共查询到20条相似文献，搜索用时 62 毫秒
1.
Alkanes at the air-sea interface were analyzed in 118 surface samples collected at five different intervals over a 12-month period from Timbalier Bay (Louisiana), ogfshore Louiisiana, and offshore Florida. The alkanes were characterized by gas chromatography and mass spectrometry. Unexpectedly, methyl branched alkanes ranging in chain length froin C(15) to C(35) and cycloalkanes were frequently the predominant components. This suggests that the alkanes are produced by natural biological sources as well as human activities. 相似文献
2.
The configurational-bias Monte Carlo technique is applied to simulate the adsorption of long chain alkanes in zeolites. This simulation technique is several orders of magnitude more efficient than conventional methods that can be used to simulate the adsorption of long chain alkanes. The calculated heats of adsorption are found to be in excellent agreement with experimental data. The results show a surprising chain length dependence of the heats of adsorption. This dependence has a simple molecular explanation in terms of preferential siting of the long chain alkanes. 相似文献
3.
We used force-clamp atomic force microscopy to measure the end-to-end length of the small protein ubiquitin during its folding reaction at the single-molecule level. Ubiquitin was first unfolded and extended at a high force, then the stretching force was quenched and protein folding was observed. The folding trajectories were continuous and marked by several distinct stages. The time taken to fold was dependent on the contour length of the unfolded protein and the stretching force applied during folding. The folding collapse was marked by large fluctuations in the end-to-end length of the protein, but these fluctuations vanished upon the final folding contraction. These direct observations of the complete folding trajectory of a protein provide a benchmark to determine the physical basis of the folding reaction. 相似文献
4.
McAuliffe C 《Science (New York, N.Y.)》1969,163(3866):478-479
A new method for equilibrating water containing alkane hydrocarbons with a gas phase and analyzing the gas for hydrocarbon content by gas chromatography extends analytical sensitivity to better than 0.1 part per billion. The solubilities at 25 degrees C of the normal C(9) (220 parts per billion) and C(10) (52 parts per billion) alkanes decrease with increasing carbon number. A discontinuity occurs at the normal C(11), alkane, probably because of a change from true solubility (molecular dispersion) to accommodation (aggregation). 相似文献
5.
Two-dimensional molecular patterns were obtained by the adsorption of long-chain alkanes, alcohols, fatty acids, and a dialkylbenzene from organic solutions onto the basal plane of graphite. In situ scanning tunneling microscopy (STM) studies revealed that these molecules organize in lamellae with the extended alkyl chains oriented parallel to a lattice axis within the basal plane of graphite. The planes of the carbon skeletons, however, can be oriented either predominantly perpendicular to or predominantly parallel with the substrate surface, causing the lamellar lattice to be either in or near registry with the substrate (alkanes and alcohols) or not in registry (fatty acids and dialkylbenzenes). In the case of the alcohols and the dialkylbenzene the molecular axes are tilted by +30 degrees or -30 degrees with respect to an axis normal to the lamella boundaries, giving rise to molecularly well-defined domain boundaries. Fast STM image recording allowed the spontaneous switch between the two tilt angles to be observed in the alcohol monolayers on a time scale of a few milliseconds. 相似文献
6.
The conformation of linear polymer chains trapped in a matrix of cross-linked polymer has been measured by neutron scattering. Three regimes were found depending on the length of the linear chain, Nl, with respect to the mesh size of the network, N(c). When N(c) > Nl, the radius of gyration, R(g), of the linear chain is the same as that observed in the uncrosslinked melt. When N(c) < Nl, R(g) shrinks according to the scaling relation R(g)(-1) approximately N(c)(-1) that has been predicted for isolated polymer chains trapped in a field of random obstacles. When N(c) < Nl the linear chains are observed to segregate. 相似文献
7.
Bergman RG 《Science (New York, N.Y.)》1984,223(4639):902-908
Alkanes, although plentiful enough to be considered for use as feedstocks in large-scale chemical processes, are so unreactive that relatively few chemical reagents have been developed to convert them to molecules having useful functional groups. However, a recently synthesized iridium (lr) complex successfully converts alkanes into hydridoalkylmetal complexes (M + R-H --> R-M-H). This is a dihydride having the formula Cp(*)(L)lrH(2), where Cp(*) and L are abbreviations for the ligands (CH(3))(5)C(5) and (CH(3))(3)P, respectively. Irradiation with ultraviolet light causes the dihydride to lose H(2), generating the reactive intermediate Cp(*)lrL. This intermediate reacts rapidly with C-H bonds in every molecule so far tested (including alkanes) and leads to hydridoalkyliridium complexes Cp(*)(L)lr(R)(H). Evidence has been obtained that this C-H insertion, or oxidative addition, reaction proceeds through a simple three-center transition state and does not involve organic free radicals as intermediates. Thus the intermediate Cp(*)lrL reacts most rapidly with C-H bonds having relatively high bond energies, such as those at primary carbon centers, in small organic rings, and in aromatic rings. This contrasts directly with the type of hydrogen-abstraction selectivity that is characteristic of organic radicals. The hydridoalkyliridium products of the insertion reactions can be converted into functionalized organic molecules-alkyl halides-by treatment with mercuric chloride followed by halogens. Expulsion (reductive elimination) of the hydrocarbon from the hydridoalkyliridium complexes can be induced by Lewis acids or heat, regenerating the reactive intermediate Cp(*)lrL. Oxidative addition of the corresponding rhodium complexes Cp(*)RhL to alkane C-H bonds has also been observed, although the products formed in this case are much less stable and undergo reductive elimination at -20 degrees C. These and other recent observations provide an incentive for reexamining the factors that have been assumed to control the rate of reaction of transition metal complexes with C-H bonds-notably the need for electron-rich metals and the proximity of reacting centers. 相似文献
8.
三种类型柑橘成熟果实表面蜡质分析 总被引：3，自引：1，他引：2
【目的】分析中国主栽柑橘类型（‘鄂柑一号’椪柑（Citrus reticulata Blanco cv.Ponkan）、尤力克柠檬（C. limon cv Eureka cv. Lemon）和沙田柚（C. grandis Osbeck cv. Shatian Yu）成熟期果实表面蜡质,建立中国柑橘果实蜡质基础数据,为科学指导柑橘采后生产提供依据。【方法】以3种类型柑橘的成熟果实为材料,采用扫描电镜观察果实表面蜡质晶体结构;用GC-MS测定果实表面蜡质成分及含量;用定量PCR检测蜡质相关基因在不同类型柑橘果皮表达特性。【结果】扫描电镜结果显示沙田柚蜡质晶体呈较大的倒伏片状,椪柑片状晶体较小,柠檬介于上述两者之间。GC-MS分析表明,3种类型柑橘果实表面外蜡中脂肪族物质主要组成成分相同,分别为：脂肪醛、饱和游离脂肪酸、饱和正链烷烃和伯醇,但品种之间各组分比例及碳原子分布有一定差异。椪柑、柠檬以及沙田柚外蜡含量分别为1.1、2.2和9.3 μg·cm-2。椪柑外蜡中醛、脂肪酸、烷烃和伯醇所占比例为50%、16%、28%和6%;柠檬中各个成分比例分别为42%、30%、18%和11%;沙田柚中则分别为50%、31%、12%和7%。椪柑中26醛和28醛含量最高;柠檬中碳原子数分配比较均匀,没有绝对占主导的成分;沙田柚脂肪族物质中高碳原子数所占比例比其他柑橘类型果实蜡质中高,尤其是32酸、31和33烷烃含量很高。椪柑和柠檬总蜡含量分别为4.3和4.6 μg·cm-2。椪柑总蜡中醛、脂肪酸、烷烃和伯醇与外蜡各成分所占比例相似,分别为57%、14%、23%和4%;柠檬总蜡各成分所占比例则分别为53%、15%、19%和8%。总蜡不同种类脂肪族物质碳原子数分配与外蜡基本相同。萜类物质种类和含量在不同品种类型之间存在较大差异：柠檬中主要的三萜类成分是软木三帖酮（friedelin）、羽扇豆醇（lupeol）、α-香树精（α-amyrin）和β-香树精（β-amyrin）;椪柑中主要含有后3种,没有检测到软木三帖酮;角鲨烯和法尼醇为紧皮柑橘（柚和柠檬）特有。定量表达分析表明,超长链脂肪酸辅酶A合成酶KCS6在不同柑橘类型之间的表达趋势与其对应的物质含量比较吻合,推测该基因造成了这些柑橘类型蜡质含量的差异。酰基转移酶CER2在沙田柚中表达量最高,推测该基因则导致了沙田柚中大于30碳原子数的蜡质成分在不同柑橘类型中占据最高比例。3种类型柑橘果实表面总蜡和外蜡中脂肪族物质均以醛为主,烷烃或酸次之,伯醇最少。总蜡和外蜡物质分配存在差异,其中伯醇和超长链脂肪酸在外蜡中占据比例更高,烷烃则是在外蜡中占据比例较低。萜类物质中三萜类和淄醇仅在总蜡中可以测定出,而角鲨烯和法尼醇则在外蜡和总蜡中都含有。【结论】蜡质成分在柑橘果实中受遗传背景和果皮结构共同影响。宽皮柑橘与紧皮柑橘具有不同的蜡质特征,本研究结果对宽皮柑橘和紧皮柑橘品种特异性的果蜡配方研发具有重要参考价值。 相似文献
9.
Anesthesia cutoff phenomenon: interfacial hydrogen bonding 总被引：6，自引：0，他引：6
Anesthesia "cutoff" refers to the phenomenon of loss of anesthetic potency in a homologous series of alkanes and their derivatives when their sizes become too large. In this study, hydrogen bonding of 1-alkanol series (ethanol to eicosanol) to dipalmitoyl-L-alpha-phosphatidylcholine (DPPC) was studied by Fourier transform infrared spectroscopy (FTIR) in DPPC-D2O-in-CCl4 reversed micelles. The alkanols formed hydrogen bonds with the phosphate moiety of DPPC and released the DPPC-bound deuterated water, evidenced by increases in the bound O-H stretching signal of the alkanol-DPPC complex and also in the free O-D stretching band of unbound D2O. These effects increased according to the elongation of the carbon chain of 1-alkanols from ethanol (C2) to 1-decanol (C10), but suddenly almost disappeared at 1-tetradecanol (C14). Anesthetic potencies of these alkanols, estimated by the activity of brine shrimps, were linearly related to hydrogen bond-breaking activities below C10 and agreed with the FTIR data in the cutoff at C10. 相似文献
10.
Solid friction between a charged polymer and fixed gel points can dramatically affect polymer mobility in gel electrophoresis. The effect is present when a polymer chain is entangled over many gel points along a portion of its length, leading to significantly different behavior than predicted by conventional theory: the mobility of the chain decreases and exhibits a stronger length dependence, which separates long linear charged polymers of different molecular weights. 相似文献
11.
Liquid alkanes with the number of carbon atoms ranging from C7 to C15 were selectively produced from biomass-derived carbohydrates by acid-catalyzed dehydration, which was followed by aldol condensation over solid base catalysts to form large organic compounds. These molecules were then converted into alkanes by dehydration/hydrogenation over bifunctional catalysts that contained acid and metal sites in a four-phase reactor, in which the aqueous organic reactant becomes more hydrophobic and a hexadecane alkane stream removes hydrophobic species from the catalyst before they go on further to form coke. These liquid alkanes are of the appropriate molecular weight to be used as transportation fuel components, and they contain 90% of the energy of the carbohydrate and H2 feeds. 相似文献
12.
采前套袋对苹果梨表皮蜡质结构和化学组分的影响 总被引：5，自引：0，他引：5
【目的】探讨采前套袋处理对苹果梨（Pyrus bretschneideri Rehd. cv. Pingguoli）果皮蜡质结构和化学组分的影响。【方法】在花后60 d对苹果梨果实进行双层套袋处理,通过扫描电镜观察和气相色谱-质谱联用（GC/MS）分析,了解套袋微域环境对果皮蜡质含量、结构和化学组分的影响。【结果】套袋处理苹果梨表皮蜡质含量与未套袋相比差异不显著;与未套袋相比,套袋果实表皮蜡质粗糙、蜡膜不均一、裂纹较多,且表面无蜡质结晶形成。对其蜡质组分分析表明,套袋与未套袋果实检出的蜡质组分数量与种类均存在差异;在含量上套袋果实烷烃较未套袋有所提高,链烷酸有所下降。同时采前套袋处理的果实蜡质组分中饱和链烷酸的碳数分布优势也发生了变化,未套袋果实以C28、C30和C26的相对丰度较大,而套袋果实仅以C26和C28的相对丰度较大,并未检测到C30饱和链烷酸。【结论】采前套袋处理对苹果梨表皮蜡质超微结构、化学组分种类和链长均具有明显的影响,但其影响机制尚需进一步研究。 相似文献
13.
枯草芽孢杆菌BAB-1产脂肽类及挥发性物质的分离和鉴定 总被引：9，自引：1，他引：8
【目的】对枯草芽孢杆菌Bacillus subtilis菌株BAB-1产生的脂肽类物质和挥发性物质进行分离鉴定和抑菌活性研究。【方法】采用HPLC和MALDI-TOF-MS对脂肽类物质进行分离纯化和鉴定;采用GC-MS对挥发性物质进行鉴定。【结果】HPLC分析表明,菌株BAB-1产生的脂肽类物质由LP-1、LP-2、LP-3三类组分组成,其中LP-2起主要抑菌作用。MALDI-TOF-MS质谱分析表明,LP-1为surfactin(C13—C15),LP-2为fengycin A(C14—C15)和fengycin B(C14—C18),LP-3成分未能确定。GC-MS结果表明,菌株BAB-1产生的挥发性物质由多种成分组成,主要为醇类、酮类、酸类、胺类和烷烃类等。【结论】枯草芽孢杆菌菌株BAB-1能够产生脂肽类物质和挥发性物质,并且这两类物质都具有较强的抑菌活性。 相似文献
14.
Ponnamperuma C Kvenvolden K Chang S Johnson R Pollock G Philpott D Kaplan I Smith J Schopf JW Gehrke C Hodgson G Breger IA Halpern B Duffield A Krauskopf K Barghoorn E Holland H Keil K 《Science (New York, N.Y.)》1970,167(3918):760-762
A sample of lunar dust was examined for organic compounds. Carbon detected in concentrations of 157 micrograms per gram had a delta(13)C per mil (PDB) value of + 20. Treatment with hydrochloric acid yielded hydrocarbons of low molecular weight, suggesting the presence of carbides. The gas chromatogram of the acylated and esterified derivatives of the hydrolyzate was similar to that obtained for the Pueblito de Allende meteorite. There were no detectable amounts of extractable high-molecular-weight alkanes, aromatic hydrocarbons, isoprenoid hydrocarbons, normal alkanes, fatty acids, amino acids, sugars, or nucleic acid bases. Traces of porphyrins were found, perhaps arising from rocket exhaust materials. 相似文献
15.
Neutron scattering gives information on molecular conformations in solid solutions of polymers of one isotope in another. Results on crystals of polyethylene grown from solution show a molecular dimension (in the form of a radius of gyration) that is almost invariant with the length of the chain. It is proposed that certain lengths of folded chains fold back onto themselves to form stacks of chain-folded ribbons ("superfolding"). 相似文献
16.
17.
Bent J Hutchings LR Richards RW Gough T Spares R Coates PD Grillo I Harlen OG Read DJ Graham RS Likhtman AE Groves DJ Nicholson TM McLeish TC 《Science (New York, N.Y.)》2003,301(5640):1691-1695
Flows of complex fluids need to be understood at both macroscopic and molecular scales, because it is the macroscopic response that controls the fluid behavior, but the molecular scale that ultimately gives rise to rheological and solid-state properties. Here the flow field of an entangled polymer melt through an extended contraction, typical of many polymer processes, is imaged optically and by small-angle neutron scattering. The dual-probe technique samples both the macroscopic stress field in the flow and the microscopic configuration of the polymer molecules at selected points. The results are compared with a recent "tube model" molecular theory of entangled melt flow that is able to calculate both the stress and the single-chain structure factor from first principles. The combined action of the three fundamental entangled processes of reptation, contour length fluctuation, and convective constraint release is essential to account quantitatively for the rich rheological behavior. The multiscale approach unearths a new feature: Orientation at the length scale of the entire chain decays considerably more slowly than at the smaller entanglement length. 相似文献
18.
Monolayers of aliphatic long-chain alcohols induced nucleation of ice at temperatures approaching 0 degrees C, in contrast with water-soluble alcohols, which are effective antifreeze agents. The corresponding fatty acids, or alcohols with bulky hydrophobic groups, induce freezing at temperatures as much as 12 degrees C lower. The freezing point induced by the amphiphilic alcohols was sensitive not only to surface area per molecule but, for the aliphatic series (C(n)H(2n + 1)OH), to chain length and parity. The freezing point for chains with n odd reached an asymptotic temperature of 0 degrees C for an upper value of n = 31; for n even the freezing point reached a plateau of -8 degrees C for n in the upper range of 22 to 30. The higher freezing point induced by the aliphatic alcohols is due to formation of ordered clusters in the uncompressed state as detected by grazing incidence synchrotron x-ray diffraction measurements. The diffraction data indicate a close lattice match with the ab layer of hexagonal ice. 相似文献
19.
The gas-phase reaction between MoO3-x and H(2)S in a reducing atmosphere at elevated temperatures (800 degrees to 950 degrees C) has been used to synthesize large quantities of an almost pure nested inorganic fullerene (IF) phase of MoS(2). A uniform IF phase with a relatively narrow size distribution was obtained. The synthesis of IFs appears to require, in addition to careful control over the growth conditions, a specific turbulent flow regime. The x-ray spectra of the different samples show that, as the average size of the IF decreases, the van der Waals gap along the c axis increases, largely because of the strain involved in folding of the lamella. Large quantities of quite uniform nanotubes were obtained under modified preparation conditions. 相似文献
20.
从短柄草中克隆获得一个2C 型蛋白磷酸酶基因,命名为BdPP2C2。序列分析结果表明,该基因ORF 的
长度为1 368 bp。进化分析结果表明,该基因所编码的蛋白与拟南芥AtPP2C56 的亲缘关系最近。实时荧光定量PCR
分析结果表明,该基因的表达显著受ABA、乙烯、双氧水3 种信号分子诱导,并且受高盐和低温胁迫诱导。这些结果
表明,BdPP2C2 是非生物逆境胁迫中的一个应答因子,并且可能受相关信号分子调控。 相似文献