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1.
Lixin Zhou 《Inorganica chimica acta》2011,376(1):44-56
The monofunctional and bifunctional bindings of the potential anticancer drug trans-isopropylaminedimethylaminedichloroplatinum (trans-IPADMADP) and its cis isomer to purine base in DNA are explored by using density functional theory and IEF-PCM solvation models. The computed lowest free energy barrier in the aqueous solution is 14.0/11.6 kcal/mol (from trans-Pt-chloroaqua complex to trans-/cis-monoadduct) for guanine(G), and 11.7/13.3 kcal/mol (from trans-Pt-chloroaqua complex to trans-/cis-monoadduct) for adenine(A). Our calculations demonstrate that the trans reactant complexes (or isolated reactants) can generate trans- or cis-monoadducts via similar trigonal bipyramidal transition state structures, suggesting that the monoadducts can subsequently close to form the bifunctional intrastrand Pt-DNA adducts and simultaneously distort DNA in the similar way as cisplatin. Our calculations show that Pt(isopropylamine)(dimethylamine)G22+ head-to-head path has the lowest free energy of activation at 17.6 kcal/mol, closely followed by the Pt(isopropylamine)(dimethylamine)GA2+ head-to-head path at 19.6 kcal/mol when the monofunctional cis-Pt-G complex serves as the reactant; while the Pt(isopropylamine)(dimethylamine)G22+ head-to-tail adduct has the lowest barrier of 20.5 kcal/mol, closely followed by the Pt(isopropylamine)(dimethylamine)GA2+ head-to-tail adduct at 23.0 kcal/mol if the monofunctional trans-Pt-G complex is the reactant.The calculated relatively lower activation energy barrier than that of cisplatin theoretically confirm that trans-[PtCl2(isopropylamine)(dimethylamine)] is a potential anticancer drug as described by experiment. 相似文献
2.
K. A. Razumova V. V. Alikaev I. S. Bondarenko A. A. Borschegovskii V. A. Vershkov A. V. Gorshkov Yu. V. Gott Yu. N. Dnestrovskij V. V. Dreval M. M. Dremin L. G. Eliseev G. S. Kirnev A. Ya. Kislov I. V. Klimanov A. S. Kozachok A. D. Komarov V. A. Krupin L. I. Krupnik S. V. Krylov S. E. Lysenko A. A. Medvedev A. V. Melnikov T. B. Myalton G. E. Notkin A. Yu. Novikov Yu. D. Pavlov D. P. Petrov V. I. Poznyak I. N. Roy P. V. Savrukhin V. V. Sannikov S. V. Soldatov A. V. Sushkov V. M. Trukhin S. M. Khrebtov V. V. Chistyakov D. A. Shelukhin 《Plasma Physics Reports》2001,27(4):273-277
Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and q L <4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth, a decrease in the Dα emission intensity, and an increase in the central electron and ion temperatures. Two deep wells on the potential profile (the first one at r/a L ≈0.6, where a L is the limiter radius, and the second one near the edge) arise during the transition. the internal barrier is formed when dq/dr~0 with q≈1 in the barrier region. 相似文献
3.
The 90-residue N-terminal Phox and Bem1p (PB1) domain of NBR1 forms an α/β ubiquitin-like fold. Kinetic analysis using stopped-flow fluorescence reveals two-state kinetics; however, nonlinear effects in the denaturant dependence of the unfolding data demonstrate changes in the position of the rate-limiting barrier along the folding coordinate as the folding conditions change. The kinetics of wt-PB1 and several mutants show that this curvature is consistent with a single-pathway mechanism involving sequential transition states (TS1 and TS2) separated by a transiently populated high-energy intermediate, rather than movement of the transition state on a broad energy plateau. We show that the two transition states within the sequential model represent structurally and thermodynamically distinct species. TS1 is a collapsed state (αTS1 = 0.71) with a large enthalpic barrier to formation that is rate-limiting under conditions that strongly favour folding. TS2 is highly native-like (αTS2 = 0.93) and represents a late entropic barrier to formation of the native state. In support of the sequential transition state mechanism, we show that the G62A helix 2 substitution stabilises TS1 and the intermediate to such an extent that the latter becomes significantly populated, leading to the observation of a fast kinetic phase representing the initial U → I transition, with TS2 (αTS2 = 0.87) becoming rate-limiting. The folding rate is not retarded by populating an intermediate, which would be expected for a misfold state, but is accelerated, suggesting that the I state is productive and on-pathway. The results show that the apparent two-state folding of the wt-PB1 domain occurs along a well-defined pathway involving structurally and thermodynamically distinct sequential transition states and an obligatory metastable intermediate that represents a productive local minimum in the energy landscape that increases the efficiency of barrier crossing through favourable effects on the entropy of activation. 相似文献
4.
Yu. N. Dnestrovskij S. V. Cherkasov A. Yu. Dnestrovskij S. E. Lysenko M. J. Walsh 《Plasma Physics Reports》2006,32(1):1-8
Models with critical gradients are widely used to describe energy balance in L-mode discharges. The so-called first critical gradient can be found from the canonical temperature profile. Here, it is suggested that discharge regimes with transport barriers can be described based on the idea of the second critical gradient. If, in a certain plasma region, the pressure gradient exceeds the second critical gradient, then the plasma bifurcates into a new state and a transport barrier forms in this region. This idea was implemented in a modified canonical profile transport model that makes it possible to describe the energy and particle balance in tokamak plasmas with arbitrary cross sections and aspect ratios. The magnitude of the second critical gradient was chosen by comparing the results calculated for several tokamak discharges with the experimental data. It is found that the second critical gradient is related to the magnetic shear s. The criterion of the transport barrier formation has the form (a 2/r)d/drln(p/p c ) > z 0 (r), where r is the radial coordinate, a is the plasma minor radius, p is the plasma pressure, p c is the canonical pressure profile, and the dimensionless function z O(r) = C O + C 1 s (with C 0i ~1, C 0e ~3, and C 1i,e ~2) describes the difference between the first and second critical gradients. Simulations show that this criterion is close to that obtained experimentally in JET. The model constructed here is used to simulate internal transport barriers in the JET, TFTR, DIII-D, and MAST tokamaks. The possible dependence of the second critical gradient on the plasma parameters is discussed. 相似文献
5.
Based on a model of protein denaturation rate limited by an entropy-related barrier, we derive a simple formula for virus inactivation time as a function of temperature. Loss of protein structure is described by two reaction coordinates: conformational disorder of the polymer and wetting by the solvent. These establish a competition between conformational entropy and hydrophobic interaction favoring random coil or globular states, respectively. Based on the Landau theory of phase transition, the resulting free energy barrier is found to decrease linearly with the temperature difference T − Tm, and the inactivation rate should scale as U to the power of T − Tm. This form recalls an accepted model of thermal damage to cells in hyperthermia. For SARS-CoV-2 the value of U in Celsius units is found to be 1.32. Although the fitting of the model to measured data is practically indistinguishable from Arrhenius law with an activation energy, the entropy barrier mechanism is more suitable and could explain the pronounced sensitivity of SARS-CoV-2 to thermal damage. Accordingly, we predict the efficacy of mild fever over a period of ∼24 h in inactivating the virus. 相似文献
6.
The decay kinetics of the aminoethanol-generated CoII-substrate radical pair catalytic intermediate in ethanolamine ammonia-lyase from Salmonella typhimurium have been measured on timescales of <105 s in frozen aqueous solution from 190 to 217 K. X-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy of the disordered samples has been used to continuously monitor the full radical pair EPR spectrum during progress of the decay after temperature step reaction initiation. The decay to a diamagnetic state is complete and no paramagnetic intermediate states are detected. The decay exhibits three kinetic regimes in the measured temperature range, as follows. i), Low temperature range, 190 ≤ T ≤ 207 K: the decay is biexponential with constant fast (0.57 ± 0.04) and slow (0.43 ± 0.04) phase amplitudes. ii), Transition temperature range, 207 < T < 214 K: the amplitude of the slow phase decreases to zero with a compensatory rise in the fast phase amplitude, with increasing temperature. iii), High temperature range, T ≥ 214 K: the decay is monoexponential. The observed first-order rate constants for the monoexponential (kobs,m) and the fast phase of the biexponential decay (kobs,f) adhere to the same linear relation on an lnk versus T−1 (Arrhenius) plot. Thus, kobs,m and kobs,f correspond to the same apparent Arrhenius prefactor and activation energy (logAapp,f (s−1) = 13.0, Ea,app,f = 15.0 kcal/mol), and therefore, a common decay mechanism. We propose that kobs,m and kobs,f represent the native, forward reaction of the substrate through the radical rearrangement step. The slow phase rate constant (kobs,s) for 190 ≤ T ≤ 207 K obeys a different linear Arrhenius relation (logAapp,s (s−1) = 13.9, Ea,app,s = 16.6 kcal/mol). In the transition temperature range, kobs,s displays a super-Arrhenius increase with increasing temperature. The change in Ea,app,s with temperature and the narrow range over which it occurs suggest an origin in a liquid/glass or dynamical transition. A discontinuity in the activation barrier for the chemical reaction is not expected in the transition temperature range. Therefore, the transition arises from a change in the properties of the protein. We propose that a protein dynamical contribution to the reaction, which is present above the transition temperature, is lost below the transition temperature, owing to an increase in the activation energy barrier for protein motions that are coupled to the reaction. For both the fast and slow phases of the low temperature decay, the dynamical transition in protein motions that are obligatorily coupled to the reaction of the CoII-substrate radical pair lies below 190 K. 相似文献
7.
Rui Xu Qi Li Jia Zhou Xiang-dong Zhou Juliy?M. Perelman Victor?P. Kolosov 《Bioscience reports》2013,33(5)
Acidic airway microenvironment is one of the representative pathophysiological features of chronic inflammatory respiratory diseases. Epithelial barrier function is maintained by TJs (tight junctions), which act as the first physical barrier against the inhaled substances and pathogens of airway. As previous studies described, acid stress caused impaired epithelial barriers and led the hyperpermeability of epithelium. However, the specific mechanism is still unclear. We have showed previously the existence of TRPV (transient receptor potential vanilloid) 1 channel in airway epithelium, as well as its activation by acidic stress in 16HBE cells. In this study, we explored the acidic stress on airway barrier function and TJ proteins in vitro with 16HBE cell lines. Airway epithelial barrier function was determined by measuring by TER (trans-epithelial electrical resistance). TJ-related protein [claudin-1, claudin-3, claudin-4, claudin-5, claudin-7 and ZO-1 (zonula occluden 1)] expression was examined by western blotting of insoluble fractions of cell extraction. The localization of TJ proteins were visualized by immunofluorescent staining. Interestingly, stimulation by pH 6.0 for 8 h slightly increased the epithelial resistance in 16HBE cells insignificantly. However, higher concentration of hydrochloric acid (lower than pH 5.0) did reduce the airway epithelial TER of 16HBE cells. The decline of epithelial barrier function induced by acidic stress exhibited a TRPV1-[Ca2+]i-dependent pathway. Of the TJ proteins, claudin-3 and claudin-4 seemed to be sensitive to acidic stress. The degradation of claudin-3 and claudin-4 induced by acidic stress could be attenuated by the specific TRPV1 blocker or intracellular Ca2+ chelator BAPTA/AM [1,2-bis-(o-aminophenoxy)ethane-N,N,N'',N''-tetra-acetic acid tetrakis(acetoxymethyl ester)]. 相似文献
8.
Current-voltage curves for DIDS-insensitive Cl− conductance have been determined in human red
blood cells from five donors. Currents were estimated from the rate of cell shrinkage using flow cytometry and differential laser light scattering. Membrane potentials were estimated from the extracellular pH of unbuffered suspensions using the proton ionophore FCCP. The width of the Gaussian distribution of cell volumes remained invariant during cell shrinkage, indicating a homogeneous Cl− conductance among the cells. After pretreatment for
30 min with DIDS, net effluxes of K+ and Cl− were induced by valinomycin and were measured in the continued
presence of DIDS; inhibition was maximal at ∼65% above 1 μM DIDS at both 25°C and 37°C. The nonlinear current-voltage curves for DIDS-insensitive net Cl− effluxes, induced by valinomycin or gramicidin at varied [K+]o,
were compared with predictions based on (1) the theory of electrodiffusion, (2) a single barrier model, (3) single
occupancy, multiple barrier models, and (4) a voltage-gated mechanism. Electrodiffusion precisely describes the
relationship between the measured transmembrane voltage and [K+]o. Under our experimental conditions (pH
7.5, 23°C, 1–3 μM valinomycin or 60 ng/ml gramicidin, 1.2% hematocrit), the constant field permeability ratio
PK/PCl is 74 ± 9 with 10 μM DIDS, corresponding to 73% inhibition of PCl. Fitting the constant field current-voltage equation to the measured Cl− currents yields P
Cl = 0.13 h−1 with DIDS, compared to 0.49 h−1 without DIDS,
in good agreement with most previous studies. The inward rectifying DIDS-insensitive Cl− current, however, is inconsistent with electrodiffusion and with certain single-occupancy multiple barrier models. The data are well described either by a single barrier located near the center of the transmembrane electric field, or, alternatively, by a
voltage-gated channel mechanism according to which the maximal conductance is 0.055 ± 0.005 S/g Hb, half the
channels are open at −27 ± 2 mV, and the equivalent gating charge is −1.2 ± 0.3. 相似文献
9.
Jinkwon Kim Sujin Han Kwang-Yong Choi Hiroyuki Nojiri Byoung Jin Suh 《Inorganica chimica acta》2007,360(8):2647-2652
The octanuclear cyano-bridged cluster [(Tp)8Fe4Ni4(CN)12] · H2O · 24CH3CN (1) (Tp = hydrotris(1-pyrazolyl)borate) showing magnetic properties of single-molecule magnet has been synthesized by reaction of [fac-Fe(Tp)(CN)3]− with {(Tp)Ni(NO3)} species formed from an equimolar reaction mixture of Ni(NO3)2 · 6H2O and KTp in MeCN. The X-ray analysis of 1 shows molecular cube structure in which FeIII and NiII ions reside in alternate corners. The average intramolecular Fe?Ni distance is 5.124 Å. Out-of-phase ac susceptibility and reduce magnetization measurements show that 1 is a single molecule magnet with ground spin state S = 6 and spin reversal energy barrier U = 14 K. Magnetic hysteresis loops were also observed by applying fast sweeping field. 相似文献
10.
A new heterometallic complex [CuMn(5-Brsap)2(MeOH)(Ac)] ⋅ CH3OH (1) (5-Brsap = 5-bromo-2-salicylideneamino-1-propanol) has been synthesized and characterized structurally as well as magnetically. Complex 1 has an alkoxo-bridged Cu(II) and Mn(III) heterobinuclear core, where the Mn(III) and Cu(II) ions have elongated octahedral and square-pyramidal geometries, respectively. In dc magnetic susceptibility measurements reveal that there is strong ferromagnetic interaction between the Mn(III) and Cu(II) ions with an exchange coupling constant J = 67.64 cm−1. The ac magnetic susceptibility measurements, frequency dependence in both the real and imaginary signals is observed, which indicates slow relaxation of magnetization. An Arrhenius plot gave the effective anisotropy barrier Δ/kB = 11.58 K and the pre-exponential factor ι0 = 1.28 × 10−6 s. 相似文献
11.
Ana C. PinheiroAna I. Bourbon Bartolomeu G. de S. MedeirosLuís H.M. da Silva Maria C.H. da SilvaMaria G. Carneiro-da-Cunha Manuel A. CoimbraAntónio A. Vicente 《Carbohydrate polymers》2012,87(2):1081-1090
The interactions between κ-carrageenan and chitosan, two oppositely charged polysaccharides, have been investigated through microcalorimetric and quartz crystal microbalance measurements. Microcalorimetric measurements show that κ-carrageenan/chitosan interaction is an exothermic process and that the alternate deposition of κ-carrageenan and chitosan results in the formation of a nanolayered coating mainly due to the electrostatic interactions existing between the two polyelectrolytes (though other types of interactions may also be involved). Quartz crystal microbalance measurements confirmed that the alternating deposition of κ-carrageenan and chitosan resulted in the formation of a stable multilayer structure. The κ-carrageenan/chitosan nanolayered coating, assembled on a polyethylene terephthalate (PET) support, was characterized in terms of its surface (contact angle measurements) and gas barrier properties (water vapor and O2 permeabilities) and analyzed by scanning electron microscopy (SEM). The water vapor permeability (WVP) and the oxygen permeability (O2P) of the κ-carrageenan/chitosan nanolayers were found to be 0.020 ± 0.002 × 10−11 and 0.043 ± 0.027 × 10−14 g m−1 s−1 Pa−1, respectively. These results contribute to a better understanding of the type of interactions that play role during the construction of this type of nanostructures. This knowledge can be used in the establishment of an approach to produce edible, biodegradable multilayered nanostructures with improved mechanical and barrier properties for application in, e.g. food and biomedical industries. 相似文献
12.
Nikhil Malewar Makarand Avachat Varsha Pokharkar Shirish Kulkarni 《AAPS PharmSciTech》2013,14(3):1178-1189
The purpose of the present study was to control in vitro burst effect of the highly water-soluble drug, ropinirole hydrochloride to reduce in vivo dose dumping and to establish in vitro–in vivo correlation. The pharmacokinetics of two entirely different tablet formulation technologies is also explored in this study. For pharmacokinetics study, FDA recommends at least 10% difference in drug release for formulations to be studied but here a different approach was adopted. The formulations F8A and F9A having similar dissolution profiles among themselves and with Requip® XL™ (f2 value 72, 77, 71 respectively) were evaluated. The Cmax of formulation F8A comprising hypromellose 100,000 cP was 1005.16 pg/ml as compared to 973.70 pg/ml of formulation F9A comprising hypromellose 4000 cP irrespective of Tmax of 5 and 5.75 h, respectively. The difference in release and extent of absorption in vivo was due to synergistic effect of complex RH release mechanism; however, AUC0–t and AUC0–∞ values were comparable. The level A correlation using the Wagner–Nelson method supported the findings where R2 was 0.7597 and 0.9675 respectively for formulation F8A and F9A. Thus, in vivo studies are required for proving the therapeutic equivalency of different formulation technologies even though f2 ≥ 50. The technology was demonstrated effectively at industrial manufacturing scale of 200,000 tablets.KEY WORDS: controlled release polymer, in vitro–in vivo correlation (IVIVC), multiple barrier layer tablets, pharmacokinetics, ropinirole hydrochloride (RH) 相似文献
13.
Darius Danusevi
ius Rta Kembryt Jurata Buchovska Virgilijus Baliuckas Darius Kavaliauskas 《Ecology and evolution》2021,11(11):6260
Tilia cordata Mill. is a valuable tree species enriching the ecological values of the coniferous‐dominated boreal forests in Europe. Following the historical decline, spreading of Tilia sp. is challenged by the elevated inbreeding and habitat fragmentation. We studied the geographical distribution of genetic diversity of Tilia cordata populations in Lithuania. We used 14 genomic microsatellite markers to genotype 543 individuals from 23 wild‐growing populations. We found that Tilia cordata retained high levels of genetic diversity (population F IS = 0–0.15, H o = 0.53–0.69, H e = 0.56–0.75). AMOVA, Bayesian clustering, and Monmonier''s barrier detection indicate weak but significant differentiation among the populations (F ST = 0.037***) into geographically interpretable clusters of (a) western Lithuania with high genetic heterogeneity but low genetic diversity, bottleneck effects, (b) relatively higher genetic diversity of Tilia cordata on rich and most soils of midland lowland, and (c) the most differentiated populations on poor soils of the coolest northeastern highland possessing the highest rare allele frequency but elevated inbreeding and bottleneck effects. Weak genetic differentiation among the Tilia cordata populations in Lithuania implies common ancestry, absence of strong adaptive gradients, and effective genetic exchange possible mediated via the riparian networks. A hypothesis on riparian networks as gene flow mediators in Tilia cordata was raised based on results of this study. 相似文献
14.
Modelling of carbohydrate conformations is a challenging task for force field developers. Three carbohydrate force fields, namely GLYCAM06, GROMOS 45a4 and OPLS were evaluated. Free energies of different ring conformations of β-d-glucopyranose were calculated using metadynamics in vacuum as well as in explicitly modelled water. All three force fields model the 4C1 conformation as the most stable by at least 6 kJ/mol, as compared to other conformations. Interconversion from the 4C1 to any other conformation is associated with a barrier of no lower than 26 kJ/mol. The free energy surface calculated in the GLYCAM06 force field is in remarkably good agreement with the recent Car-Parrinello metadynamics study. The effect of a water environment is relatively low and analogous in all tested force fields. Namely, the presence of water stabilizes the upper-left (3,OB) versus bottom-right (B3,O) area of Stoddard’s plot, relative to the situation in vacuum. Comparison of free and potential surfaces is also provided for vacuum calculations. 相似文献
15.
V. V. Alikaev A. A. Borshchegovskii V. A. Vershkov V. V. Volkov A. V. Gorshkov Yu. V. Gott S. A. Grashin M. M. Dremin L. G. Eliseev Yu. V. Esipchuk V. A. Zhuravlev A. M. Kakurin G. S. Kirnev N. A. Kirneva A. Ya. Kislov D. A. Kislov I. V. Klimanov V. A. Kochin V. A. Krupin S. V. Krylov A. V. Melnikov T. B. Myalton A. Yu. Novikov G. E. Notkin Yu. D. Pavlov V. V. Piterskii V. I. Poznyak I. N. Roi P. V. Savrukhin V. V. Sannikov S. V. Soldatov M. M. Stepanenko A. V. Sushkov K. N. Tarasyan V. M. Trukhin E. V. Trukhina L. N. Khimchenko V. V. Chistyakov D. A. Shelukhin 《Plasma Physics Reports》2000,26(11):917-930
An improved confinement regime with an external transport barrier (H-mode) is obtained during electron-cyclotron resonance heating of a plasma in the T-10 tokamak. A characteristic feature of this regime is a spontaneous density growth accompanied by a drop in the intensity of Dα line and an increase in βp by a factor of ~1.6. The threshold power for the L-H transition is close to that predicted by the ITER scaling. The best characteristics of the H-mode are achieved with decreasing q L to 2.2. It is shown that the external transport barrier arises for electrons, whereas the heat transport barrier insignificantly contributes to improved confinement. 相似文献
16.
The mechanism of the nitric oxide reduction in a bacterial nitric oxide reductase (NOR) has been investigated in two model systems of the heme-b3-FeB active site using density functional theory (B3LYP). A model with an octahedral coordination of the non-heme FeB consisting of three histidines, one glutamate and one water molecule gave an energetically feasible reaction mechanism. A tetrahedral coordination of the non-heme iron, corresponding to the one of CuB in cytochrome oxidase, gave several very high barriers which makes this type of coordination unlikely. The first nitric oxide coordinates to heme b3 and is partly reduced to a more nitroxyl anion character, which activates it toward an attack from the second NO. The product in this reaction step is a hyponitrite dianion coordinating in between the two irons. Cleaving an NO bond in this intermediate forms an FeB (IV)O and nitrous oxide, and this is the rate determining step in the reaction mechanism. In the model with an octahedral coordination of FeB the intrinsic barrier of this step is 16.3 kcal/mol, which is in good agreement with the experimental value of 15.9 kcal/mol. However, the total barrier is 21.3 kcal/mol, mainly due to the endergonic reduction of heme b3 taken from experimental reduction potentials. After nitrous oxide has left the active site the ferrylic FeB will form a μ-oxo bridge to heme b3 in a reaction step exergonic by 45.3 kcal/mol. The formation of a quite stable μ-oxo bridge between heme b3 and FeB is in agreement with this intermediate being the experimentally observed resting state in oxidized NOR. The formation of a ferrylic non-heme FeB in the proposed reaction mechanism could be one reason for having an iron as the non-heme metal ion in NOR instead of a Cu as in cytochrome oxidase. 相似文献
17.
Despite highly specialized pollination strategies, hybridization is a common phenomenon among Mediterranean deceptive orchids. Food-deceptive species sire a progeny of F1 unfertile plants, which work as a late post-zygotic barrier. Conversely, when pre-zygotic barriers of sexually deceptive (Ophrys) species are absent, the hybrids are fertile and an extensive introgression may occur. Here, we have performed molecular analysis and hand pollination treatments to characterize a hybrid zone of two food-deceptive species, O. mascula and O pauciflora. Hybrids (called O. × colemanii) have shown different amounts of parental nrDNA, strongly supporting that they are F2 and/or successive hybrid generations. Comparable high levels of reproductive success have been detected in natural conditions and in experimental crosses suggesting the absence of effective reproductive barriers either between hybrids, either between hybrids and parental species. In light of ecological and distributional features of O. × colemanii across its distribution range, we hypothesize that these populations have originated by secondary contact in the periglacial belt of Apennines. Moreover, the rare and localized O. pauciflora could benefit a genetic enrichment by hybridizing with a widespread related species. O. × colemanii is not a dead end population, but may have a role as potential reserve of adaptive variability and is an unusual stage along the speciation process. 相似文献
18.
During running, muscles of the lower limb act like a linear spring bouncing on the ground. When approaching an obstacle, the overall stiffness of this leg-spring system (kleg) is modified during the two steps preceding the jump to enhance the movement of the center of mass of the body while leaping the obstacle. The aim of the present study is to understand how kleg is modified during the running steps preceding the jump. Since kleg depends on the joint torsional stiffness and on the leg geometry, we analyzed the changes in these two parameters in eight subjects approaching and leaping a 0.65 m-high barrier at 15 km h−1. Ground reaction force (F) was measured during 5–6 steps preceding the obstacle using force platform and the lower limb movements were recorded by camera. From these data, the net muscular moment (Mj), the angular displacement (θj) and the lever arm of F were evaluated at the hip, knee and ankle. At the level of the hip, the Mj–θj relation shows that muscles are not acting like torsional springs. At the level of the knee and ankle, the Mj–θj relation shows that muscles are acting like torsional springs: as compared to steady-state running, the torsional stiffness kj decreases from ~1/3 two contacts before the obstacle, and increases from ~2/3 during the last contact. These modifications in kj reflect in changes in the magnitude of F but also to changes in the leg geometry, i.e. in the lever arms of F. 相似文献
19.
The static and dynamic structures of DNA duplexes affected by 5S-Tg (Tg, Thymine glycol) epimers were studied using MD simulations and Markov State Models (MSMs) analysis. The results show that the 5S,6S-Tg base caused little perturbation to the helix, and the base-flipping barrier was determined to be 4.4 kcal mol−1 through the use of enhanced sampling meta-eABF calculations, comparable to 5.4 kcal mol−1 of the corresponding thymine flipping. Two conformations with the different hydrogen bond structures between 5S,6R-Tg and A19 were identified in several independent MD trajectories. The 5S,6R-Tg:O6HO6•••N1:A19 hydrogen bond is present in the high-energy conformation displaying a clear helical distortion, and near barrier-free Tg base flipping. The low-energy conformation always maintains Watson–Crick base pairing between 5S,6R-Tg and A19, and 5S-Tg base flipping is accompanied by a small barrier of ca. 2.0 KBT (T = 298 K). The same conformations are observed in the MSMs analysis. Moreover, the transition path and metastable structures of the damaged base flipping are for the first time verified through MSMs analysis. The data clearly show that the epimers have completely different influence on the stability of the DNA duplex, thus implying different enzymatic mechanisms for DNA repair. 相似文献
20.
Discussions of active transport usually assume stoichiometry between the rate of transport J+ and the metabolic rate Jr. However, the observation of a linear relationship between J+ and Jr does not imply a stoichiometric relationship, i.e., complete coupling. Since coupling may possibly be incomplete, we examine systems of an arbitrary degree of coupling q, regarding stoichiometry as a limiting case. We consider a sodium pump, with J+ and Jr linear functions of the electrochemical potential difference, -X+, and the chemical affinity of the metabolic driving reaction, A. The affinity is well defined even for various complex reaction pathways. Incorporation of a series barrier and a parallel leak does not affect the linearity of the composite observable system. The affinity of some region of the metabolic chain may be maintained constant, either by large pools of reactants or by regulation. If so, this affinity can be evaluated by two independent methods. Sodium transport is conveniently characterized by the open-circuit potential (Δψ)I=0 and the natural limits, level flow (J+)X+=0, and static head X0+ = (X+)J+=0. With high degrees of coupling -X0+/F approaches the electromotive force ENa (Ussing); -X0+/F cannot be identified with ((RT/F) ln f)X+=0, where f is the flux ratio. The efficiency η = -J+X+/JrA is of significance only when appreciable energy is being converted from one form to another. When either J+ or -X+ is small η is low; the significant parameters are then the efficacies εJ+ = J+/JrA and εX+ = -X+/JrA, respectively maximal at level flow and static head. Leak increases both J+ and εJ+ for isotonic saline reabsorption, but diminishes -X0+ and εX♀. Electrical resistance reflects both passive parameters and metabolism. Various fundamental relations are preserved despite coupling of passive ion and water flows. 相似文献