首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 46 毫秒
1.
The longitudinal proton magnetic relaxation times T1 were measured for ferri (met)-and carbonmonoxy-bovine haemoglobin and equine myoglobin in 0.1 M KH2PO4 aqueous solutions near pH 6 at 5°C and 35°C from 1.5- to 60-MHz Larmor frequencies. It is concluded that the correlation time τC for the dipole–dipole interaction of electron and nuclear spins is in fact the electron (ferric) spin relaxation time τS being close to 1.5 × 10?10 sec for both metHb and metMb at 5°C. At 35°C the paramagnetic relaxation rates are not determined solely by the relaxation of protons exchanging from the haem pocket with bulk solvent. Hence, τC at 35°C cannot be calculated from the dispersion data obtained at this temperature. The relevance of this for the determination of interspin distances r is discussed.  相似文献   

2.
The complex permittivity of sonicated aqueous solutions of purified dimyristoylphosphatidylcholine has been measured as a function of frequency between 3 kHz and 40 GHz. The dielectric spectrum of the samples shows two dispersion/absorption regions, one centered at about 80 MHz the other at about 20.GHz (30°C). Otherwise than in previous studies no additional dispersion/absorption process has been found at frequencies below 10 MHz.The complex dielectric spectrum of the samples is discussed with respect to the dynamical state of solvent water in solutions of single-bilayer vesicles. The main relaxation time of the solvent water, τ1 ((2πτ1)?1 ≈ 20 GHz), is smaller than that of pure water, τW, at the same temperature. This effect results from the action of internal depolarizing fields which obviously overcompensate and enhancement of τ1 due to specific solute/solvent interactions (hydration) as had been previously found with micellar solutions of lysolecithins.It cannot be excluded, that some solvent water shows unusual dynamical behaviour. If there exists a substantial amount of such motionally perturbed water, however, it must be characterized by a relaxation time close to that of the phosphorylcholine zwitterions, τ2 ((2πτ2)?1 ≈ 80 MHz).  相似文献   

3.
We have developed a new model describing the relationship between plasma and red cell tracers flowing through the lung. The model is the result of an analysis of the transport of radiolabeled plasma albumin between two flowing phases and shows that differences between red cell and plasma tracer curves are related to microvascular hematocrit. The model was tested in an isolated, blood-perfused dog lung preparation in which we injected51Cr-labeled red cells and125I-labeled plasma albumin into the pulmonary artery. From the tracer concentration-time curves at the venous outflow, we calculatedh r, the ratio of microvascular hematocrit to large-vessel hematocrit. In 18 baseline experiments,h r=0.92±0.01 (mn±sem) at a blood flow rate of 10.7±0.3 ml s−1. We determined the effects of (a) glass bead embolization, (b) alloxan, and (c) lobe ligation onh r. Embolization attenuated the separation between plasma and red cells (increasedh r), probably as a consequence of passive vasodilation. Alloxan enhanced separation of plasma and red cells (decreasedh r), possibly as a result of arteriolar vasoconstriction. Ligation of a fraction of the perfused tissue at constant flow did not cause significant change inh r in the remaining perfused tissue. The model assumes that large-vessel transit times are uniform and that all dispersion occurs in the microvasculature. A theoretical analysis apportioning dispersion between large and small vessels disclosed that the error associated with these assumptions is likely to be less than 15% of the measuredh r. We conclude from this study that the microvascular hematocrit model describes experimental plasma and red cell curves. The results imply thath r can be readily deduced from tagged red cells and plasma and can be accounted for in calculating permeability-surface area in diffusing tracer experiments.  相似文献   

4.
Dielectric relaxation of DNA in aqueous solutions.   总被引:1,自引:0,他引:1  
Using a four-electrode cell and a new electronic system for direct detection of the frequency differences specturm of solution impedance, the complex dielectric constant of calf thymus DNA (Mr = 4 × 106) in aqueous NaCl at 10°C is measured at frequencies ranging from 0.2 Hz to 30 kHz. The DNA concentrations are Cp = 0.01% and 0.05%, and the NaCl concentrations are varied from Cs = 10?4 M to 10?3 M. A single relaxation regions is found in this frequency range, the relaxation frequency being 10 Hz at Cp = 0.01% and Cs = 10?3 M. At Cp = 0.05% it is evidenced that the DNA chains have appreciable intermolecular interactions. The dielectric relaxaton time τd at Cp = 0.01% agrees well with the rotational relaxation time estimated from the reduced visocisty on the assumption that the DNA is not representable as a rigid rod but a coiled chain. It is concluded that the dielectric relaxiatioinis ascribed to the rotation of the molecule. Observed values of dielectric increment and other experimental findings are reasonably explained by assuming that the dipole moment of DNA results from the slow counterion fluctuation which has a longer relaxation time than τd.  相似文献   

5.
The complex dielectric constant of collagen in aqueous solutions (polymer concentration, Cp = 0.02–0.2%) was measured at 10°C in the frequency range from 3 Hz to 30 kHz. The loss peak for Cp = 0.02% is located at 90 Hz and the dielectric relaxation time τD is estimated to be 1.8 ± 0.3 msec. The τD agrees well with the rotational relaxation time estimated from the reduced viscosity, and the relaxation is ascribed to the end-over-end rotation of the molecule. The Cp dependence of τD and the dielectric increment Δε are interpreted in terms of the aggregation of molecules. The dipole moment of a molecule, obtained from Δε at Cp = 0.02% and pH 6.5, is (5.2 ± 0.2) × 104D, which is explained by the asymmetrical distribution of the ionized side chains of the molecule.  相似文献   

6.
Real and imaganiry parts of complex dielectric constant of dilute solutions of DNA in 10?3M NaCl with molecular weight ranging from 0.4 × 106 to 4 × 106 were measured at frequencies from 0.2 Hz to 30 kHz. Dielectric increments Δε were obtained from Cole-Cole plots and relaxation times τD from the loss maximum frequency. The τD of all samples agrees well with twice of the maximum viscoelastic relexation time in the Zimm theory, indicating that the low-frequency dielectric relaxiation should be ascribed to be the rotation of DNA. The rms dipole moment, which was obtained from Δε, agree well with that calculated from the counterion fluctuation theory. The dielectric increment was found to be greatly depressed in MgCl2, which is resonably interpreted in terms of a strong binding of Mg++ ions with DNA.  相似文献   

7.
Polyadenylic acid forms a 2:1 complex with the C-nucleoside formyein B at both pH 7.0, 0.15 m-Na+ and pH 6.0, 0.15 M-Na+. The formation of this complex has been followed by equilibrium dialysis, and by optical rotatory dispersion measurements in the range 333 to 450 nm. At pH 7, melting curves for thermal dissociation of the complex (followed by the optical rotation at 345 nm) show a strongly co-operative helix-coil transition. From the variation of Tm with the free formyein B concentration at this temperature, the partial molar enthalpy of formation of the complex, at the mid-point of the transition, has been calculated as -12.8 kcal./mol of formyein B. Viscometry and optical rotatory dispersion measurements indicate that the structure of the complex at pH 6 is the same as at pH 7, and that it may be formed in preference to the double-helical acid form of poly (A). The structure and properties of the complex are discussed.  相似文献   

8.
G Schwarz  J Seelig 《Biopolymers》1968,6(9):1263-1277
Dielectric relaxation of poly(γ-benzyl L -glutamate) in solution has been studied in the 5 kcps-10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*max = 5 × 10?7 sec. The elementary process of helical growth turns out to be practically diffusion-controlled (with a rate constant of hydrogen bond formation of 1.3 × 1010 sec?1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action of an electric field.  相似文献   

9.
The single-strand helix-coil transition in various oligo- and polyadenylates is characterized by means of an improved cable temperature-jump technique. In all the polymers studied {poly(rA), poly(dA), poly[A(m2′)] and poly[A(e2′)]} helix-coil relaxation is observed in the time range from 30 to 1000 nsec. Relaxation-time constants observed at wavelengths λ<280 nm (τα) are different from those found at λ >280 nm (τβ), indicating the presence of more than two conformational states. The time constants τα increase in the series poly(dA), poly[A(m2′)], constants τβα is approximately 2.5, except in poly(dA) where τβα ≈ 9. Relaxation measurements with r(A)n- oligomers show a decrease in conformational mobility with increasing chain length. The relaxation curves also demonstrate that “internal” residues have lower reaction rates than residues at the ends of the oligomer chain. Measurement in D2O reveal a solvent isotope effect for τα of +87% for poly(rA), and of +53% for poly(dA), whereas no isotope effect is found in τβ. The absence of “slow” relaxation processes in the model compound 9,9′ -trimethylenebisadenine shows that the relatively low rate of the single-strand helix-coil transitions is due to the coupling of base stacking with the folding of the sugar–phosphate chain. The absence of a seprate relaxation process (corresponding to τβ) in 9,9′-trimethylenebisadenine, as well as in the dinucleotides ApC and CpA, suggests that this relaxation process is dependent upon the presence of both the sugar–phosphate chain and of adjacent adenine bases. The experimental data provide evidence that there is more than one ordered conformation in various single-stranded oligo- and polyadenylates and that the transition between these conformations is influenced by the sugar conformation.  相似文献   

10.
The real and imaginary parts of complex viscosity, η′ and η″, are measured for dilute solutions of poly(γ-benzyl-L -glutamate) in m-cresol, a helicogenic solvent. The frequency range is 2.2–525 kHz; the concentration range 0.2–5 g/dl; the temperature 30°C, and the molecular weights Mr are 6.4 × 104–17 × 104. The dispersion curve of extrapolated intrinsic dynamic viscosity [η′] of samples with Mr > 105 is interpreted in terms of three mechanisms appearing from low to high frequencies: end-over-end rotation, flexural deformation, and side-chain motion. For a sample with Mr < 105, the flexural relaxation disappears and a plateau of [η′] is distinctly observed between rotational and side-chain relaxations. Rotational relaxation times of all the samples obey the Kirkwood–Auer theory. The strong concentration dependence of rotational relaxation time is explained by collisions of molecules rather than association. Flexural relaxation times calculated from a theory by assuming the persistence length as 1200 Å are consistent with observed dispersion curves of [η′].  相似文献   

11.
Structure–dynamics interrelationships are important in understanding protein function. We have explored the empirical relationship between rotational correlation times (τc and the solvent accessible surface areas (SASA) of 75 proteins with known structures. The theoretical correlation between SASA and τc through the equation SASA = Krτc (2/3) is also considered. SASA was determined from the structure, τc calc was determined from diffusion tensor calculations, and τc expt was determined from NMR backbone13 C or 15N relaxation rate measurements. The theoretical and experimental values of τc correlate with SASA with regression analyses values of Kr as 1696 and 1896 m2s-(2/3), respectively, and with corresponding correlation coefficients of 0.92 and 0.70.  相似文献   

12.
Genetic parameters for meat quality traits and their relationships with body weight and breast development were estimated for a total of 420 male turkeys using REML. The birds were slaughtered in a commercial plant and the traits measured included pH at 20 min (pH20) and 24 h post-mortem (pHu) and colour of the breast and thigh meat. The heritabilities of the rate and the extent of the pH fall in the breast muscle were estimated at h2 = 0.21 ± 0.04 and h2 = 0.16 ± 0.04, respectively. Heritabilities ranging from 0.10 to 0.32 were obtained for the colour indicators in the breast muscle. A marked negative genetic correlation (rg = -0.80 ± 0.10) was found between pH20 and lightness (L*) of breast meat, both traits corresponding to PSE indicators. The pH20 in the thigh muscle had a moderate heritability (h2 = 0.20 ± 0.07) and was partially genetically related to pH20 in the breast muscle (rg = 0.45 ± 0.17). Body weight and breast yield were positively correlated with both initial and ultimate pH and negatively with the lightness of breast meat.  相似文献   

13.
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   

14.
Axial dispersion of the liquid phase was investigated in a concentric-tube airlift bioreactor (RIMP: V L=0.70?m3) as a whole and in the separate zones (riser, downcomer, gas-separator) using the axial dispersion model. The axial dispersion number Bo and the axial dispersion coefficient, D ax were determined from the output curves to an initial Dirac pulse, using the tracer response technique. They were analyzed in relation to process and geometrical parameters, such as: gas superficial velocity, νSGR; top clearance, h S; bottom clearance, h B, and resistances at downcomer entrance expressed as A d/A R ratio. Correlations between Bodenstein numbers in the overall bioreactor and riser and downcomer sections (BoT,BoR,BoD) and the geometrical and process parameters were developed, which can allow to assess the complex influence of these parameters on liquid axial dispersion.  相似文献   

15.
T.J. Lea  C.C. Ashley 《BBA》1982,681(1):130-137
CO2-induced acidosis in barnacle muscle fibres prolongs the relaxation phase of the electrically stimulated contraction (Ashley, C.C., Franciolini, F., Lea, T.J. and Lignon, J. (1979) J. Physiol. 296, 71P). In order to test if this effect is due to a direct action of H+ on the relaxation kinetics of the myofilaments, isolated myofibrillar bundles were contracted and relaxed in Ca2+ buffer solutions at pH 6.0 and 7.1, in the presence of 20 mM caffeine to inactivate the sarcoplasmic reticulum. At pH 7.1, the relaxation half-time was reduced from 1.5 to 0.3 s as the EGTA concentration in the relaxing solution was progressively increased from 0.3 to 50 mM. The resulting curve was shifted in the direction of increasing EGTA concentration by lowering the pH to 6.0. This effect could be explained by the reduction in affinity of Ca2+ for EGTA at pH 6.0, since relaxation half-times for a given relaxing pCa (calculated from the contaminating Ca2+ concentrations in the relaxing solutions) were shorter (by about 40%) at pH 6.0 compared with 7.1. However, similar experiments using the new Ca2+-chelating agent 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), which is much less pH sensitive than EGTA, indicated that there was no significant difference between relaxation half-times at pH 6.0 and 7.1 for a given relaxing pCa. It is concluded that because no prolongation of relaxation of the myofibrils was observed on lowering the pH from 7.1 to 6.0, the effect of CO2 on the relaxation of intact muscle fibres is probably due to a modification of sarcoplasmic reticulum activity.  相似文献   

16.
15N NMR relaxation measurements have been used to study the dynamic behaviour of the main-chain of hen lysozyme in a partially folded state, formed in a 70% (v/v) trifluoroethanol (TFE)/30% water mixture at 37°C and pH 2. This state is characterised by helical secondary structure in the absence of extensive tertiary interactions. The NMR relaxation data were interpreted by mapping of spectral density functions and by derivation of segmental as well as global order parameters. The results imply that the dynamics of lysozyme in TFE can, at least for the great majority of residues, be adequately described by internal motions which are superimposed on an overall isotropic tumbling of the molecule. Although the dynamic behaviour shows substantial variations along the polypeptide chain, it correlates well with the conformational preferences identified in the TFE state by other NMR parameters. Segments of the polypeptide chain which are part of persistent helical structures are highly restricted in their motion (S2> 0.8, with effective internal correlation times τe< 200 ps) but are also found to experience conformational exchange on a millisecond timescale. Regions which are stabilised in less persistent helical structure possess greater flexibility (0.6 <S2< 0.8, 200 ps < τe< 1 ns) and those which lack defined conformational preferences are highly flexible (S2< 0.6, τe∼1 ns). The dynamic behaviour of the main-chain was found to be correlated with other local features of the polypeptide chain, including hydrophobicity and the position of the disulphide bridges. Despite the absence of extensive tertiary interactions, preferential stabilisation of native-like secondary structure by TFE results in a pattern of main-chain dynamics which is similar to that of the native state.  相似文献   

17.
The effects of salts (NaCl, LiCl, Me4NCl, AgNO3, MgCl2, CuCl2 and MnCl2) and dyes (acridine orange and methylene blue) on the low-frequency dielectric relaxation (0.1 Hz–30 kHz) of dilute aqueous solutions of DNA were investigated with varying salt or dye concentrations. Both the dielectric relaxation time τD and the rotational relaxation time τ estimated from the reduced viscosity decrease in quite parallel ways with increasing M/P (M/P being the normality ratio of cation to phosphate residue), reflecting the contraction of DNA molecule due to electrostatic shielding and cation binding. The agreement between τD and τ through the whole range of M/P supports our previous conclusion that the low-frequency relaxation of DNA arises from rotation of the molecule. The dielectric increment Δε also decreases with increasing M/P on account of both the contraction of DNA and the decrease in effective degree of dissociation of DNA. Δε as a function of M/P is interpreted in terms of a quasi-permanent dipole due to counterion fluctuation. These effects of cations are the strongest for divalent cations and rather weak for Na+, Li+, and Me4N+. Effects of dye on τD and Δε are also well explained by the rotation of DNA molecule with a quasi-permanent dipole due to counterion fluctuation on the basis of intercalation of dye at D/P < 0.2 (D/P being the molarity ratio of dye to phosphate residue) and external binding at 0.2 < D/P < 1.0.  相似文献   

18.
The death of the hybridoma VO 208 in a continuous culture at pH 7 and 6.8 was investigated by measuring both the appearance of visible dead cells which do not exclude the trypan blue dye and the release of lactate dehydrogenase (LDH) in the culture medium. The intracellular LDH was found to be completely released either when live cells lysed or when they were transformed into visible dead cells. No significant lysis of blue dead cells could be observed at the two different pH. Using a LDH balance over the culture system, cell lysis was found negligible at pH 7, but accounted for 20% of the total cell death at pH 6.8. A methodology is proposed to evaluate the rate constants of hybridoma lysis and total death. For the investigated cell line in continuous culture, the calculated total cell death rate constant was found to increase from 0.002 h–1 to 0.01 h–1 when decreasing the pH from 7 to 6.8.Abbreviations D dilution rate (h–1) - kb specific trypan-blue dead cells appearance rate (h–1) - kL specific lysis rate of viable cells (h–1) - kd specific death rate (h-1) - LDH0 lactate dehydrogenase activity in the feed culture medium (IU.l–1) - LDH lactate dehydrogenase activity in the outlet culture medium (IU.l–1) - LDHi intracellular lactate dehydrogenase activity of viable cells (IU.10–9 cells) - rLDH total rate of LDH release (IU.h–1.L–1) - rb transformation rate of viable cells into blue dead cells (109 cells.h–1.L–1) - xv viable cell concentration (109 cells.l–1) - xb trypan-blue dead cell concentration (109 cells.l–1)  相似文献   

19.
M Sakamoto  R Hayakawa  Y Wada 《Biopolymers》1979,18(11):2769-2782
As a continuation of previous papers [Biopolymers (1976) 15 , 879; (1978) 17 , 1508], the low-frequency dielectric relaxation of DNA solutions was studied with a four-electrode cell and the simultaneous two-frequency measurement. Below a critical concentration, the dielectric relaxation time agrees with the rotational relaxation time estimated from the reduced viscosity and is almost independent of DNA concentration Cp, and the dielectric increment is proportional to Cp. The critical concentration is approximately 0.02% of DNA for molecular weight Mr 2 × 106 and 0.2% for Mr 4.5 × 105 in 1 mM NaCl. Dielectric relaxations are compared for samples before and after deproteinization, and the protein contamination is found to have a minor effect on the dipole moment of DNA. The effect of a mixed solvent of water and ethanol on the dielectric relaxation of DNA is well interpreted in terms of changes in viscosity and the dielectric constant of the solvent, assuming that the relaxation arises from rotation of the molecule with a quasi-permanent dipole due to counterion fluctuation.  相似文献   

20.
Dielectric constant and dielectric loss of heavy meromyosin (HMM) were measured with varying pH. HMM showed a broader dispersion pattern than that with a single relaxation time especially on the high-frequencey side. The dielectric increment increased sharply with pH, above pH 6, whereas the mean relaxation time and whole dispersion pattern were unchanged in the same region. The values of the increment and the mean relaxation time were much larger than those of usual globular proteins. The dispersion profile, pH dependence, and values of the increment are well explained by Oosawa's counterion fluctuation theory. Other mechanisms are more or less inadequate to our results. In the low pH region below the isoelectric precipitation region, both the increment and the mean relaxation time decreased; this is probably due to partial denaturation and suppression of the dissociation of carboxyl groups. An experiment on a urea-denatured sample supports this assumption. The biological significance of the pH dependence is discussed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号