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1.
Monte Carlo simulations are employed to investigate the thermodynamics of the first transition in writhe of a circular model filament corresponding to a 468 base-pair DNA. Parameters employed in these simulations are the torsional rigidity, C = 2.0 × 10−19 dyne cm2, and persistence length, P = 500 Å. Intersubunit interactions are modeled by a screened Coulomb potential. For a straight line of subunits this accurately approximates the nonlinear Poisson-Boltzmann potential of a cylinder with the linear charge density of DNA. Curves of relative free energy vs writhe at fixed linking difference (Δ1) exhibit two minima, one corresponding to slightly writhed circles and one to slightly underwrithed figure-8's, whenever Δ1 lies in the transition region. The free energies of the two minima are equal when Δ1c = 1.35, which defines the midpoint of the transition. At this midpoint, the free energy barrier between the two minima is found to be ΔGbar = (0.20) kBT at 298 K. Curves of mean potential energy vs writhe at fixed linking difference similarly exhibit two minima for Δ1 values in the transition region, and the two minimum mean potential energies are equal when Δ1 = 1.50. At the midpoint writhe, Δ1c = 1.35, the difference in mean potential energy between the minimum free energy figure-8 and circle states is (1.3) kBT, and the difference in their entropies is 1.3 kB. Thus, the entropy of the minimum free energy figure-8 state significantly exceeds that of the circle at the midpoint of the transition. The first transition in writhe is found to occur over a rather broad range of Δ1 values from 0.85 to 1.85. The twist energy parameter (ET), which governs the overall free energy of supercoiling, undergoes a sigmoidal decrease, while the translational diffusion coefficient undergoes a sigmoidal increase, over this same range. The static structure factor exhibits an increase, which reflects a decrease in radius of gyration associated with the circle to figure-8 transition. © 1996 John Wiley & Sons, Inc. 相似文献
2.
We investigate the utility of an improved isotopic method to partition the net ecosystem exchange of CO 2 ( F) into net photosynthesis ( FA) and nonfoliar respiration ( FR). Measurements of F and the carbon isotopic content in air at a high‐elevation coniferous forest (the Niwot Ridge AmeriFlux site) were used to partition F into FA and FR. Isotopically partitioned fluxes were then compared with an independent flux partitioning method that estimated gross photosynthesis (GEE) and total ecosystem respiration (TER) based on statistical regressions of night‐time F and air temperature. We compared the estimates of FA and FR with expected canopy physiological relationships with light (photosynthetically active radiation) and air temperature. Estimates of FA and GEE were dependent on light as expected, and TER, but not FR, exhibited the expected dependence on temperature. Estimates of the isotopic disequilibrium D , or the difference between the isotopic signatures of net photosynthesis ( δA, mean value ?24.6‰) and ecosystem respiration ( δR, mean value ?25.1‰) were generally positive ( δA>δR). The sign of D observed here is inconsistent with many other studies. The key parameters of the improved isotopic flux partitioning method presented here are ecosystem scale mesophyll conductance ( gm) and maximal vegetative stomatal conductance ( gcmax). The sensitivity analyses of FA, FR, and D to gcmax indicated a critical value of gcmax (0.15 mol m ?2 s ?1) above which estimates of FA and FR became larger in magnitude relative to GEE and TER. The value of D decreased with increasing values of gm and gcmax, but was still positive across all values of gm and gcmax. We conclude that the characterization of canopy‐scale mesophyll and stomatal conductances are important for further progress with the isotope partitioning method, and to confirm our anomalous isotopic disequilibrium findings. 相似文献
3.
We used a combination of eddy flux, chamber and environmental measurements with an integrated suite of models to analyse the seasonality of net ecosystem carbon uptake ( FCO2) in an 8-year-old, closed canopy Pinus radiata D.Don plantation in New Zealand (42°52′ S, 172°45′ E). The analyses utilized a biochemically based, big-leaf model of tree canopy photosynthesis ( Ac), coupled to multiplicative environmental-constraint functions of canopy stomatal conductance ( Gc) via environmental measurements, a temperature-dependent model of ecosystem respiration ( Reco), and a soil water balance model. Available root zone water storage capacity at the measurement site is limited to about 50 mm for the very stony soil, and annual precipitation is only 660 mm, distributed evenly throughout the year. Accordingly the site is prone to soil moisture deficit throughout the summer. G c and Ac obtained maximum rates early in the growing season when plentiful soil water supply was associated with sufficient quantum irradiance ( Qabs), and moderate air saturation deficit ( D) and temperature ( T). From late spring onwards, soil water deficit and D confined Gc and Ac congruously, which together with the solely temperature dependency of Reco resulted in the pronounced seasonality in FCO2. Reflecting a light-limitation of Ac in the closed canopy, modelled annual carbon (C) uptake was most sensitive to changes in Qabs. However, Qabs did not vary significantly between years, and changes in annual FCO2 were mostly due to variability in summer rainfall and D. Annual C-uptake of the forest was 717 g C m –2 in a near-average rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m –2). 相似文献
4.
The cooperativity in hemoglobin can be described by the Hill parameter n, the free energy of interaction Δ F1 and the allosteric free energy Δ FA. By this latter is meant here the free energy change associated with the transition from the deoxy to the oxy conformation in hemoglobin. In this paper some general relations between n, Δ F1 and Δ FA are given. A method is presented by which Δ FA can be calculated from oxygenation data. 相似文献
5.
For helix-coil transitions of polypeptide in binary mixtures consisting of helix-forming solvent and coil solvent, the transition enthalpy Δ H( T, x) has been found to depend significantly on temperature ( T) and solvent composition ( x). For such systems, calorimetric measurements may yield some averages of Δ H( T, x) which are no longer amenable to direct comparison with Δ H itself. Theoretical equations relating calorimetric data to Δ H( T, x) are derived and tested favorably with experimental data. It is demonstrated that the transition enthaply from heat capacity measurements is approximately equal to Δ Hcfm, while those from heat of dilution and heat of solution measurements are equal to Δ Hc. Here Δ Hc denotes the value of Δ H at the transition point and fm represents the maximum helical content attained in a thermally induced transition. The discrepancies among calorimetric data are also discussed. 相似文献
6.
We review a recent paper 20 in which a specific enhancement factor (i.e., a phase transition into a condensed Bose mode) is proposed to account for the observed amplification of the ground state energies of the L - and D -amino acid enantiomers; the difference between these energies is assumed to be due to the neutral parity-violating electroweak interaction. This physical effect initially shifts the enantiomer energies by about 3 × 10 ?19 eV. The proposed phase transition is characterized by a critical temperature Tc, which may be studied theoretically by enlarging the standard electroweak theory to include either the top quark or supersymmetry 21. Possible experimental means of finding Tc are discussed. 相似文献
7.
Summary Attenuated total reflection infrared spectroscopy has been used to determine the equilibrium distribution of the peptide antibiotic alamethicin R
F30 between dipalmitoyl phosphatidylcholine bilayers and the aqueous environment. The distribution coefficient K=c
eq
W
/ c
eq
M
turned out to be concentration dependent, pointing to alamethicin association in the membrane with increasing concentration in the aqueous phase ( c
eq
W
). This concentration was varied within 28 and 310 nm, i.e., in a range typical for black film experiments. Furthermore, diffusion coefficients of alamethicin in the hydrophobic phase of the membrane ( D
M) and across the membrane/water interface ( D
I) have been estimated from the time course of the equilibration process. It was found that the diffusion rate of the uncharged analogue R
F50 is about 10 times higher than that of the R
F30 component, exhibiting one negative charge at the C-terminus. The time constants for transmembrane diffusion of alamethicin R
F30 varied between 2.2 hr at low concentration and 3.2 hr at higher concentration. The corresponding low concentration value of the R
F50 component was found to be 0.25 hr. 相似文献
8.
A new antigenic system in the rat homologous to the Qa/TL antigen system in the mouse has been characterized. It was detected by antibodies raised in donor-recipient combinations
that were matched for the RT1. A, B, D, E loci in the major histocompatibility complex (MHC): (R11×BN)F 1 anti-BN.1L(LEW), (R18×BN)F 1 anti-BN.1L, and BN.1LV1(F344) anti-BN.1L. Absorption analyses using these antisera and a variety of inbred, congenic and
recombinant strains identified three alleles, RT1.G
a
, G
b
, G
c
, of which G
c
is a null allele. The strain distribution of these alleles was determined, using 37 strains of rats representative of all
of the prototypic haplotypes and a number of congenic and recombinant strains. The use of the congenic and recombinant strains
showed that the RT1.G locus was linked to the MHC and that the most probable gene order was A-E-G. Testcross analysis showed that the map distance between A and G was 1.4 cM(4/285 recombinants). The RT1.G antigen has a heavy chain of M
r 46 000 and is present on both T and B cells. 相似文献
9.
Short‐term measurements of carbon dioxide, water, and energy fluxes were collected at four locations along a mean annual precipitation gradient in southern Africa during the wet (growing) season with the purpose of determining how the observed vegetation–atmosphere exchange properties are functionally related to the long‐term climatic conditions. This research was conducted along the Kalahari Transect (KT), one in the global set of International Geosphere‐Biosphere Program transects, which covers a north–south aridity gradient, all on a homogenous sand formation. Eddy covariance instruments were deployed on a permanent tower in Mongu, Zambia (879 mm of rainfall per year), as well as on a portable tower in Maun (460 mm yr ?1), Okwa River Crossing (407 mm yr ?1), and Tshane (365 mm yr ?1), Botswana for several days at each site. The relationships between CO 2 flux, Fc, and photosynthetically active radiation were described well by a hyperbolic fit to the data at all locations except for Mongu, the wettest site. Here, there appeared to be an air temperature effect on Fc. While daytime values of Fc routinely approached or exceeded ?20 μmol m ?2 s ?1 at Mongu, the magnitude of Fc remained less than ?10 μmol m ?2 s ?1 when the air temperature was above 27°C. Canopy resistances to water vapor transfer, rc, displayed an overall decline from the wetter sites to the more arid sites, but the differences in rc could be almost exclusively accounted for by the decrease in leaf area index (LAI) from north to south along the KT. Ecosystem water use efficiency (WUE), defined as the ratio of net carbon flux to evapotranspiration, showed a general decrease with increasing vapor pressure deficit, D, for all of the sites. The magnitudes of WUE at a given D, however, were dissimilar for the individual sites and were found to be stratified according to the position of the sites along the long‐term aridity gradient. For example, Mongu, which has the wettest climate, has a much lower WUE for like levels of D than Tshane, which historically has the most arid climate. Given the similar inferred stomatal resistances between the sites, the disparate carbon uptake behavior for the grass vs. woody vegetation is the likely cause for the observed differences in WUE along the aridity gradient. The short‐term flux measurements provide a framework for evaluating the vegetation's functional adaptation to the long‐term climate and provide information that may be useful for predicting the dynamic response of the vegetation to future climate change. 相似文献
10.
Neurotransmitters, hormones, or dyes may be released from vesicles via a fusion pore, rather than by full fusion of the vesicle with the plasma membrane. If the lifetime of the fusion pore is comparable to the time required for the substance to exit the vesicle, only a fraction of the total vesicle content may be released during a single pore opening. Assuming 1), fusion pore lifetimes are exponentially distributed ( τP), as expected for simple single channel openings, and 2), vesicle contents are lost through the fusion pore with an exponential time course ( τD), we derive an analytical expression for the probability density function of the fraction of vesicle content released ( F): dP/ dF = A (1 − F) (A-1), where A = τD/ τP. If A > 1, the maximum of the distribution is at F = 0; if A < 1, the maximum is at F = 1; if A = 1, the distribution is perfectly flat. Thus, the distribution never has a peak in the middle (0 < F < 1). This should be considered when interpreting the distribution of miniature synaptic currents, or the fraction of FM dye molecules lost during a single fusion pore opening event. 相似文献
11.
A kinetic-LED-array-spectrophotometer (Klas) was recently developed for measuring in vivo redox changes of P700, plastocyanin (PCy), and ferredoxin (Fd) in the near-infrared (NIR). This spectrophotometer is used in the present work for in vitro light-induced measurements with various combinations of photosystem I (PSI) from tobacco and two different cyanobacteria, spinach plastocyanin, cyanobacterial cytochrome c6 (cyt. c6), and Fd. It is shown that cyt. c6 oxidation contributes to the NIR absorption changes. The reduction of (FAFB), the terminal electron acceptor of PSI, was also observed and the shape of the (FAFB) NIR difference spectrum is similar to that of Fd. The NIR difference spectra of the electron-transfer cofactors were compared between different organisms and to those previously measured in vivo, whereas the relative absorption coefficients of all cofactors were determined by using single PSI turnover conditions. Thus, the (840 nm minus 965 nm) extinction coefficients of the light-induced species (oxidized minus reduced for PC and cyt. c6, reduced minus oxidized for (FAFB), and Fd) were determined with values of 0.207?±?0.004, –?0.033?±?0.006, –?0.036?±?0.008, and –?0.021?±?0.005 for PCy, cyt. c6, (FAFB) (single reduction), and Fd, respectively, by taking a reference value of +?1 for P700+. The fact that the NIR P700 coefficient is larger than that of PCy and much larger than that of other contributing species, combined with the observed variability in the NIR P700 spectral shape, emphasizes that deconvolution of NIR signals into different components requires a very precise determination of the P700 spectrum. 相似文献
12.
ATPases with unusual membrane-embedded rotor subunits were found in both F 1F 0 and A 1A 0 ATP synthases. The rotor subunit c of A 1A 0 ATPases is, in most cases, similar to subunit c from F 0. Surprisingly, multiplied c subunits with four, six, or even 26 transmembrane spans have been found in some archaea and these multiplication events were
sometimes accompanied by loss of the ion-translocating group. Nevertheless, these enzymes are still active as ATP synthases.
A duplicated c subunit with only one ion-translocating group was found along with “normal” F 0 c subunits in the Na + F 1F 0 ATP synthase of the bacterium Acetobacterium woodii. These extraordinary features and exceptional structural and functional variability in the rotor of ATP synthases may have
arisen as an adaptation to different cellular needs and the extreme physicochemical conditions in the early history of life. 相似文献
13.
BaO‐B 2O 3‐P 2O 5 glasses doped with a fixed concentration of Tb 3+ ions and varying concentrations of Al 2O 3 were synthesized, and the influence of the Al 3+ ion concentration on the luminescence efficiency of the green emission of Tb 3+ ions was investigated. The optical absorption, excitation, luminescence spectra and fluorescence decay curves of these glasses were recorded at ambient temperature. The emission spectra of terbium ions when excited at 393 nm exhibited two main groups of bands, corresponding to 5D 3 → 7F j (blue region) and 5D 4 → 7F j (green region). From these spectra, the radiative parameters, viz., spontaneous emission probability A, total emission probability AT, radiative lifetime τ and fluorescent branching ratio β, of different transitions originating from the 5D 4 level of Tb 3+ ions were evaluated based on the Judd‐Ofelt theory. A clear increase in the quantum efficiency and luminescence of the green emission of Tb 3+ ions corresponding to 5D 4 → 7F 5 transition is observed with increases in the concentration of Al 2O 3 up to 3.0 mol%. The improvement in emission is attributed to the de‐clustering of terbium ions by Al 3+ ions and also to the possible admixing of wave functions of opposite parities. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
14.
Rare earth ions (Eu 3+ or Tb 3+)‐activated Ca 3 Ga 2 Si 3O 12 (CaGaSi) phosphors were synthesized by using a sol–gel method. Photoluminescence spectra of Eu 3+:CaGaSi phosphors exhibited five emission bands at 578, 592, 612, 652 and 701 nm, which were assigned to the transitions ( 5D 0 → 7F 0, 7F 1, 7F 2, 7F 3 and 7F 4), respectively, with an excitation wavelength of λexci = 392 nm. Among these, the transition 5D 0 → 7F 2 (612 nm) displayed bright red emission. In the case of Tb 3+:CaGaSi phosphors, four emission bands were observed at 488 ( 5D 4 → 7F 6), 543 ( 5D 4 → 7F 5), 584 ( 5D 4 → 7F 4) and 614 nm ( 5D 4 → 7F 3) from the measurement of PL spectra with λexci = 376 nm. Among these, the transition 5D 4 → 7F 5 at 543 nm displayed bright green emission. The structure and morphology of the phosphors were studied from the measurements of X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy‐dispersive X‐ray analysis (EDAX) results. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
15.
The endotherm enthalpy changes Δ HD and temperatures TD of thermal denaturation of tropocollagen fibers were measured by DSC calorimetry as functions of water content. The denaturation temperatures decrease with increasing water content. The enthalpy change values increase sharply in the range 0–28% of water content, where a maximum of 14.3 cal g ?1 is reached. The effect of water uptake on the enthalpy term is explained by water bridge formation within the collagen triple helix. Evidence is given for the existence of approximately three intercatenary water bridges per triplet at the enthalpy maximum, their H-bond energy amounting to approximately 4000 kcal/mol of protein. In the 30–60% range of water content, Δ HD decreases by 2 cal ?1 probably due to interactions between secondary water structures and the stabilizing intrahelical water bonds. The influence of two neutral potassium salts, with a structure-stabilizing and a structure-breaking anion (F ? and I ?), on the hydration dependence of Δ HD and TD was also studied. It was shown that the primary hydration is not influenced by these ions, but that TD and Δ HD are altered in an ion specific way in the presence of interface and bulk water. Hydrophobic interactions do not explain the experimental results. A reaction mechanism of the effects of ions upon the structural stability of collagen is proposed and discussed in terms of interactions of the medium water molecules with the intrahelical water bonds, and in terms of proton-donor/proton-acceptor equilibria between peptide groups, hydrated ions, and intrahelical water molecules. 相似文献
16.
The effect of salt on the intercalation of acridine dyes and DNA is rather well explained by the Gouy-Chapman double-layer theory as applied to a cylinder model of the DNA–dye complex. The free energy of transfer of a dye ion from the bulk solution to the complex is divided into several parts, one of which, Δ F0, accounts for the short-range, nonelectrostatic interactions. The assumption that Δ F0 should not depend on the amount of dye in the complex leads to an internal dielectric constant of the cylinder of about Di = 7. The scatter in Δ F0 values, as calculated from individual experimental points, is of order 0.5 kT per dye ion. This scatter is large enough to mask possible effects of heterogeneity in DNA sequences. The calculations are made for a long cylinder with radius 10 Å, with the DNA phosphate charges smeared uniformly at the surface, a uniform spacing of dye charges at the cylinder axis, and a length of b = 3.37 Å per base pair. Each intercalated dye ion also adds a length b to the total length of the cylinder. The salt-dependent part of the electric free energy of intercalation, Δ F1, is tabulated for complexes with r = 0–0.24 dye ions per DNA phosphate in 0.002–0.2 M monovalent salt and dye solutions. 相似文献
17.
Objective: Obese patients without clinically apparent heart disease may have a high output state and elevated total and central blood volumes. Central circulatory congestion should result in elevated pulmonary diffusing capacity (D LCO) and capillary blood volume (V c) reflecting pulmonary capillary recruitment; however, the effect on membrane diffusion (D m) is uncertain. We examined D LCO and its partition into V c and D m in 13 severely obese subjects (BMI = 51 ± 14 kg/m 2) without manifest cardiopulmonary disease before and after surgically induced weight loss. Research Methods and Procedures: D LCO and its partition into V c and D m [referenced to alveolar volume (V A)] as described by Roughton and Forster, total body water by tritiated water, and fat distribution by waist‐to‐hip ratio were performed. Results: Despite normal D LCO (mean 98 ± 16% predicted), V c/V A was increased (mean 118 ± 30% predicted), and D m/V A was reduced (mean 77 ± 34% predicted). Nine of 13 subjects were restudied after weight loss (mean 52 ± 43 kg); V c/V A decreased to 89 ± 18% predicted ( p = 0.01), and D m/V A increased to 139 ± 30% predicted ( p < 0.01). Increasing total body water was associated with both increasing V c ( r = 0.74, p = 0.01) and increasing waist‐to‐hip ratio ( r = 0.65, p = 0.02), indicating that circulatory congestion increases with increasing central obesity. Discussion: Severely obese subjects without manifest cardiopulmonary disease may have increased V c indicating central circulatory congestion and reduced D m suggesting associated alveolar capillary leak, despite normal D LCO. Reversibility with weight loss is in accord with reversibility of the hemodynamic abnormalities of obesity. 相似文献
18.
This study investigated the effects of cinnamic acid (CA) on growth, biochemical and physiological responses of Lactuca sativa L. CA (0.1, 0.5, 1.0 and 1.5 mM) treatments decreased plant height, root length, leaf and root fresh weight, but it did not affect the leaf water status. CA treatment (1.5 mM) significantly reduced Fv, Fm, photochemical efficiency of PSII ( Fv/ Fm) and quantum yield of PSII (ΦPSII) photochemistry in L. sativa. The photochemical fluorescence quenching (qP) and non-photochemical quenching (NPQ) were reduced after treatment with 1.5 mM CA. Fraction of photon energy absorbed by PS II antennae trapped by “open” PS II reaction centers ( P) was reduced by CA (1.5 mM) while, portion of absorbed photon energy thermally dissipated ( D) and photon energy absorbed by PSII antennae and trapped by “closed” PSII reaction centers ( E) was increased. Carbon isotope composition ratios (δ 13C) was less negative (−27.10) in CA (1.5 mM) treated plants as compared to control (−27.61). Carbon isotope discrimination (Δ 13C) and ratio of intercellular CO 2 concentration ( ci/ ca) from leaf to air were also less in CA treated plants. CA (1.5 mM) also decreased the leaf protein contents of L. sativa as compared to control. 相似文献
19.
Eu 3+-doped-bismuth-based phosphate glasses with chemical equation (60 − x)P 2O 5–20Bi 2O 3−10Na 2CO 3–10SrF 2– xEu 2O 3 (PBNSEu), (where x = 0, 0.1, 0.5, 1.0, 1.5 and 2 mol%) were fabricated using the melt-quenching method. Obtain X-ray diffraction (XRD), energy-dispersive X-ray (EDAX), and Fourier transform infrared (FTIR) spectra were used to characterize the structure of the prepared PBNSEu glass. The J–O (Judd–Ofelt) intensity parameters (Ω 2, Ω 4) were estimated using photoluminescence emission spectra. When excited with a xenon lamp at λ exc = 394 nm, the most intense red-emission transition occurred at ~612 nm ( 5D 0→ 7F 2). J–O intensity parameters were used to calculate radiative properties, whereas the radiative branching ratio ( βR), radiative transition probability ( AR), radiative lifetime (τ R), and total radiative transition rate ( Aτ) were calculated for the transitions 5D 0→ 7F J (where J = 0–4) and were obtained in the emission spectra for europium ion-doped in the current glass. Using the CIE1931 chromaticity coordinates axes, the colours of various concentrations of Eu 3+ ion-doped PBNS glass were evaluated using the emission spectra. Temperature-dependent luminescence spectra were recorded for the optimized PBNSEu20 glass to calculate the activation energy. These results strongly suggested red components in w-LEDs and visible display laser applications. 相似文献
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