfaces and proteins very likely involve specific contacts between the lattice and the protein side chains, so elucidation of the nature of interactions between protein side chains and their corresponding inorganic mineral surfaces will provide insight into the recognition and regulation of hard tissue growth. Isotropic chemical shifts, chemical shift anisotropies (CSAs), NMR line-width information, (13)C rotating frame relaxation measurements, as well as direct detection of correlations between (13)C spins on protein side chains and (31)P spins in the crystal surface with REDOR NMR show that, in the peptide fragment derived from the N-terminal 15 amino acids of salivary statherin (i.e., SN-15), the side chain of the phenylalanine nearest the C-terminus of the peptide (F14) is dynamically constrained and oriented near the surface, whereas the side chain of the phenylalanine located nearest to the peptide's N-terminus (F7) is more mobile and is oriented away from the hydroxyapatite surface. The relative dynamics and proximities of F7 and F14 to the surface together with prior data obtained for the side chain of SN-15's unique lysine (i.e., K6) were used to construct a new picture for the structure of the surface-bound peptide and its orientation to the crystal surface.     

Investigation of the association and flexibility of cationic lipidic peptide dendrons by NMR spectroscopy

The cationic peptide dendrons synthesized and studied are lower generation polylysine-based partial dendrimers with or without lipid chains in the core. The dendrons with lipidic chains can be utilized as protein and liposomal mimics because of their unique structural properties. The full assignments of three different dendrons (L)7(NH2)8, (C14)1(L)7(NH2)8 and (C14)3(L)7(NH2)8 were obtained in D2O and H2O/D2O using a 500 MHz NMR spectrometer. The hydrophobic lipidic core of branched polylysine dendrons was found to induce aggregation upon increasing concentration. Because non-lipidic dendrons do not self-assemble, the behaviour and internal structural features of two different dendrons with one and three C14 hydrocarbon chains were explored. The critical association concentration clearly depends on the number of core hydrophobic residues and the association starts at 0.025 mM for (C14)1(L)7(NH2)8 and 0.05 mM for (C14)3(L(7(NH2)8. Chemical shift analysis also revealed that the hydrophobic chains of the dendrons associate in the core, whereas the polar head groups (NH2) are mainly located at the surfaces of the aggregates. The T1 relaxation time measurements showed that the mobility of the hydrocarbon chain is greater with the monomeric form of dendron (C14)1(L)7(NH2)8) than that of monomer (C14)3(L)7(NH2)8. The inter-chain hydrophobic interactions restrict the flexibility of the dendron with three hydrocarbon chains. As expected, the flexibility of the monomeric form is higher than that of the aggregated state for both of the dendrons.     

介质和力场协同作用对纳米纤维素形貌结构的调控

纤维素是一种由直链多聚糖通过糖苷键连接而成的巨型线性高分子,纤维素分子链通过氢键紧密排列形成纤维素晶体.由于纤维素晶体具有优良的化学可修饰性和机械性能等优点,纳米化加工的纤维素可广泛应用于日常生活和工业生产的各个领域.本文主要介绍了本课题组在机械剪切力作用下,实现纤维素纳米化并同时进行亲水或疏水改性的研究进展,重点介绍了介质极性对纤维素分子链之间相互作用的影响,并通过改变分子链之间的相互作用来调控纳米化纤维素的形貌和亲、疏水性.提出机械外力和环境极性协同作用下,晶面导向剥离纤维素的理论.     

Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces

Iron oxyhydroxides (FeOOH) are highly reactive minerals of widespread occurrence in natural and industrial settings. These minerals chiefly occur as nano- to submicron-sized particles and are covered by hydroxyl functional groups coordinated to one (-OH), two (μ-OH), or three (μ(3)-OH) underlying iron atoms. These groups are reaction centers for gases, solutes as well as solvents and thereby play important roles in the fate and transformation of natural and industrial compounds. In this work we provide tools to identify hydroxyl groups on distinct crystallographic planes of two important FeOOH polymorphs, namely lepidocrocite (γ-FeOOH) and goethite (α-FeOOH). Fourier transform infrared spectroscopy was used to monitor O-H stretching vibrations of groups on particles with well-defined and distinct morphologies. Spectral responses to proton loadings and thermal gradients were used to assign bands to hydroxyl groups. These efforts were facilitated by the extraction of pure spectral components obtained by multivariate curve resolution. Molecular dynamics simulations of dominant crystallographic planes of the particles guided band assignment procedures by identifying feasible hydrogen bond networks between surface groups. Our findings provide new possibilities for molecular-scale resolution of important gas-phase processes on the surfaces of these important minerals.     

Oligosaccharide Analogues of Polysaccharides. Part 26

The anthraquinone derivatives T‐x‐x ( x = 2, 4, and 8), possessing two cellobiosyl, cellotetraosyl, and cellooctaosyl chains, respectively, C‐glycosidically bonded at C(1) and C(8) were synthesised as potential mimics of cellulose I. The anthraquinone template enforces a parallel orientation of the cellodextrin chains at a distance corresponding to the one between the crystallographically independent chains of cellulose I, and the ethynyl and buta‐1,3‐diynyl linker units ensure an appropriate phase shift between them. The H‐bonding of the T‐x‐x mimics was analysed and compared to the one of the mono‐chained analogues T‐x and of the known cellulose II mimics N‐x‐x and N‐x where one or two cellodextrin chains are O‐glycosidically bonded to naphthalene‐1,8‐diethanol, or to naphthalene‐1‐ethanol. The OH signals of T‐x and T‐x‐x in solution in (D₆)DMSO were assigned on the basis of DQFCOSY, HSQC, and TOCSY (only of T‐4, T‐4‐4 , and T‐8‐8 ) spectra and on a comparison with the spectra of N‐x and N‐x‐x. Hydrogen bonding was analysed on the basis of the chemical shift of OH groups and its temperature dependence, coupling constants, SIMPLE ¹H‐NMR experiments, and ROESY spectra. T‐4‐4 and T‐8‐8 in (D₆)DMSO appear to adopt a V‐shape arrangement of the cellosyl chains, avoiding inter‐chain H‐bond interactions. The well‐resolved solid‐state CP/MAS ¹³C‐NMR spectra of the mono‐chained T‐x ( x = 1, 2, 4, and 8) show that only T‐8 is a close mimic of cellulose II. While the solid‐state CP/MAS ¹³C‐NMR spectrum of the C₁‐symmetric diglucoside T‐1‐1 is well‐resolved, the spectra of T‐2‐2 and T‐4‐4 show broad signals, and that of T‐8‐8 is rather well resolved. The spectrum of T‐8‐8 resembles that of cellulose I_β. A comparison of the X‐ray powder‐diffraction spectra of T‐8‐8 and T‐8 with those of celluloses confirms that T‐8‐8 is a H‐bond mimic of cellulose I and T‐8 one of cellulose II. Surprisingly, there is little difference between the CP/MAS ¹³C‐NMR spectra of the acetyl protected mono‐chained C‐glycosylated anthraquinone derivatives A‐x and the double‐chained A‐x‐x ( x = 2, 4, and 8). The spectra of A‐4 and A‐4‐4 resemble strongly the one of cellulose triacetate I ( CTA I ). The (less well‐resolved) spectra of the cellooctaosides A‐8 and A‐8‐8 , however, resemble the one of CTA II . The similarity between the solid‐state CP/MAS ¹³C‐NMR spectra of A‐4 and A‐4‐4 to the one of CTA I , and of A‐8 and A‐8‐8 to the one of CTA II is opposite to the observations in the acetylated cellodextrin series. The mono‐chained A‐x cellulose triacetate mimics 21 ( A‐2 ), 32 ( A‐4 ), and 55 ( A‐8 ) were synthesised by Sonogashira coupling of the cellooligosyl‐ethynes 15, 28 , and 50 , followed by selective deacetylation. Complete deacetylation provided the corresponding T‐x mimics. The double‐chained A‐x‐x mimics 24 ( A‐2‐2 ), 35 ( A‐4‐4 ), and 58 ( A‐8‐8 ) were prepared from A‐x by triflation and Sonogashira coupling with the cellosyl‐buta‐1,3‐diynes 19, 31 , and 53 . Their deacetylation provided the corresponding T‐x‐x mimics 25, 36 , and 59 . The cellooligosyl‐ethynes and cellooligosyl‐buta‐1,3‐diynes required for the Sonogashira coupling were prepared by stepwise glycosylation of the partially O‐benzylated β‐cellobiosyl‐ethyne and β‐cellobiosyl‐buta‐1,3‐diyne 13 and 17 , respectively, with the cellobiosyl donor 2 and the cellohexaosyl donor 47 .     

CP/MAS (13)C NMR study of cellulose and cellulose derivatives. 1. Complete assignment of the CP/MAS (13)C NMR spectrum of the native cellulose

The precise assignments of cross polarization/magic angle spinning (CP/MAS) (13)C NMR spectra of cellulose I(alpha) and I(beta) were performed by using (13)C labeled cellulose biosynthesized by Acetobacter xylinum (A. xylinum) ATCC10245 strain from culture medium containing D-[1,3-(13)C]glycerol or D-[2-(13)C]glucose as a carbon source. On the CP/MAS (13)C NMR spectrum of cellulose from D-[1,3-(13)C]glycerol, the introduced (13)C labeling were observed at C1, C3, C4, and C6 of the biosynthesized cellulose. In the case of cellulose biosynthesized from D-[2-(13)C]glucose, the transitions of (13)C labeling to C1, C3, and C5 from C2 were observed. With the quantitative analysis of the (13)C transition ratio and comparing the CP/MAS (13)C NMR spectrum of the Cladophora cellulose with those of the (13)C labeled celluloses, the assignments of the cluster of resonances which belong to C2, C3, and C5 of cellulose, which have not been assigned before, were performed. As a result, all carbons of cellulose I(alpha) and I(beta) except for C1 and C6 of cellulose I(alpha) and C2 of cellulose I(beta) were shown in equal intensity of doublet in the CP/MAS spectrum of the native cellulose, which suggests that two inequivalent glucopyranose residues were contained in the unit cells of both cellulose I(alpha) and I(beta) allomorphs.     

High-Resolution <Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR of Glycyrrhizic Acid and Its Esters

Resonances for protons and C atoms in the ¹H and ¹³C NMR spectra of glycyrrhizic acid and its esters were assigned using high-resolution ¹H (600 MHz) and ¹³C (150 MHz) NMR methods. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 347–350, July–August, 2005.     

300 Mhz 1H NMR spectra and conformations of biotin and related hexahydrothienoimidazolone derivatives

¹H NMR spectra of biotin and four related hexahydrothienoimidazolones in which the endo pentanoate side chain of biotin is replaced by another endo or exo substituent, and the urea nitrogen atoms are substituted with benzyl groups, have been obtained at 300 MHz. Vicinal coupling constants differentiate cis and trans proton pairs. The generalized Karplus equation was utilized to calculate dihedral angles from vicinal proton-proton coupling constants. The conformation of biotin in solution, calculated from coupling constants, is in good agreement with solid state X-ray crystallographic data.     

Superhydrophobic surfaces from surface-hydrophobized cellulose fibers with stearoyl groups

In this report, surface-hydrophobized cellulose fibers by stearoyl groups were used for the construction of superhydrophobic surfaces. The product after the synthesis contains two components: cellulose microfibers as the major component and nanoscaled segments in small amounts. The crystalline structure of cellulose was maintained after surface modification based on solid-state ¹³C NMR spectroscopy. Superhydrophobic surfaces showing static water contact angles of >150° were fabricated using freshly prepared products containing both components via the facile route, e.g., solvent casting. The cellulose types, microcrystalline cellulose or cotton linter cellulose fibers, did not significantly affect the chemical modification of cellulose fibers, but the superhydrophobic surfaces using surface-hydrophobized cotton linters as starting materials exhibited higher surface hydrophobicity and better impact stability in comparison to shorter microcrystalline cellulose. Due to the presence of a crystalline cellulose skeleton, the obtained superhydrophobic surfaces are stable during the heat treatment at 80 °C.     

Structural analysis of polymer-brush-type cellulose β-ketoesters by molecular dynamics simulation

The reason for anomalous NMR patterns of cellulose β-ketoesters, which were prepared by reaction between cellulose and ketene dimers having long alkyl chains (AKDs) under homogeneous conditions using a cellulose solvent system, was studied by molecular dynamics simulation. Cellulose/AKD β-ketoester models with degree of substitution (DS) 2.0 and degree of polymerization (DP) 5, 10, 20 or 40, and cellulose/fatty acid ester models with DS 3.0 and DP 5, 10, 20 or 40 were assembled in the simulation. The calculated results were compared with those obtained by NMR and conformation analyses of the actually prepared cellulose derivatives. The molecular dynamics simulation data showed that the average velocities of anhydroglucose units in cellulose/AKD β-ketoesters were approximately one tenth of those in cellulose/fatty acid esters. Thus, cellulose chains in the cellulose/AKD β-ketoesters are extremely restricted in motion by the β-ketoester substituents. The solid-like behavior of cellulose chains in cellulose/AKD β-ketoesters in solution state is, therefore, explainable by strong restriction in motion of cellulose chains by long, branched and bulky substituents introduced into cellulose hydroxyls in high densities via β-ketoester bonds.     

预测[Si, C, S]＋和[Si, C, S]－体系的稳定异构体

采用DFT, QCISD及CCSD(T)方法分别对二重态的[Si, C, S]＋和[Si, C, S]－体系势能面进行理论计算, 用QCISD/6-311＋G(d)方法, 在[Si, C, S]＋和[Si, C, S]－体系中, 我们分别得到了2个过渡态连接的3个稳定体和2个过渡态连接的4个稳定体, 经热力学及动力学分析发现, [Si, C, S]＋体系只有二重态线性的离子[Si—C—S]＋可能稳定存在, 而[Si, C, S]－体系有二重态线性的离子[Si—C—S]－和三元环c-[SiCS]－可能稳定存在.     

Interaction tensors and local dynamics in common structural motifs of nitrogen: a solid-state (14)n NMR and DFT study

(14)N solid-state NMR powder patterns have been obtained at high field (21.1 T) using broadband, frequency-swept pulses and a piecewise acquisition method. This approach allowed the electric field gradient (EFG) tensor parameters to be obtained from model organic and inorganic systems featuring spherically asymmetric nitrogen environments (C(Q) values of up to ca. 4 MHz). The advantages and limitations of this experimental approach are discussed, and the observation of (14)N T(2) relaxation anisotropy in certain systems is also reported, which can shed light on dynamic processes, allowing motional geometries and jump rates to be probed. In particular, we show that observable effects of dynamics on (14)N spectra can be mediated by modulation of either the EFG tensor or heteronuclear dipolar couplings. It is demonstrated that the QCPMG protocol can be used to selectively enhance certain types of nitrogen environments on the basis of differences in T(2). We also present the results of extensive density functional theory calculations on these systems, which show remarkably good correlation with the experimental results and allow the prediction of tensor orientations, assignment of parameters to crystallographic sites, and a rationalization of the origin of the EFG tensors in terms of contributions from individual molecular orbitals. This work demonstrates that ultra-wideline (14)N solid-state NMR can, under favorable circumstances, be a straightforward, useful, and informative probe of molecular structure and dynamics.     

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.     

Syntheses, structure, and a Mössbauer and magnetic study of Ba(4)Fe(2)I(5)S(4)

The compound Ba4Fe2I5S4 has been prepared at 1223-1123 K by the "U-assisted" reaction of FeS, BaS, S, and U with BaI2 as a flux. A more rational synthesis was also found; however, the presence of U appears to be essential for the formation of single crystals suitable for X-ray diffraction studies. Ba4Fe2I5S4 crystallizes in a new structure type with two formula units in space group I4/m of the tetragonal system. The structure consists of a Ba-I network penetrated by (1)infinity[Fe2S4] chains. Each Fe atom, which is located on a site with 4 symmetry, is tetrahedrally coordinated to four S atoms. The FeS4 tetrahedra edge-share to form linear (1)infinity[Fe2S4] chains in the [001] direction. The Fe-Fe interatomic distance in these chains is 2.5630(4) A, only about 3% longer than the shortest Fe-Fe distance in -Fe metal. Charge balance dictates that the average formal oxidation state of Fe in these chains is +2.5. The M?ssbauer spectra obtained at 85 and 270 K comprise a single quadrupole doublet that has hyperfine parameters consistent with an average Fe oxidation state of +2.5. The M?ssbauer spectrum obtained at 4.2 K consists of a single magnetic sextet with a small hyperfine field of -15.5 T. This spectrum is also consistent with rapid electron delocalization and an average Fe oxidation state of +2.5. The molar magnetic susceptibility of Ba4Fe2I5S4, obtained between 3.4 and 300 K, qualitatively indicates the presence of weak pseudo-one-dimensional ferromagnetic exchange within a linear chain above 100 K and weak three-dimensional ordering between the chains at lower temperatures.     

Molecular dynamics simulation of cellulose-coated oil-in-water emulsions

The behaviors of cellulose chains and cellulose mini-crystal in oil-in-water emulsions were studied by molecular dynamics simulations to investigate the coating states and the structural features of cellulose in these emulsions. In oil-in-water emulsion, dispersed cellulose chains gradually assemble during the progress of the simulation, eventually surrounding the octane droplet. In case of a cellulose mini-crystal, the cellulose chain at the corner of the crystal first contacts with the octane droplet through its hydrophobic surface. The other cellulose chains along the crystal plane then gradually move toward the octane molecules. In both emulsions, the cellulose was found to interact with both water and octane surfaces with specific conformations that allow the CH groups of the glucose rings to contact with octane molecules, while the OH groups of these rings contact with water molecules to form hydrogen bonds. The cellulose chains on the octane droplet also contact with each other through lateral hydrogen bonding between chains. These interactions stabilize the emulsion formed by cellulose molecules as surfactants.