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Biomedical subjects

Xiulan Xie

Publications and source records attributed to Xiulan Xie.

7 recordsLinked to original sources
PubMed2005

Synthesis, characterization and cytotoxicity of poly(ethylene glycol)-graft-trimethyl chitosan block copolymers.

PEGylated trimethyl chitosan (TMC) copolymers were synthesized in an attempt to both increase the solubility of chitosan in water, and improve the biocompatibility of TMC. A series of copolymers with different degrees of substitution were obtained by grafting activated poly(ethylene glycol)s (PEG) of different MW onto TMC via primary amino groups. Structure of the copolymers was characterized using 1H, 13C NMR spectroscopy and GPC. Solubility experiments demonstrated that PEG-g-TMC copolymers were completely water-soluble over the entire pH range of 1-14 regardless of the PEG MW, even when the graft density was as low as 10%. Using the methyl tetrazolium (MTT) assay, the effect of TMC molecular weight, PEGylation ratio, PEG and TMC molecular weight in the copolymers, and complexation with insulin on the cytotoxicity of TMC was examined, and IC50 values were calculated with L929 cell line. All polymers exhibited a time- and dose-dependent cytotoxic response that increased with molecular weight. PEGylation can decrease the cytotoxicity of TMC to a great extent in the case of low molecular weight TMCs. According to the cytotoxicity results, PEG 5 kDa is superior for PEGylation when compared to PEG 550 Da at similar graft ratios. Complexation with insulin further increased cell viability. In addition, Lactate dehydrogenase (LDH) assays were performed to quantify the membrane-damaging effects of the copolymers, which is in line with the conclusion drawn from MTT assay. Moreover, the safety of the copolymers was corroborated by observing the morphological change of the cells with inverted phase contrast microscopy. Based upon these results PEG-g-TMC merits further investigations as a drug delivery vehicle.

Animals↗
PubMed2005

An asymmetric ion channel derived from gramicidin A. Synthesis, function and NMR structure.

The biological ion channel gramicidin A (gA) was modified by synthetic means to obtain the tail-to-tail linked asymmetric gA-derived dimer compound 3. Single-channel current measurements for 3 in planar lipid bilayers exhibit an Eisenman I ion selectivity for alkali cations. The structural asymmetry does not lead to an observable functional asymmetry. The structure of 3 in solution without and with Cs cations was investigated by 1H-NMR spectroscopy. In CDCl3/CD3OH (1 : 1, v/v), 3 forms a mixture of double-stranded beta-helices. Upon addition of excess CsCl, the double-stranded species are converted completely into one new conformer: the right-handed single-stranded beta-helix. A combination of DQF-COSY and TOCSY was used for the assignment of the 1H-NMR spectrum of the Cs-3 complex in CDCl3/CD3OH (1 : 1, v/v). A total of 69 backbone, 27 long-range, and 64 side-chain distance restraints were obtained from NOESY together with 25 phi and 14 chi1 torsion angles obtained from coupling constants. These data were used as input for structure calculation with dyana built in sybyl 6.8. A final set of 11 structures with an average rmsd for the backbone of 0.45 A was obtained (PDB: 1TKQ). The structure of the Cs-3 complex in solution is equivalent to the bioactive channel conformation in the membrane environment.

Amino Acid Sequence↗
PubMed2004

Gs-HSQC-NOESY versus gs-NOESY-HSQC experiments: signal attenuation due to diffusion; application to symmetrical molecules.

Gradient-selected (gs) HSQC-NOESY type experiments are often applied in order to obtain NOE cross peaks in symmetrical molecules or in complex organic molecules, e.g. carbohydrates. Since the coherence-selecting gradients in these pulse sequences are separated by several delays, including the mixing time, the gs-HSQC-NOESY spectra exhibit severe signal attenuation due to diffusion effects. The respective NOE information unaffected by diffusion effects can be obtained by X-half-filtered gs-NOESY-HSQC spectra. In this paper, an X-half-filtered version of the gs-NOESY-HSQC experiment is presented and the NOE cross peak integrals of its spectra are compared with those of a standard gs-HSQC-NOESY experiment. With phenanthrene as an example of symmetrical molecules, it is shown that the relative signal attenuation of gs-HSQC-NOESY cross peaks versus gs-NOESY-HSQC cross peaks is caused by diffusion effects and follows the Stejskal-Tanner equation. In contrast, the X-half-filtered gs-NOESY-HSQC experiment provides NOE cross peaks with reasonable signal-to-noise ratios even for long-range interactions in the presence of quadrupolar relaxation. This is demonstrated by the spectra of lithium dimethylcuprate.

Journal Article↗
PubMed2003

Dimethyl- and bis[(trimethylsilyl)methyl]cuprates show aggregates higher than dimers in diethyl ether: molecular diffusion studies by PFG NMR and aggregation-reactivity correlations.

The molecular sizes of higher aggregates of dimethylcuprates (Me(2)CuLi (1), 1.LiI, and 1.LiCN) and bis[(trimethylsilyl)methyl]cuprates ((Me(3)SiCH(2))(2)CuLi (2), 2.LiI, and 2.LiCN) in diethyl ether (Et(2)O) were determined by pulsed field gradient (PFG) NMR diffusion measurements. The obtained diffusion coefficients show molecular sizes larger than those of dimers for all systems. In these higher aggregates, steric hindrance and dilution reduce aggregation, whereas LiCN increases it. The molecular sizes were first determined by a spherical model-free approach and then refined by structure models of higher aggregates. These models were built by a combination of diffusion results, known NMR studies, and crystal structures. Thus, polymeric chains with homodimeric cores connected by solvent (salt-free case) or solvent and salt (salt-containing case) were proposed. These models were confirmed by a solvation analysis, whereby the number of solvent molecules attached to the aggregates was determined by a weighted average study. On the basis of these structure models, the number of repetition units (length index) was determined to be between 1.3 and 5.2, with the general trends in aggregation independent of the structure model used. A combined analysis of the determined length indices and known relative reactivities led for the first time to a correlation between higher aggregation and reactivity of dimethylcuprates in the addition reaction with enones: aggregates higher than dimers reduce the reactivity. Consequently, despite their consistent homodimeric core structures, for the first time the remaining reactivity differences between iodo- and cyanodimethylcuprates in Et(2)O are explained by the difference in their aggregation.

Journal Article↗
PubMed1998

Ferro- and Antiferromagnetic Exchange in Decamethylbimetallocenes.

With the aim of studying next-neighbor magnetic interactions in polymeric metallocenes the paramagnetic decamethylbimetallocenes (M'M') have been chosen as most simple model compounds. They have been synthesized for vanadium, cobalt, and nickel (to yield V'V', Co'Co', and Ni'Ni', respectively) by starting from dilithium and dithallium salts of the fulvalene dianion. The latter have been characterized by (13)C NMR spectroscopy. Decamethylbiferrocene has been synthesized as a diamagnetic standard compound, and decamethylbicobaltocenium hexafluorophosphate, as a precursor to Co'Co'. While the methylated M'M' species were stable when protected from air, the synthesis of the parent binickelocene (Ni'Ni') was accompanied by the formation of the ternickelocene NiNiNi. According to (1)H NMR spectroscopy NiNi and NiNiNi were antiferromagnetic and underwent ligand exchange to nickelocene and bisfulvalenedinickel. Unlike the usually green nickelocenes Ni'Ni' was deep red-violet owing to a new band at 528 nm. Measurements of the magnetic susceptibility (chi(m)) and the magnetization established a rare example of ferromagnetic interaction within a purely organometallic compound for Co'Co'. By contrast, V'V' and Ni'Ni' were antiferromagnetic (J = -1.6 and -180 cm(-)(1), respectively, with H = -JS(A).S(B)). The (1)H and (13)C NMR spectra confirmed the expected structures of Co'Co' and Ni'Ni', while the synthesis of V'V'-d(8) and (2)H NMR spectroscopy were necessary to fully establish the vanadium compound. Temperature-dependent measurements of the (1)H NMR signal shifts and of chi(m) yielded similar J values for Ni'Ni'. MO calculations were carried out for M'M', and the results were converted into theoretical NMR spectra of the bridging fulvalene ligand depending on the spin-carrying MO. This allowed the full assignment of the NMR signals and showed that the spin is delocalized to more than one MO. The MOs were shown to have different magnetic coupling capabilities, and the different magnetic behavior of M'M' was attributed to the near-degeneracy of the magnetic orbitals.

Journal Article↗