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1.
Nuclear magnetic resonance (NMR) relaxation times were studied in acclimated and nonacclimated Kharkov winter wheat ( Triticum aestivum L.) crowns and acclimated cell aggregates to determine if membrane permeability was altered by freezing. The NMR water signal decay consisted of two exponential components: a short one arising from extracellular water, and a long one arising from intracellular water. A slow freezethaw treatment of nonacclimated and 1-week acclimated crowns decreased the long relaxation time, suggesting membrane injury. Similar results were obtained for nonacclimated and acclimated crowns killed directly in liquid N 2. A significant increase in plasma membrane permeability to Mn2+ was observed in acclimated freeze-killed crowns and cell aggregates. Freezing injury to plant tissue appears to be a membrane-related phenomenon, but more extensive injury occurs to nonacclimated and acclimated tissue with a high water content (cell aggregates) compared to acclimated tissue with a low water content (crowns). 相似文献
2.
Proton and phosphorus-31 nuclear spin–lattice relaxation times T1 and spin–spin relaxation times T2 have been measured on the single-stranded polyriboadenylic acid [poly(A)]–Mn 2+ system in a neutral D 2O solution in the temperature range 10°–90°C at 100 and 40.5 MHz, respectively, with the Fourier transform nmr method. Minimum values of T1 have been found for all these nuclei, which have enabled the exact estimation of apparent distances from Mn 2+ to H 2, H 8, H 1′, and the phosphorus nucleus to be 4.7, 4.1, 5.2, and 3.0 Å, respectively. The electron spin of Mn 2+ penetrates into the phosphorus nucleus, giving 31P hyperfine coupling of more than 10 6 Hz. Evidence of penetration of the electron spin into H 8 and H 2 is also obtained, suggesting direct coordination of nitrogen atoms of the adenine ring to the Mn 2+ Ion. Combined with the result from proton relaxation enhancement of water, it is concluded that every Mn 2+ ion added is bound directly to two phosphate groups with a Mn 2+–phosphorus distance of 3.3 Å, while a part of the Mn 2+ ions are simultaneouly bound to the adenine ring. It is estimated that 39 ± 13% and 13 ± 5% of Mn 2+ are coordinated by N 7 and N 3 (or N 1), respectively. The motional freedom of poly(A) in the environment of the Mn 2+ binding site has been found to be quenched to the extent that the rotational motion becomes several times slower than that of the corresponding Mn 2+–free poly(A). The activation energies for the molecular motion are, however, practically unchanged from those for Mn 2+–free poly(A), and are found to be 8.3, 8.5, 6.1, and 8.7 kcal/mol for H 8, H 2, H 1′, and phosphorus, respectively. T2 of phosphorus is determined by the dissociation rate ( k?1) of Mn 2+ from the phosphate group for the whole temperature range studied with activation enthalpy of 6.5 kcal/mol. The dissociation rates of Mn 2+ from the adenine ring are also estimated from proton T2 values below 50°C. 相似文献
3.
To study the influence of cell wall polyuronide structure on bound paramagnetic ion interactions, spin-spin coupling measurements were made on intact cell walls exchanged with a wide range fo Mn 2+ and Cu 2+ concentrations. These experiments were performed so that dimer-only intercationic nearest neighbor distances ( d) and lattice constants (κ) could be calculated from the linewidth-concentration dependency. d values were estimated to be 12 and 14 Å for Cu 2+ and Mn 2+, respectively. At the maximal bound ion concentration, κ was 2.3–2.6, indicating that about 5–7 paramagnetic ion nearest neighbor spin-spin interactions occur per dipole in the nearly filled lattice. This latter observation strongly argues for the egg-box model of the cell wall-polyuronide lattice structure. Mn 2+ linewidths of hydrated cell wall-bound paramagnetic ions displayed an unusual temperature dependency, whereby linewidths increased between 20°C and the temperature at which maximal linewidths were observed ( Tmax). Tmax was inversely proportional to the degree of lattice hydration, indicating that the temperature dependency was not associated with the freezing of bound water. The relative change in Mn 2+ linewidths, between 20°C and Tmax, was affected by binding site-associated 1H spin-lattice relaxation times, indicating that the temperature dependency is at least partially controlled by cell wall polyuronide structure. 相似文献
4.
Transverse relaxation times ( T2) of tissue water ( 1H) in leaves and suspension cultured cells of grape hybrids ( Vitis spp. cv `Venus' and `Veeblanc') were measured by nuclear magnetic resonance at various temperatures. The tissue water was characterized by two T2 time constants. A sharp decrease in T2 for the major fraction of tissue water was observed in association with heat injury, as measured by electrolyte leakage and triphenyltetrazolium chloride reduction in both leaves and suspension cultured cells. The changes in T2 as a result of heat injury were irreversible, as indicated by a temperature dependent hysteresis of T2. Studies using a paramagnetic probe (Mn +2) indicated that the plasma membrane was irreversibly damaged at the killing temperature, resulting in a loss of cell compartmentalization. Tissue water in heat-killed samples was characterized by only a single T2. 相似文献
5.
The binding of Mn 2+ to the anthranilate synthetase-phosphoribosyltransferase enzyme complex from Salmonella typhimurium was examined by electron paramagnetic resonance studies. Two types of binding sites were observed: one to two tight sites with a dissociation constant of 3–5 μm and five to six weaker sites with a dissociation constant of 40–70 μm. The activator constant for Mn 2+ was found to be 9 μm for the glutamine-linked anthranilate synthetase activity and 4 μm for the phosphoribosyltransferase activity. These values are both in the range of the dissociation constant for the tight sites. Water proton relaxation rate measurements showed that the binary enhancement values for both classes of sites were equivalent, ?b = 10.7 ± 2.0. The addition of chorismate to the Mn 2+-enzyme complexes when predominantly the tight Mn 2+ sites were occupied resulted in a large decrease in the observed enhancement ( ?T = 2.0). Addition of 5-phosphoribosyl-1-pyrophosphate to the enzyme-Mn 2+ complexes caused large decreases in the water proton relaxation rate ( ?T = 1.5) when tight or tight plus weaker Mn 2+ sites were occupied. No changes in the water proton relaxation rate were observed when glutamine, pyruvate, or anthranilate were added; a small decrease was observed when enzyme-Mn 2+ was titrated with tryptophan. Tryptophan significantly altered the effect of the binding of chorismate but not of 5-phosphoribosyl-1-pyrophosphate. The effect of tryptophan on the water proton relaxation rate of a Mn 2+-enzyme-chorismate complex using a variant enzyme complex which is tryptophan hypersensitive ( P. D. Robison, and H. R. Levy, 1976, Biochim. Biophys. Acta. 445, 475–485) occurred at lower concentrations than for the normal enzyme complex. The uncomplexed anthranilate synthetase subunit was titrated with Mn 2+ and found to have one to two binding sites with a dissociation constant of 300 ± 100 μm. This dissociation constant is much larger than the activator constant for Mn 2+ for uncomplexed anthranilate synthetase which was determined to be 4 μm. These results indicate that the Mn 2+-binding sites on anthranilate synthetase are altered when the enzyme complex is formed and that both chorismate and 5-phosphoribosyl-1-pyrophosphate interact closely with enzyme-bound Mn 2+ or cause a large effect upon its environment. 相似文献
6.
Carbon-13 spin-lattice relaxation times, T 1, have been measured for aqueous solutions of L-aspartic acid, L-alanine, O-phospho-L-serine, and 2-mercapto-L-succinic acid in the presence of the paramagnetic metal ions, Cu 2+ and Mn 2+, and Mg 2+ as a diamagnetic control, at ambient temperature and neutral pH. Nitrogen-15, oxygen-17 and proton relaxation times were also obtained for L-aspartic acid and phosphorus-31 relaxation times for O-phospho-L-serine under similar conditions. The structures of these complexes in solution were determined from the various metal ion-nuclei distances calculated from the paramagaetically-induced relaxation. These results indicate that the Cu 2+ interaction with L-aspartic acid is through α-amino and β-carboxyl groups while Mn 2+ coordinates most strongly through α-and β-carboxyl groups, with the possibility of a weak interaction through the amino group.An examination of the coordination of these divalent metal ions to an analog of L-aspartic acid in which the β-carboxyl group is replaced by a phosphate group ( O-phospho-L-serine) indicated that Cu 2+ coordination is now probably through the α-amino and phosphate groups, while this analog is a monodentate ligand for Mn 2+ coordinating through the phosphate group. Removal of the β-carboxyl group (L-alanine) also results in Cu 2+ coordination through the α-carboxyl and α-amino groups, and the same ligand interactions are observed with Mn 2+. Replacement of the α-amino group of L-aspartic acid with an - SH group (2-mercapto-L-succinate) is sufficient to eliminate any specific coordination with either Cu 2+ or Mn 2+. 相似文献
7.
The water proton spin-lattice relaxation rate (1/ T1) was measured in suspensions of erythrocytes adsorbed with the hemagglutinating Echovirus type 11. The observed 1/ T1 enhancement, which was displaying a linear dependence on the virus concentration, was taken as a proof that the adsorption phase of the virus-cell interaction accounts, at least partially, for the previously observed phenomena in virus-infected HEp-2 cells. By injecting the Echovirus type 11 into suspensions of erythrocytes doped with Mn 2+ or treated with concanavalin A, cylochalasin D and colchicinc. it was suggested that the observed 1/ T1 enhancement could be related to modifications of the permeability properties of the red cell membrane, as well as to movements of the cell surface receptors. 相似文献
8.
Nuclear magnetic resonance (NMR) microimaging and proton relaxation times were used to monitor differences between the hydration
state of the nucleus and cytoplasm in the Rana pipiens oocyte. Individual isolated ovarian oocytes were imaged in a drop of Ringer's solution with an in-plane resolution of 80
μm. Proton spin echo images of oocytes arrested in prophase I indicated a marked difference in contrast between nucleoplasm
and cytoplasm with additional intensity gradations between the yolk platelet-rich region of the cytoplasm and regions with
little yolk. Neither shortening τ e (spin echo time) to 9 msec (from 18 msec) nor lengthening τ r (spin recovery time) to 2 sec (from 0.5 sec) reduced the observed contrast between nucleus and cytoplasm. Water proton T 1 (spin-lattice) relaxation times of oocyte suspensions indicated three water compartments that corresponded to extracellular
medium (T 1= 3.0 sec), cytoplasm (T 1= 0.8 sec) and nucleoplasm (T 1= 1.6 sec). The 1.6 sec compartment disappeared at the time of nuclear breakdown. Measurements of plasma and nuclear membrane
potentials with KCl-filled glass microelectrodes demonstrated that the prophase I oocyte nucleus was about 25 mV inside positive
relative to the extracellular medium. A model for the prophase-arrested oocyte is proposed in which a high concentration of
large impermeant ions together with small counter ions set up a Donnan-type equilibrium that results in an increased distribution
of water within the nucleus in comparison with the cytosol. This study indicates: (i) a slow exchange between two or more
intracellular water compartments on the NMR time-scale, (ii) an increased rotational correlation time for water molecules
in both the cytoplasmic and nuclear compartments compared to bulk water, and (iii) a higher water content (per unit dry mass)
of the nucleus compared to the cytoplasm, and (iv) the existence of a large (about 75 mV positive) electropotential difference
between the nuclear and cytoplasmic compartments.
Received: 18 January 1996/Revised: 29 April 1996 相似文献
9.
The electric quadrupole moment of the deuterium nucleus provides a nuclear magnetic resonance (NMR) probe of electric field gradients, and thereby of organization of tissue water. 8-17% of H 2O in rat muscle and brain was replaced by D 2O from 50% deuterated drinking water. The peak height of the steady-state NMR spectrum of D in muscle water was 74% lower than that of an equal concentration of D 2O in liquid water. Longitudinal NMR relaxation times ( T1) of D in water of muscle and brain averaged 0.092 and 0.131 sec, respectively, compared with 0.47 sec in D 2O in liquid water. Transverse NMR relaxation times ( T2) averaged 0.009 and 0.022 sec in D 2O of muscle and brain, respectively, compared with 0.45 sec in D 2O in liquid water. These differences cannot be explained by paramagnetic ions or by magnetic inhomogeneities, which leaves increased organization of tissue water as the only tenable hypothesis. Evidence was also obtained that 27% of muscle water and 13% of brain water exist as a separate fraction with T2 of D 2O less than 2 × 10 -3 sec, which implies an even higher degree of structure. Each of the two fractions may consist of multiple subfractions of differing structure. 相似文献
10.
The dormant cysts of Artemia undergo cycles of hydration-dehydration without losing viability. Therefore, Artemia cysts serve as an excellent intact cellular system for studying the dynamics of water-protein interactions as a function of hydration. Deuterium spin-lattice ( T1) and spin-spin ( T2) relaxation times of water in cysts hydrated with D 2O have been measured for hydrations between 1.5 and 0.1 g of D 2O per gram of dry solids. When the relaxation rates (I/ T1, I/ T2) of 2H and 17O are plotted as a function of the reciprocal of hydration (1/H), an abrupt change in slope is observed near 0.6 g of D 2O (or H 2 17O)/gram of dry solids, the hydration at which conventional metabolism is activated in this system. The results have been discussed in terms of the two-site and multisite exchange models for the water-protein interaction as well as protein dynamics models. The 2H and 17O relaxation rates as a function of hydration show striking similarities to those observed for anisotropic motion of water molecules in protein crystals. It is suggested here that although the simple two-site exchange model or n-site exchange model could be used to explain our data at high hydration levels, such models are not adequate at low hydration levels (<0.6 g H2O/g) where several complex interactions between water and proteins play a predominant role in the relaxation of water nuclei. We further suggest that the abrupt change in the slope of I/T1 as a function of hydration in the vicinity of 0.6 g H2O/g is due to a change in water-protein interactions resulting from a variation in the dynamics of protein motion. 相似文献
11.
Natural abundance 13C nuclear magnetic resonance spectroscopy ( 13C NMR) was used to study the mode of binding of Mn 2+ and Cu 2+ to the cyclitol, cis-inositol. Resonance linewidths and the electron nuclear relaxation rates [( T1e) ?1 values] were used to establish that a unique binding site exists for these metal-ions on this cyclitol involving only the three axial hydroxyl groups. This work may aid in the development of new organometallic complexes used as paramagnetic relaxation agents in magnetic resonance imaging research. 相似文献
12.
The effect of channel opening in the tonoplast by d- myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P 3] has been examined on red beet ( Beta vulgaris) vacuoles. Patch-clamp measurements of the vacuolar potential and current were performed on vacuoles isolated in 0.1 micromolar free Ca 2+ medium. With vacuoles clamped at +30 millivolts, the Ins(1,4,5)P 3 induced changes in current were depending on the Ca 2+ buffer strength in the external medium. The spontaneous depolarization of vacuoles in which H +-pumps were activated by 5 millimolar MgATP was increased from +6 to +18 millivolts by 1 micromolar Ins(1,4,5)P 3. We have interpreted our data by assuming that even with 2 millimolar EGTA to buffer Ca 2+ at 0.1 micromolar in the external medium, Ins(1,4,5)P 3 released enough Ca 2+ from the vacuole to produce an accumulation of this ion near the tonoplast. Apart from their dependency with free Ca 2+ in the cytoplasm, the electrical properties of the tonoplast could be depending on the Ins(1,4,5)P 3 and Ca 2+ buffer values in the cytoplasm. 相似文献
13.
Magnesium-dependent adenosine triphosphatase, purified from sheep kidney medulla using digitonin, has been characterized in a series of kinetic and magnetic resonance studies. Kinetic studies of divalent metal activation using either Mg 2+ or Mn 2+ indicate a biphasic response to divalent cations. Apparent Km values of 23 μm for free Mg 2+ and 3.3 μm for free Mn 2+ are obtained at low levels of added metal, while Km values of 0.50 mm for free Mg 2+ and 0.43 mm for free Mn 2+ are obtained at much higher levels of divalent cations. In all cases the kinetic data indicate that the binding of divalent metals is independent of the substrate, ATP. Kinetic studies of the substrate requirements of the Mg 2+-ATPase also yield biphasic Lineweaver-Burk plots. At low ATP concentrations, kinetic studies yield apparent Km values for free ATP of 6.0 and 1.4 μm with Mg 2+ and Mn 2+, respectively, as the activating divalent metals. At much higher levels of ATP the response of the enzyme to ATP changes so that Km values for free ATP of 8.0 and 2.0 mm are obtained for Mg 2+ and Mn 2+, respectively. In both cases, however, the binding of ATP is independent of added metal. ADP inhibits the Mg 2+-ATPase and the kinetic data indicate that ADP competes with ATP at both the high and low affinity sites. Dixon plots of the data are consistent with competitive inhibition at both ATP sites, with Ki values of 10.5 μm and 4.5 mm. Electron paramagnetic resonance and water proton relaxation rate studies show that the enzyme binds 1 g ion of Mn 2+ per 469,000 g of protein. The Mn 2+ binding studies yield a KD for Mn 2+ at the single high affinity site of 2 μm, in good agreement with the kinetically determined activator constant for Mn 2+ at low Mn 2+ levels. Moreover, the EPR binding studies also indicate the existence of 34 weak sites for Mn 2+ per single high affinity Mn 2+ site. The KD for Mn 2+ at these sites is 0.55 mm, in good agreement with the kinetic activator constant for Mn 2+ of 0.43 mm, consistent with additional activation of the enzyme by the large number of weaker metal binding sites. The enhancement of water proton relaxation by Mn 2+ in the presence of the enzyme is also consistent with the tight binding of a single Mn 2+ ion per 469,000 Mr protein and the weaker binding of a large number of divalent metal ions. Analysis of the data yields a value for the enhancement for bound Mn 2+ at the single tight site, ?b, of 5 and an enhancement at the 34 weak sites of 11. The frequency dependence of water proton relaxation by Mn 2+ at the single tight site yields a dipolar correlation time (constant from 8–60 MHz) of 3.18 × 10 ?9 s. The kinetics and metal binding studies, together with the effect of temperature on ATPase activity at high and low levels of ATP, are consistent with the existence in this preparation of a single Mg 2+-ATPase, with high and low affinity sites for divalent metals and for ATP. Observations of both high and low affinities for ATP have been made with two other purified ATPases. The similarities of these systems to the Mg 2+-ATPase described here are discussed. 相似文献
14.
The binding of Mn 2+, Ca 2+, and rare earth ions to apoconcanavalin A has been studied by water proton relaxation enhancement, electron paramagnetic resonance spectroscopy, and fluorescence spectroscopy. An electron paramagnetic resonance and water proton relaxation rate study of the titration of apoconcanavalin A with Mn 2+ gives evidence of two equivalent binding sites per monomer with KD = 50 μm ± 4 μm. When a similar Mn 2+ titration of apoconcanavalin A is performed in the presence of Ca 2+ ion, very little free Mn 2+ is detected by electron paramagnetic resonance until the two Mn 2+ binding sites per monomer are filled. The substitution of a rare earth ion for Ca 2+ ion in the above experiment often resulted in a slight displacement of Mn 2+ from the transition metal site as detected by electron paramagnetic resonance. A water proton relaxation rate study of the titration of apoconcanavalin A with Gd 3+ reflects two binding sites with a KD = 40 μm ± 4 μm and two with a KD = 200 μm ± 50 μm. The fluorescence emission spectrum of concanavalin A (λ em = 340 nm) is slightly quenched by the addition of Tb 3+ while Tb 3+ fluorescence is greatly enhanced. A fluorometric titration of apoconcanavalin A with Tb 3+ also reflects two sites with a KD = 40 μm ± 15 μm and two with a KD = 270 μm ± 50 μm. 相似文献
15.
Solvent 1H T?11 values at magnetic fields between 2.3 × 10 ?4 and 1.2 T were measured for ethyleneglycol solutions of manganese(II), cobalt(II), nickel(II), copper(II), and gadolinium(III) salts in the temperature range ?10–+40 °C. The T?11 profiles were interpreted on the ground of the available theories according to whether the electronic relaxation times or the rotational times are the correlation times for the interaction. The comparison among systems experiencing different correlation times and electronic structures (metal ions in water, ethyleneglycol, and in proteins) allowed us to test the general theoretical approach regarding nuclear relaxation times in paramagnetic systems. Such theoretical approaches allow researchers to extract structural parameters and information on the correlation times and dynamic processes operative in the various investigated systems. 相似文献
16.
Measurements were made of the water proton relaxation rate ( T?12 = R2), electron spin resonance (ESR) six-line signal of ‘free’ Mn 2+, and O 2-evolution activity in thylakoid membranes from pea leaves. The main results are: (1) Aging of thylakoids at 35°C causes a parallel decrease in O 2-evolution activity, in R2 and in the content of bound Mn, suggesting that R2 may be related to the loosely bound Mn involved in O 2 evolution. (2) Treatment of thylakoids with tetraphenylboron (TPB) at [TPB] > 2 mM produces a 2-fold increase in R2, without release of Mn 2+. The titration curve exhibits three sharp end points. The first end point occurs at a of 1.25, at which the O 2 evolution is completely inhibited. (3) Treatment of thylakoids with NH 2OH also increases R2 by nearly 2-fold, either by the reduction of the higher oxidation states of Mn to Mn 2+ and / or by exposing the Mn to solvent protons. Also, progressive release of bound Mn occurs at [NH 2OH] ≥ 1 mM as shown by an increase increase in the Mn 2+ ESR signal and a decrease in R2. (4) Addition of H 2O 2 (0.1–1.0%) to thylakoids causes an enhancement of R2 similar to that by NH 2OH, but without the release of Mn 2+. (5) Heat treatment of thylakoids at 40–50°C releases Mn 2+ and increases R2. Conversely, pH values of 7 to 4 release Mn 2+ without changing R2 while pH values of 7–9 increase R2 without releasing Mn 2+. Thus, both high and low pH values as well as the heat treatment cause structural changes enhancing the relaxivity of the bound Mn or of other paramagnetic species. 相似文献
17.
Measurements of water proton spin relaxation enhancements (ε) can be used to discriminate high-affinity binding of Mn 2+ or Gd 3+ to biological membranes, from low-affinity binding. In rat liver mitochondria, εb values of approx. 11 are observed upon binding of Mn 2+ to the inner membrane, while internal or low-affinity binding remains invisible to this technique. Energy-driven Mn 2+ uptake by liver mitochondria results in the subsequent decay of ε 1.Comparison of ε 1 with the initial velocity of Mn 2+ uptake in rat liver mitochondria reveals a linear correlation, which holds at all temperatures between 0 °C and 40 °C, regardless of the mitochondrial protein concentration. Consequently, enhancement appears to reflect the binding of Mn 2+ to the divalent cation pump.Binding of Mn 2+ to blowfly flight muscle also results in substantial ε 1, which is associated with the glycerol-1-phosphate dehydrogenase instead of divalent cation transport. Consequently, no decay in ε 1 due to uptake occurs after Mn 2+ is bound.Lanthanide ions are also bound and transported by mitochondria. Addition of Gd 3+ to pigeon heart or rat liver mitochondria results in εb ≈ 5–6, which decays with similar kinetics in both systems. The uptake velocity of Gd 3+ in rat liver mitochondria is about the rate with which Mn 2+ is transported. Lanthanides also diminish ε 1 due to the addition of Mn 2+, and greatly retard the Mn 2+ uptake kinetics. The presence of carbonylcyanide- p-trifluoromethoxyphenylhydrazone depresses ε 1 upon addition of Mn 2+ or Gd 3+ and also uncouples energy-driven uptake. On the other hand, prolonged anaerobic incubation in the presence of antimycin and rotenone exhausts the mitochondria of their energy stores, blocks the uptake of Mn 2+, but does not affect ε 1 significantly. Evidently, the uncoupler-induced disappearance of divalent cation binding sites is not the result of “de-energization”.Measurements of ε 1 at several NMR frequencies indicate a correlation time ( τb) for carrier-bound Mn 2+ in rat liver mitochondria between 20 ns and 4 ns as one varies the temperature between 10 °C and 30 °C. The 13 Kcal/mole activation energy for τb suggests that the 11 ns time constant at room temperature represents the movement of the Mn II-carrier complex. On the other hand, τb is probably approx. 100 times too short to represent the rotational motion of a carrier protein. Apparently, Mn 2+ binds to a small arm of the carrier which moves independently of the main body of any protein.In addition to Mn(H 2O) 62+, other complexes of Mn 2+ may also be bound and transported by rat liver mitochondria. Only a small increase in ε 1 occurs upon addition of MnHPO 4, yet this species is accumulated by the mitochondria. Consequently, the carrier does not recognize divalent metal ions on the basis of charge. 相似文献
18.
Vesicle suspensions of up to 5 % egg lecithin and 2.5 % cholesterol have been found to have no effect on the NMR relaxation times of 17O from water. Addition of 1–5 mM Mn 2+ to an equimolar vesicle suspension of egg lecithin and cholesterol permitted resolution of the free induction decay into two exponential components, a fast one arising from the external water and a slow one arising from the intravesicular fluid. From the rates of relaxation the mean life time of the water molecules within the vesicles was calculated to be 1±0.1 ms at 22°C. The size of the vesicle was estimated from electron micrographs to be about 500 Å in diameter. These data yield an equilibrium water permeability, Pw, of about 8 μs −1 for the vesicle membranes. From the temperature dependence of Pw an activation energy of 12±2 kcal/mol was obtained. The longitudinal relaxation time ( T1) of water within vesicles remained the same as in pure water. 相似文献
19.
Measurement by two nuclear magnetic resonance (NMR) techniques of the mean residence time τ a of water molecules inside Chlorella vulgaris (Beijerinck) var. “viridis” (Chodot) is reported. The first is the Conlon and Outhred (1972 Biochim Biophys Acta 288: 354-361) technique in which extracellular water is doped with paramagnetic Mn 2+ ions. Some complications in application of this technique are identified as being caused by the affinity of Chlorella cell walls for Mn 2+ ions which shortens the NMR relaxation times of intra- and extracellular water. The second is based upon observations of effects of diffusion on the spin echo of intra- and extracellular water. Echo attenuation of intracellular water is distinguished from that of extracellular water by the extent to which diffusive motion is restricted. Intracellular water, being restricted to the cell volume, suffers less echo attenuation. From the dependence of echo amplitude upon gradient strength at several values of echo time, the mean residence time of intracellular water can be determined. From the mean residence time of intracellular water, the diffusional water permeability coefficient of the Chlorella membrane is calculated to be 2.1 ± 0.4 × 10 −3 cm sec −1. 相似文献
20.
Careful experiments on the measurement of the intensity of the deuterium NMR signal for 2H 2O in muscle and in its distillate were performed, and they showed that all 2H 2O in muscles is “NMR visible.”The spin-lattice relaxation time ( T1) of the water protons in the muscle and liver of mice and in egg white has been studied at six frequencies ranging from 4.5 to 6.0 MHz over the temperature range of +37 to −70°C. T1 values of deuterons in 2H 2O of gastrocnemius muscle and liver of mice have been measured at three frequencies (4.5, 9.21 and 15.35 MHz) over the temperature range of +37 to −20°C. Calculations on T1 for both proton and deuteron have been made and compared with the experimental data. It is suggested that the reduction of the T1 values compared to pure water and the frequency dependence of T1 are due to water molecules in the hydration layer of the macromolecules, and that the bulk of water molecules in the biological tissues and egg white undergoes relaxation like ordinary liquid water. 相似文献
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