共查询到20条相似文献,搜索用时 989 毫秒
1.
Multi-proton spin-echo images were collected from cold-acclimated winter wheat crowns (Triticum aestivum L.) cv. Cappelle Desprez at 400 MHz between 4 and ?4 °C. Water proton relaxation by the spin-spin (T2) mechanism from individual voxels in image slices was found to be mono-exponential. The temperature dependence of these relaxation rates was found to obey Arrhenius or absolute rate theory expressions relating temperature, activation energies and relaxation rates, Images whose contrast is proportional to the Arrhenius activation energy (Ea), Gibb's free energy of activation (ΔG?), and the entropy of activation (ΔS?) for water relaxation on a voxel basis were constructed by post-image processing. These new images exhibit contrast based on activation energies rather than rules of proton relaxation. The temperature dependence of water proton T2 relaxation rates permits prediction of changes in the physical state of water in this tissue over modest temperature ranges. A simple model is proposed to predict the freezing temperature kof various tissue in wheat crowns. The average Ea and ΔH? for water proton T2 relaxation over the above temperature range in winter wheat tissue were ?6.4 ± 14.8 and ?8.6 ± 14.8kj mol?1, respectively. This barrier is considerably lower than the Ea for proton translation in ice at 0°C, which is reported to be between 46.0 and 56.5 kj mol?1 相似文献
2.
The structures of metal-nucleotide complexes bound to rabbit muscle creatine kinase have been studied by making measurements of paramagnetic effects of two dissimilar activating paramagnetic cations, Mn(II) and Co(II), on the spin-relaxation rates of the 31P nuclei of ATP and ADP in these complexes. The experiments were performed on enzyme-bound complexes, thereby limiting the contributions to the observed relaxation rate to two exchanging complexes (with and without the cation). Measurements were made as a function of temperature in the range 5-35 degrees C and at three 31P NMR frequencies, 81, 121.5, and 190.2 MHz, in order to determine the effect of exchange on the observed relaxation rates. The relaxation rates in E X MnADP and E X MnATP are independent of frequency, and their temperature variation yields activation energies (delta E) in the range 5-8 kcal/mol; in the transition-state analogue complex E X MnADP X NO3- X Cre (Cre is creatine), delta E is increased to 17.3 kcal/mol. These results demonstrate that the relaxation rates in the Mn(II) complexes are exchange limited and are incapable of providing structural data. It is shown further that use of line-width measurements to estimate the lifetime of the paramagnetic complex leads to incorrect results. The relaxation rates in E X CoADP and E X CoATP exhibit frequency dependence and delta E values in the range 1-3 kcal/mol; i.e., these rates depend on the Co(II)-31P distances, whereas those in the E X CoADP X NO3- X Cre complex have delta E approximately 18 kcal/mol and are significantly contributed by exchange.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
3.
Dielectric dispersion of DNA was studied in the frequency range 100 Hz–100 kHz at four different temperatures (6–30°C). The dielectric increment ε0–ε∞ increased with the rise of temperature. The relaxation time, on the other hand, decreased. Both the increase in dielectric increment and the decrease in relaxation time could not be explained on the basis of the counterion polarization theory. Dipole moment was estimated from Kirkwood theory. It was found to decrease systematically with temperature. Even at 0°C there was a dipole moment of 104D. 相似文献
4.
P. Bertrand J. M. Janot H. Benosman J. P. Gayda F. Labeyrie 《European biophysics journal : EBJ》1987,14(5):273-278
E.P.R. experiments and spin-lattice relaxation time measurements have been performed on Flavocytochrome b
2in the range 10 K to 100 K, to obtain information on the distance between the two prosthetic groups of the protein, flavin and heme. We have used the stabilization effect of pyruvate on the semiquinone form of the flavin, to compare the E.P.R. spectral shape and the relaxation properties of the radical when the heme is either in the ferrous form or in the ferric form. When the heme is ferric, no significant increase of the line broadening or enhancement of the relaxation rate of the radical can be detected in the range 10 K to 100 K. From these results, a minimum intercentre distance of 18 to 20 Å can be estimated. 相似文献
5.
The proton and deuterium longitudinal relaxation rates were Studied at room temperature up to the highest protein concentrations in oxyhaemoglobin solutions of different H2O/D2O composition. The deuterium relaxation rates followed the experimentally well known single linear dependence on protein concentration, the slopes being little influenced by solvent (D2O/H2O) composition. The proton ralaxation rates show two different liner dependences on haemoglobin concentration. The entire concentration range is described by two straight lines with the threshold concentration about 11 mM (in haem), The ratio of the slopes is 1.6 (high-to-low Hb-conc.). Only in the higher concentration range two T1's were observed if the solvent contained more than half of D2O. The slow relaxation phase of protons has T1's similar to those measured in solutions with less than half of D2O. The relaxation of the other phase was ten times faster. The ratio of the proton populations in these two phases was equal to 2 (slow-to-fast) and independent of protein concentration. The fast relaxing protons are attributed to water molecules encaged within two or more haemoglobin molecules which associate for times long enough on the PMR time-scale. 相似文献
6.
Populations at the warm range margins of the species distribution may be at the greatest risks of extinction from global warming unless they can tolerate extreme environmental conditions. Yet, some studies suggest that the thermal behavior of some lizard species is evolutionarily rigid. During two successive years, we compared the thermal biology of two populations of Liolaemus pictus living at the northern (warmer) and one population living at the southern (colder) range limits, thus spanning an 800 km latitudinal distance. Populations at the two range margins belong to two deeply divergent evolutionary clades. We quantified field body temperatures (Tb), laboratory preferred body temperatures (PBT), and used operative temperature data (Te) to calculate the effectiveness of thermoregulation (E). During one year in all populations, we further exposed half of the lizards to a cold or a hot acclimation treatment to test for plasticity in the thermal behavior. The environment at the southern range limit was characterized by cooler weather and lower Te. Despite that, females had higher Tb and both males and females had higher PBT in the southernmost population (or clade) than in the northernmost populations. Acclimation to cold conditions led to higher PBT in all populations suggesting that plastic responses to thermal conditions, instead of evolutionary history, may contribute to geographic variation. Lizards regulated moderately well their body temperature (E≈0.7): they avoided warm microhabitats in the northern range but capitalized on warm microhabitats in the southern range. We review literature data to show that Liolaemus species increase their thermoregulation efficiency in thermally challenging environments. Altogether, this indicates that habitats of low thermal quality generally select against thermoconformity in these lizards. 相似文献
7.
17β-Estradiol relaxes cholecystokinin- and KCl-induced tension in male guinea pig gallbladder strips
Estrogen has been shown to have an inhibitory effect on the contractility of gastrointestinal smooth muscle, including the gallbladder. Since estrogen and progesterone levels are elevated during pregnancy, a biliary stasis may develop during pregnancy that is characterized by an increase in the fasting and residual volumes and by a decrease in emptying capacity. This study investigates the effect of 17β-estradiol (E2) on contraction in male guinea pig gallbladder strips. E2 induced a concentration-dependent relaxation of either CCK-induced tension or KCl-induced tension. Pretreatment of the strips with PKA inhibitor 14-22 amide myristolated had no significant effect on the E2-induced relaxation. Pretreatment of strips with 2-APB, and inhibitor of IP3 induced Ca2+ release, produced a significant (p < 0.001) increase in the amount of E2-induced relaxation when either CCK or KCl were used to induce tension. KT5823, an inhibitor of PKG, also significantly (p < 0.001) increased the amount of E2-induced relaxation. Genistein, an inhibitor of protein tyrosine kinase, had no significant effect on the E2-induced relaxation. Bisindolymaleimide IV and chelerythrine Cl- when used in combination had no significant effect on the amount of CCK-induced tension, but significantly (p < 0.001) increased the amount of E2-induced relaxation. When E2 was added to the chambers prior to either CCK or KCl, a significant decrease (p < 0.001) in the amount of tension generated was observed. The inhibition of extracellular Ca2+ entry mediates the E2-induced relaxation of CCK- and KCl-induced tension in male guinea pig gallbladder strips. 相似文献
8.
Hubert L. Dulieu 《Mutation research》1974,25(3):289-304
A greenish-yellow mutant was obtained after treatment of seeds of Nicotiana tabacum L. var. Xanthi n.c. with ethyl methanesulfonate (EMS). Two genetically independent mutations (a1 and a2) were isolated. The first mutation (a1) antagonizes the function of its partially dominant a1+ allele. The second mutation (a2) is amorphous but strongly interacts with a1.Among the nine possible genotypes at the two loci, five varied in somatic cells. The heterozygous state a1+/a1 strongly increased the frequency of both spontaneous and induced variations. However, two homozygotes also showed variations.Variants were isolated from induced and spontaneous non-reciprocal and reciprocal variations within paliside tissues by bud induction in vitro. They were genetically tested. In this first paper, only non-reciprocal variations are reported.Green variants from the greenish-yellow (J1) dihybrid a1+/a1a2+/a2 clone had two genotypes: the first was due to true reversions of a1 to a1+, whereas the second was due to amorphous a10 mutations from a1. These a10 mutations may well be deletions.The lightest yellow variants from J1 were due to mutations either from a1+ into a1 or from a2+ into a2.Deletions at the a1+?a1 locus led to either yellow variations when a1+ was lost, or to false reversions when the antagonistic allele a1 was lost.Amorphous alleles at the a1+?a1 locus were also isolated from tissues other than J+. They gave zygotic lethality (s) that probably varied with the size of the deletions. Thus, true reversions and deletions at the a1+?a1 locus could be distinguished from one another by progeny tests.Other variants showed higher frequencies of spontaneous variations (instability). Somatic changes observed in these unstable systems were due to modifications at the marker loci. The genetic nature of this instability is not yet known.There is strong evidence that the genetic events involved in these non-reciprocal variations were deletions, conversions and point mutations. True reversions from a1 into a1+ and new mutations from a1+ into a1 were obtained only from a1+/a1. It was therefore supposed that the changes observed took place only in heterozygotes, and the conversion hypothesis was made. Attempts are being made to prove that conversions do exist in higher plants, and to find out if this process, as deletions, is induced by radiation. 相似文献
9.
The storage and loss shear moduli, G, and G?, have been measured for solutions of three samples of poly-γ-benzyl-L -glutamate with molecular weights from 16 to 57 × 104, by use of the Birnboim-Schrag multiple-lumped resonator. The frequency range was 106 to 6060 Hz, the concentration range 0.0015–0.005 g/ml, and the temperature 25°C. Two helicogenic solvents with widely different viscosities, dimethylformamide and m-cresol, were used to provide a broader effective frequency range. The intrinsic moduli, extrapolated to infinite dilution, were compared with the predictions of the theory of Ullman for rigid rods; agreement was rather good at the lowest frequencies, but unsatisfactory at high frequencies. The data over the entire frequency range of three of logarithmic decades could be described closely by a relaxation spectrum consisting of one terminal relaxation time separated by a gap from a sequence of relaxtion times spaced as in the Zimm theory. The terminal time agrees approximately with that calculated for end-over-end rotation of a rigid rod. The additional relaxation mechanisms are tentatively attributed to modes of flexural deformation of the helix. 相似文献
10.
Rigidity of myosin and myosin rod by electric birefringence 总被引:1,自引:0,他引:1
The rotational relaxation times of rabbit myosin and myosin rod have been determined by electric birefringence measurement. The relaxation time of myosin measured in 10 mM pyrophosphate buffers in a pH range of 7.6–9.5 was found to have substantial concentration and pH dependences. The infinite-dilution limit of the relaxation time, τ°, was determined as 38 ± 2 μs, and it was found to be independent of pH. For myosin rod, a possible thermally induced conformational change was investigated in a temperature range of 1–43°C. The rotational relaxation time of myosin rod shows no clear indication of conformational change in this temperature range, and the radius of gyration measurement by light scattering was shown to be consistent with this observation. The steady-state birefringence, however, decreases substantially above around 40°C. This, the myosin rod appears to be only slightly flexible even at physiological temperature, but the possibility of a “melting” or “hinging” of the myosin rod cannot completely be ruled out on the basis of these experiments. 相似文献
11.
Borrego Carles M. Arellano Juan B. Abella Carles A. Gillbro Tomas Garcia-Gil Jesús 《Photosynthesis research》1999,60(2-3):257-264
We have determined the molar extinction coefficient of bacteriochlorophyll (BChl) e, the main light-harvesting pigment from brown-coloured photosynthetic sulfur bacteria. The extinction coefficient was determined using pure [Pr,E]BChl eF isolated by reversed-phase HPLC from crude pigment extracts of Chlorobium (Chl.) phaeobacteroides strain CL1401. The extinction coefficients at the Soret and Qy bands were determined in four organic solvents. The extinction coefficient of BChl e differs from those of other related Chlorobium chlorophylls (BChl c and BChl d) but is similar to that of chlorophyll b. The determined extinction coefficient was used to calculate the stoichiometric BChl e to BChl a and BChl e to carotenoids ratios in whole cells and isolated chlorosomes from Chl. phaeobacteroides strain CL1401 using the spectrum-reconstruction method (SRCM) described by Naqvi et al. (1997) (Spectrochim Acta A Mol Biomol Spectrosc 53: 2229–2234) . In isolated chlorosomes, BChl a content was ca. 1% of the total BChl content and the stoichiometric ratio of BChl e to carotenoids was 6. In whole cells, however, BChl a content was 3–4%, owing to the presence of BChl a-containing elements, i.e. FMO protein and reaction centre. An average of 5 BChl e molecules per carotenoid was determined in whole cells. 相似文献
12.
Xueying Wang Raju Gautam Pablo J. Pinedo Linda J. S. Allen Renata Ivanek 《Journal of mathematical biology》2014,69(2):501-532
Many infectious agents transmitting through a contaminated environment are able to persist in the environment depending on the temperature and sanitation determined rates of their replication and clearance, respectively. There is a need to elucidate the effect of these factors on the infection transmission dynamics in terms of infection outbreaks and extinction while accounting for the random nature of the process. Also, it is important to distinguish between the true and apparent extinction, where the former means pathogen extinction in both the host and the environment while the latter means extinction only in the host population. This study proposes a stochastic-differential equation model as an approximation to a Markov jump process model, using Escherichia coli O157:H7 in cattle as a model system. In the model, the host population infection dynamics are described using the standard susceptible-infected-susceptible framework, and the E. coli O157:H7 population in the environment is represented by an additional variable. The backward Kolmogorov equations that determine the probability distribution and the expectation of the first passage time are provided in a general setting. The outbreak and apparent extinction of infection are investigated by numerically solving the Kolmogorov equations for the probability density function of the associated process and the expectation of the associated stopping time. The results provide insight into E. coli O157:H7 transmission and apparent extinction, and suggest ways for controlling the spread of infection in a cattle herd. Specifically, this study highlights the importance of ambient temperature and sanitation, especially during summer. 相似文献
13.
The relaxation behaviour of the sodium salt of poly (L -glutamic acid) in the solid state has been examined by means of dynamic mechanical spectroscopy. Bound water was found to exert a profound influence on the relaxation behaviour and on a bulk property, the rigidity. Certain features of the loss spectrum have been identified with the hydration-dependent β-to-α conformational transition. Thus two side-chain relaxations are observed below ambient temperature, one associated with the β from (β1β) and a second at a lower temperature associated with the α form (β1α). The greater rigidity of the α form below the relaxation temperature and the larger rigidity drop accompanying the β1α can be explained in terms of the structural differences of the two conformations. 相似文献
14.
The glass transition and its related dynamics of myoglobin in water and in a water–glycerol mixture have been investigated by dielectric spectroscopy and differential scanning calorimetry (DSC). For all samples, the DSC measurements display a glass transition that extends over a large temperature range. Both the temperature of the transition and its broadness decrease rapidly with increasing amount of solvent in the system. The dielectric measurements show several dynamical processes, due to both protein and solvent relaxations, and in the case of pure water as solvent the main protein process (which most likely is due to conformational changes of the protein structure) exhibits a dynamic glass transition (i.e. reaches a relaxation time of 100 s) at about the same temperature as the calorimetric glass transition temperature Tg is found. This glass transition is most likely caused by the dynamic crossover and the associated vanishing of the α-relaxation of the main water relaxation, although it does not contribute to the calorimetric Tg. This is in contrast to myoglobin in water–glycerol, where the main solvent relaxation makes the strongest contribution to the calorimetric glass transition. For all samples it is clear that several proteins processes are involved in the calorimetric glass transition and the broadness of the transition depends on how much these different relaxations are separated in time. 相似文献
15.
Using broadband dielectric spectroscopy, we have studied different types of relaxation processes, namely, primary (α), secondary (β), and another sub-Tg process called γ-process, in the supercooled state of d-lyxose, over a wide frequency (10-2–) and temperature range (120–340 K). In addition, the same sample was analyzed by differential scanning calorimeter. The temperature dependence of the relaxation times as well as the dielectric strength of different processes has been critically examined. It has been observed that the slower secondary relaxation (designated as β-) process shifts to lower frequencies with increasing applied pressure, but not the faster one. This pressure dependence indicates that the observed slower secondary relaxation (β-) is Johari–Goldstein relaxation process and faster one (γ-process) is probably the rotation of hydroxymethyl (–CH2OH) side group attached to the sugar ring, that is, of intramolecular origin. 相似文献
16.
The paramagnetic effects of Mn(II) and Co(II) on the spin-lattice relaxation rates of 31P nuclei of ATP and ADP and of Mn(II) on the spin-lattice relaxation rate of the delta protons of arginine bound to arginine kinase from lobster tail muscle have been measured. Temperature variation of 31P relaxation rates in E.MnADP and E.MnATP yields activation energies (delta E) in the range 6-10 kcal/mol. Thus, the 31P relaxation rates in these complexes are exchange limited and cannot provide structural information. However, the relaxation rates in E.CoADP and E.CoATP exhibit frequency dependence and delta E values in the range 1-2 kcal/mol; i.e., these rates depend upon 31P-Co(II) distances. These distances were calculated to be in the range 3.2-4.5 A, appropriate for direct coordination between Co(II) and the phosphoryl groups. The paramagnetic effect of Mn(II) on the 1H spin-lattice relaxation rate of the delta protons of arginine in the E.MnADP.Arg complex was also measured at three frequencies (viz., 200, 300, and 470 MHz). These 1H experiments were performed in the presence of sufficient excess of arginine to be observable over the protein background but with MnADP exclusively in the enzyme-bound form so that the enhancement in the relaxation rates of the delta protons of arginine arises entirely from the enzyme-bound complex. Both the observed frequency dependence of these rates and the delta E less than or equal to 1.0 +/- 0.3 kcal/mol indicate that this rate depends on the 1H-Mn(II) distances.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
17.
Inhibition of bovine brain hexokinase by its product, glucose 6-phosphate, is considered to be a major regulatory step in controlling the glycolytic flux in the brain. Investigations on the molecular basis of this regulation, i.e. allosteric or product inhibition, have led to various proposals. Here, we attempt to resolve this issue by ascertaining the location of the binding sites for glucose and glucose 6-phosphate on the enzyme with respect to a divalent-cation-binding site characterized previously [Jarori, G. K., Kasturi, S. R. & Kenkare, U. W. (1981) Arch. Biochem. Biophys. 211, 258-268]. The paramagnetic effect of enzyme-bound Mn(II) on the spin-lattice relaxation rates (T-1(1] of ligand nuclei (1H and 31P) in E.Mn(II).Glc and E.Mn(II).Glc6P complexes have been measured. The paramagnetic effect of Mn(II) on the proton relaxation rates of C1-H alpha, C1-H beta and C2-H beta of glucose in the E.Mn(II).Glc complex was measured at 270 MHz and 500 MHz. The temperature dependence of these rates was also studied in the range of 5-30 degrees C at 500 MHz. The ligand nuclear relaxation rates in E.Mn(II).Glc are field-dependent and the Arrhenius plot yields an activation energy (delta E) of 16.7-20.9 kJ/mol. Similar measurements have also been carried out on C1-H alpha, C1-H beta and C6-31P at 270 MHz (1H) and 202.5 MHz (31P) for the E.Mn(II).Glc6P complex. The temperature dependence of 31P relaxation rates in this complex was measured in the range 5-30 degrees C, which yielded delta E = 9.2 kJ/mol. The electron-nuclear dipolar correlation time (tau c), determined from the field-dependent measurements of proton relaxation rates in the E.Mn(II).Glc complex, is 0.22-1.27 ns. The distances determined between Mn(II) and C1-H of glucose and glucose 6-phosphate are approximately 1.1 nm and approximately 0.8 nm, respectively. These data, considered together with our recent results [Mehta, A., Jarori, G. K. & Kenkare, U. W. (1988) J. Biol. Chem. 263, 15492-15498], suggest that glucose and glucose 6-phosphate may bind to very nearly the same region of the enzyme. The structure of the binary Glc6P.Mn(II) complex has also been determined. The phosphoryl group of the sugar phosphate forms a first co-ordination complex with the cation. However, on the enzyme, the phosphoryl group is located at a distance of approximately 0.5-0.6 nm from the cation. 相似文献
18.
Proton and phosphorus magnetic relaxation studies on the dynamic structure of single-stranded polyriboadenylic acid 总被引:1,自引:0,他引:1
K Akasaka 《Biopolymers》1974,13(11):2273-2280
Proton and phosphorus-31 nuclear spin-lattice relaxation times (T1) have been measured with the Fourier-transform method at 100 and 40.5 MHz, respectively, on single-stranded polyriboadenylic acid (poly(A)) in a neutral D2O solution in the temperature range of 14–82°C. T1 minimum is observed around 35–45°C for H(8), H(1′), and phosphorus resonances. Rotational correlation times have been deduced from the T1 data, which indicate that the sugar–phosphate backbone as well as the base–sugar segment is undergoing rapid internal motion of 10?8–10?10 sec range. The molecular motion of the sugar–phosphate backbone as deduced from the phosphorus relaxation is well-characterized by a single activation enthalpy of 8.1 kal/mole for the whole temperature range of 14–82°C. Activation enthalpies of similar magnitude have been obtained for the motion of the adenine–ribose moiety from H(8) and H(1′) relaxation. The relative magnitude of T1 for H(8) and H(1′) infers that the poly(A) nucleotide exists on the average as anti in the single-stranded form. The phosphorus T1 value is consistent with a conformation such that both C(4′)–C(5′) and C(4′)–C(3′) bonds are nearly trans to their connected O–P bonds. 相似文献
19.
目的:研究不同温度条件下血管舒缩功能变化及哌唑嗪、山莨菪碱扩张血管作用变化特征,评价VitE在低温条件下的内皮保护作用,探讨上述药物在冻伤预防过程中的应用前景。方法:利用血管条技术,观察小鼠尾动脉血管在8℃、16℃、25℃、37℃四个温度条件下的收缩及舒张反应特点,比较哌唑嗪、山莨菪碱在不同温度条件下扩血管作用差异。在冷暴露处理的同时预敷Vit E,观察其对低温条件下血管内皮依赖性舒张功能的改善作用。结果:①不同温度条件下苯肾上腺素诱发的血管收缩反应存在明显差异,温度越低,收缩幅度越小;②硝普钠浓度依赖的扩血管作用随着温度的降低明显增强;③与硝普钠作用特点类似,哌唑嗪、山莨菪碱在低温条件下的扩血管作用强于37℃组;④低温能够降低乙酰胆碱内皮依赖的扩血管作用,Vit E能够剂量依赖地对抗低温的影响。结论:随着温度的降低,苯肾上腺素作用下的血管收缩明显减弱,平滑肌靶点扩血管药物的作用显著增强。乙酰胆碱内皮依赖的扩血管作用随温度的下降有所降低,Vit E能够在一定程度上减小低温对乙酰胆碱扩血管作用的影响。 相似文献
20.
The potency of various fragments of γ-endorphin (β-LPH61–77) was compared on their ability to facilitate extinction of pole-jumping avoidance behavior and their effects in two “grip tests” used as measures of neuroleptic-like activity. It appeared that β-LPH66–77 is the shortest sequence which shows potencies in the three tests comparable to that of DTγE (β-LPH62–77). The activity of β-LPH67–77 was less. It is proposed that β-LPH66–77 rather than DTγE represents an endogenous neuroleptic-like neuropeptide which may play a key role in psychopathology. 相似文献