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991.
Cellulose: the structure slowly unravels 总被引:5,自引:0,他引:5
ANTOINETTE C. O'SULLIVAN 《Cellulose (London, England)》1997,4(3):173-207
This article attempts to bring together basic and complex information which has been gathered on cellulose structure, principally
that of native cellulose, over the last few decades. Even though advances have been made in the field of crystallography,
powder crystallography cannot yield a definitive cellulose structure and single crystal diffraction is not possible due to
the lack of suitable crystals. Knowledge obtained on the biosynthesis of native cellulose and on the polymorphy of cellulose
and its derivatives help our understanding of ultrastructure. Many inconsistencies between early crystallographic studies
of native cellulose have been clarified by the discovery that two polymorphs (α and β) of cellulose I exist. Models of the
possible ultrastructural arrangements within native cellulose have been put forward over the decades; with advancement in
technology, computer simulations of small and large systems are being created to test the viability of these ultrastructural
models. It is hoped that this review will aid in the understanding of the complexity and uncertainties that still exist in
this subject.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
992.
The Brønsted acidity of the various Si(nAl) sites present in zeolites is evaluated from proton binding energy and LUMO energy calculated by the semiempirical MNDO quantum chemical method. The two calculated energy values both exhibit a linear correlation with the existing 29Si NMR chemical shift and the IR hydroxyl stretching frequency data. The inter-convertibility between different Si(nAl) sites during an alumination or dealumination process is also evaluated based on the calculated substitution energy. The results indicate that alumination processes are less favorable to occur in zeolites than dealumination processes and the latter is more likely to occur for Si(nAl) clusters that contain the maximum number of aluminum nearest neighbors. 相似文献
993.
A new optimization based adaptive control strategy for simulated moving beds (SMBs) is proposed. A linearized reduced order model, which accounts for the periodic nature of the SMB process, is used for online optimization and control. The manipulated variables are the four inlet flow rates, the outputs are the raffinate and extract concentrations. Concentration measurements at the raffinate and extract outlets are used as the feedback information. The state estimate from the periodic Kalman filter is used for the prediction of the outlet concentrations over a chosen horizon. Predicted outlet concentrations are the basis for the calculation of the optimal input adjustments, which maximize the productivity and minimize the desorbent consumption subject to constraints on product purities. The realization of this concept is discussed and the implementation on a virtual eight column SMB platform is assessed, in the case of binary linear systems. For a whole series of typical plant disturbances it is shown that the proposed approach is effective in minimizing off-spec products and in achieving optimal SMB operation, also in the case where there are significant model uncertainties. 相似文献
994.
995.
996.
Investigation of the Interactions of β‐Peptides with DNA Duplexes by Circular Dichroism Spectroscopy
Kenji Namoto James Gardiner Thierry Kimmerlin Dieter Seebach 《Helvetica chimica acta》2006,89(12):3087-3103
The interaction of β‐peptides with the DNA duplexes of dA20dT20 and a GCN4‐binding CRE sequence was examined. To gauge the factors that govern these interactions, two β‐pentadecapeptides, 1 and 2 , a β‐dodecapeptide, 3 , three β‐decapeptides, 4 – 6 , three β‐heptapeptides, 7 – 9 , and β‐octaarginine 10 were designed and synthesized. The β‐peptides were conceived to adopt a β‐peptide 314 helix, in which the side chains at position i and i + 3 are aligned vertically along one side of the helix. The side chains of Lys, Asn, and Arg were positioned such that potential H‐bonding sites were created for a helical conformation to interact with the base pairs of DNA. CD Analysis showed that β‐peptides 1, 2 , and 10 interacted with dA20dT20. In addition, β‐peptides 1 and 2 showed significant interaction with a DNA‐duplex 20mer containing the ATF/CREB recognition sequence for the regulatory protein GCN4. It is impossible, at this stage of the investigation, to make a safe proposal about the actual nature of the interaction of the structures(s) of the complexes, the formation of which is suggested by the CD spectra reported herein. 相似文献
997.
考虑了烯烃、醇与酸的再吸附及其非本征效应(烯烃、醇与酸在催化剂孔道中的扩散作用、物理吸附及溶解度效应)对产物分布的影响,推导了基于详细反应机理的亚甲基插入的烷基机理F-T合成校正综合动力学模型.利用文献数据对动力学模型进行了回归,获得了与文献报道结果相一致的动力学参数.由校正动力学模型计算的烷烃、烯烃、醇与酸产物分布及烯烃比、醇烃比及酸烃比与实验数据较好地吻合.动力学计算结果表明,在铁锰催化剂上,烷烃、烯烃、醇与酸生成的反应是平行竞争反应,烯烃、醇与酸在催化剂表面的再吸附及二次反应导致产物分布偏离了ASF分布.动力学研究还表明,相同碳数的醇与酸产物在催化剂表面上再吸附及二次反应的机会比相同碳数的烯烃大.通过比较相同碳数的烯烃、醇与酸的分子体积及沸点,指出了在铁锰催化剂上,低碳数的烯烃、醇与酸的再吸附及二次反应对产物分布影响的非本征效应中,烯烃、醇与酸的扩散阻力不是主导效应. 相似文献
998.
Oliveira SC Paiva TC Visconti AE Giudici R 《Applied biochemistry and biotechnology》1998,74(3):161-172
Discrimination between different rival models for describing the inhibitory effect of ethanol both on yeast growth and on
fermentation was studied for a continuous process of alcoholic fermentation in a tower reactor with recycling of flocculating
cells. Models tested include linear, parabolic, hyperbolic, exponential, and generalized nonlinear power-law types. The best
expressions were identified under the criteria that all the kinetic parameters should assume acceptable values in a feasible
range and should result in the best fit of the experimental data. The kinetic parameters were estimated from steady-state
data of several sugar concentrations in feeding stream (S0 = 160, 170, 180, 190, 200 g/L), constant dilution rate (D = 0.2 h-1), recycle ratio (α = 13.6), and temperature (T = 30°C). The best model for the yeast growth was of power-law type, whereas
for the product formation the best model was of linear type. These models were able to reproduce the trends of the process
variables satisfactorily. 相似文献
999.
Dabek-Zlotorzynska E Aranda-Rodriguez R Graham L 《Journal of separation science》2005,28(13):1520-1528
Urban fine airborne particulate matter (PM2.5) and vehicle emission samples were studied for water-soluble low-molecular-weight carboxylic acids using CE with indirect UV detection. Further identification of these acids was achieved using GC-MS as their butyl esters (after derivatization with BF3/butanol). Several dicarboxylic acids in the range C2-C10 including straight-chain, branched-chain, cis- and trans-unsaturated, and aromatic acids were confirmed by GC-MS. In addition, aromatic acids such as benzoate, phthalate, terephthalate, isophthalate, and 4-methylphtalate were present in such samples, but some of these were not well resolved by the used CE method. Oxocarboxylic acids (Cn(w) with n > 4) were also identified by GC-MS but not determined by CE due to lack of standards. The rapidity and simplicity of the CE method were clearly demonstrated, and the method was observed to be advantageous for routine monitoring of water-soluble organic acids in airborne PM2.5 and vehicle emission at low microg/L levels. 相似文献
1000.
用原子吸收光谱法测定延边地区317名65~108岁老人头发中锌、铜、铁、锰、镁含量,观察不同民族、性别、年龄老人这些元素的差异。结果表明:1.朝鲜旗发锌低于汉族,满族发铜低于汉族、朝鲜族,汉族发镁高于朝鲜族、满族,发铁、发锰在汉、朝、满民族之间无区别。2.汉族、朝鲜族男发锌高于女性,汉族女性发铜高于男性,发铁、发锰、发镁在性别之间无区别。3、90岁组朝鲜族发锰、发镁低于65岁、76岁组,朝鲜族发锌、发铜、发铁在65~108岁之间无统计学意义。 相似文献