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901.
902.
The paper describes molecular dynamics (MD) simulations on the crystal structures of the Iβ and II phases of cellulose. Structural
proposals for each of these were made in the 1970s on the basis of X-ray diffraction data. However, due to the limited resolution
of these data some controversies remained and details on hydrogen bonding could not be directly obtained. In contrast to structure
factor amplitudes in X-ray diffraction, energies, as obtained from MD simulations, are very sensitive to the positions of
the hydroxyl hydrogen atoms. Therefore the latter technique is very suitable for obtaining such structural details. MD simulations
of the Iβ phase clearly shows preference for one of the two possible models in which the chains are packed in a parallel orientation.
Only the parallel-down mode (in the definition of Gardner and Blackwell (1974) J Biopolym 13: 1975-2001) presents a stable
structure. The hydrogen bonding consists of two intramolecular hydrogen bonds parallel to the glycosidic linkage for both
chains, and two intralayer hydrogen bonds. The layers are packed hydrophobically. All hydroxymethyl group are positioned in
the tg conformation. For the cellulose II form it was found that, in contrast to what seemed to emerge from the X-ray fibre
diffraction data, both independent chains had the gt conformation. This idea already existed because of elastic moduli calculations
and 13C-solid state NMR data. Recently, the structure of cellotetraose was determined. There appear to be a striking similarity
between the structure obtained from the MD simulations and this cellotetraose structure in terms of packing of the two independent
molecules, the hydrogen bonding network and the conformations of the hydroxymethyl group, which were also gt for both molecules.
The structure forms a 3D hydrogen bonded network, and the contribution from electrostatics to the packing is more pronounced
than in case of the Iβ structure. In contrast to what is expected, in view of the irreversible transition of the cellulose
I to II form, the energies of the Iβ form is found to be lower than that of II by 1 kcal mol-1 per cellobiose.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
903.
Recently there has been a moderate resurgence in the use of flax-seed in a variety of ways including bread. The scientific basis of its use is very limited. There is some claim for beneficial effects in cancer and lupus nephritis. These claims could be due to its ability to scavenge oxygen radicals. However, its antioxidant activity is not known. Recently a method has been developed to isolate secoisolariciresinol diglucoside (SDG) from defatted flax-seed in large quantity (patent pending). We investigated the ability of SDG to scavenge úOH using high pressure liquid chromatography (HPLC) method. úOH was generated by photolysis of H2O2 (1.25-10.0 \sgmaelig;moles/ml) with ultraviolet light and was trapped with salicylic acid which is hydroxylated to produce úOH-adduct products 2,3-dihydroxybenzoic acid (DHBA) and 2,5-DHBA. H2O2 produced a concentration-dependent úOH as estimated by 2,3-DHBA and 2,5-DHBA. A standard curve was constructed for known concentrations of 2,3-DHBA and 2,5-DHBA against corresponding area under the peaks which then was used for measurement of 2,3-DHBA and 2,5-DHBA generated by UV irradiation of H2O2 in the presence of salicylic acid. SDG in the concentration range of 25, 50, 100, 250, 500, 750, 1000 and 2000 \sgmaelig;g/ml (36.4, 72.8, 145.6, 364.0, 728.0, 1092.0, 1456.0 and 2912.0 \sgmaelig;M respectively) produced a concentration-dependent decrease in the formation of 2,3-DHBA and 2,5-DHBA, the inhibition being 4 and 4.65% respectively with 25 \sgmaelig;g/ml (36.4 \sgmaelig;M) and 82 and 74% respectively with 2000 \sgmaelig;g/ml (2912.0 \sgmaelig;M). The decrease in úOH-adduct products was due to scavenging of úOH not and by scavenging of formed 2,3-DHBA and 2,5-DHBA. SDG prevented the lipid peroxidation of liver homogenate in a concentration-dependent manner in the concentration range from 319.3-2554.4 \sgmaelig;M. These results suggest that SDG scavenges úOH and therefore has an antioxidant activity. 相似文献
904.
Andreas Janshoff Claudia Steinem Manfred Sieber A. el Bayâ Marcus Alexander Schmidt H.-J. Galla 《European biophysics journal : EBJ》1997,26(3):261-270
The binding of cholera toxin, tetanus toxin and pertussis toxin to ganglioside containing solid supported membranes has been
investigated by quartz crystal microbalance measurements. The bilayers were prepared by fusion of phospholipid-vesicles on
a hydrophobic monolayer of octanethiol chemisorbed on one gold electrode placed on the 5 MHz AT-cut quartz crystal. The ability
of the gangliosides GM1, GM3, GD1a, GD1b, GT1b and asialo-GM1 to act as suitable receptors for the different toxins was tested by measuring the changes of quartz resonance frequencies.
To obtain the binding constants of each ligand-receptor-couple Langmuir-isotherms were successfully fitted to the experimental
adsorption isotherms. Cholera toxin shows a high affinity for GM1 (Ka = 1.8 ⋅ 108M–1), a lower one for asialo-GM1 (Ka = 1.0 ⋅ 107 M–1) and no affinity for GM3. The C-fragment of tetanus toxin binds to ganglioside GD1a, GD1b and GT1b containing membranes with similar affinity (Ka∼106 M–1), while no binding was observed with GM3. Pertussis toxin binds to membranes containing the ganglioside GD1a with a binding constant of Ka = 1.6 ⋅ 106 M–1, but only if large amounts (40 mol%) of GD1a are present. The maximum frequency shift caused by the protein adsorption depends strongly on the molecular structure of
the receptor. This is clearly demonstrated by an observed maximum frequency decrease of 99 Hz for the adsorption of the C-fragment
of tetanus toxin to GD1b. In contrast to this large frequency decrease, which was unexpectedly high with respect to Sauerbrey's equation, implying
pure mass loading, a maximum shift of only 28 Hz was detected after adsorption of the C-fragment of tetanus toxin to GD1a.
Received: 14 January 1997 / Accepted: 15 April 1997 相似文献
905.
H. Etienne M. Lartaud N. Michaux-Ferriére M. P. Carron M. Berthouly C. Teisson 《In vitro cellular & developmental biology. Plant》1997,33(2):81-87
Summary A culture procedure using temporary immersion in a liquid medium was tested for somatic embryogenesis of Hevea brasiliensis (Müll. Arg.). Embryogenic callus was placed under regeneration conditions, either on a gelled medium (Phytagel, Sigma, St.
Louis, MO) or in a container designed for temporary immersion. The latter technique has some advantages over the use of a
gelled medium during both the early steps of somatic embryogenesis, i.e., embryo development, and later on, i.e., during maturation,
desiccation and germination. Somatic embryo production in a liquid medium was three to four times greater than on a semi-solid
medium: 400 embryos/g fresh weight under the best embryogenesis induction conditions. Somatic embryogenesis had to be initiated
on a gelled medium before the embryogenic callus was transferred to temporary immersion, and the amounts of 3,4- dichlorophenoxyacetic
acid and N6-benzyladenine had to be reduced. Temporary immersion resulted in substantially more consistent, synchronized somatic embryo
development, reducing the number of abnormal embryos by half and stimulating germination. All of the late events could be
carried out in the temporary immersion container. Effective drying conditions were achieved after 12 wk without immersion
and without selection of the embryos. Temporary immersion during germination greatly stimulated root development (+60%) and
epicotyl emergency (+35%), combined with increased synchronization and a substantially reduced workload. 相似文献
906.
本文报导了金蛇王营养液(GKS)对老龄鼠细胞因子产生的影响。实验表明,细胞因子分泌功能下降的老龄鼠饲喂金蛇王营养液后,IL-2和IFN-r水平明显增加。进而证明金蛇王营养液是一种良好的免疫增强剂,能增强老龄机体抗肿瘤、抗感染等作用 相似文献
907.
Optimizing nutritional conditions for the liquid culture production of effective fungal biological control agents 总被引:1,自引:0,他引:1
M A Jackson 《Journal of industrial microbiology & biotechnology》1997,19(3):180-187
Spores of fungal pathogens of weeds and insects are unique in their ability to actively infect and kill their pest host.
While these capabilities are advantageous in terms of their use as a contact biological control agent, or biopesticide, they
also require special consideration during spore production. Directed approaches to medium optimization must consider not
only spore yield but also spore qualities such as desiccation tolerance, stability as a dry preparation, and biocontrol
efficacy. Nutritional conditions during culture growth and sporulation should direct the accumulation of appropriate endogenous
reserves so that newly formed spores possess these advantageous qualities. Studies with the bioherbicidal fungus Colletotrichum truncatum and with the bioinsecticidal fungus Paecilomyces fumosoroseus have demonstrated the impact of nutrition on spore ‘fitness’ for use as a biological control agent. The optimization strategy
used in these nutritional studies as well as a comparison of the results are presented.
Received 06 February 1997/ Accepted in revised form 29 May 1997 相似文献
908.
909.
The microdialysis technique was used to study the effect of nitric oxide synthase (NOS) activity on taurine release. Taurine release was characterized in rat striatum that was excitotoxically lesioned compared to normal conditions. The basal taurine level of the dialysate decreased during quinolinate (QUIN) lesion in parallel to the cell degeneration process. The K+-stimulated taurine concentration also decreased during QUIN-lesion, but to an extent that was different from that of basal values. K+-stimulated taurine levels were further markedly lowered by coapplication of the NOS inhibitor L-NAME in control and in lesioned animals up to 30 days after QUIN-injection. Postdegenerative tissue did not show any NOS-dependency in K+-induced taurine release. We conclude that a substantial part of K+-induced taurine release depends on NOS-activity both in normal brain tissue and in excitotoxically induced neurodegeneration. The main source of K+-induced taurine release in control rats are neurons but in lesioned animals are activated astroglial cells. 相似文献