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1.
Crystals of cholesteryl octanoate (C35H60O2) are monoclinic, space group P21, with a = 12.80(3), b = 9.20(2), c = 14.12(3)A?, β = 93.81(3)° and 2 molecules per unit cell. The structure has been determined by Patterson rotation and translation methods from the X-ray intensities (Mo-Kα radiation) of 1320 reflections (sinθ/λ < 0.59 A??1) measured with a diffractometer. Refinement by block diagonal least squares and Fourier methods gave R = 0.096. The molecules are arranged in monolayers with their long axes antiparallel and severely tilted (28°). There is a close packing of cholesteryls within the monolayers, but the octanoate chains which form the layer interface regions are conformationally and thermally disordered. The crystal structure is quite different from that of cholesteryl nonanoate, as expected from the discontinuity in thermodynamic properties and phase behaviour which occurs at this point in the homologous series.  相似文献   

2.
Theory is presented relating to the reversible interaction of an f-valent acceptor, A, with a bivalent ligand, B, which leads to the formation of a series of complexes comprising networks of alternating A and B molecules. An explicit expression is derived for the overall extent of reaction in terms of the total molar concentrations of reactants (mA and mB), the valency of the acceptor and the site-binding constant, k, governing the equilibria. It is shown by differentiation of this expression holding mA (or mB) fixed that relations are available for the independent evaluation of f and k from a combination of precipitin and radioimmunoassay experiments. Moreover, it is established that dilution with solvent (mA/mB fixed) cannot lead to the appearance of a precipitate with this type of crosslinking system. The latter observation forms the background for the development of theory pertaining to the joint operation of ligand dimerization, 2B?B2, and crosslinking of the multivalent acceptor with bivalent B2. The theoretical examination of this system is developed in terms of site-probability functions and involves the delineation of unique solutions for the extent of crosslinking reaction aided by the definition of the extent of binding in defined limits. It is shown with the use of numerical examples that the system involving self-associating ligand may result in the appearance of a precipitate on dilution with solvent and the conditions for the operation of this phenomenon are elucidated. It is noted that other types of ligand self-interaction may lead to similar effects in crosslinking systems, and the general principles emerging from this study are discussed in terms of systems in which antibody ligands are known to be involved in association reactions or are suspected to be so involved on the basis of precipitation effects observed on dilution with solvent.  相似文献   

3.
Consideration is given to the reversible interaction of a bivalent ligand, B, with a multivalent acceptor, A (possessing f reactive sites) which leads to the formation of a series of complexes, AiBj, comprising networks of alternating acceptor and ligand molecules. A binding equation is derived on the basis of a site association constant, k, defined in terms of reacted site probability functions. This equation, which relates the binding function, r (the moles of ligand bound per mole of acceptor) to the concentration of unbound ligand, mb, is used to show that plots of r vs. 2kmB constructed with fixed but different values of kmA intersect at the point (mB = 12k, r = f2) where the extent of reaction and the concentrations of those complexes for which ji = f2 attain maximal values. Corresponding Scatchard plots are shown by numerical example to be non-linear, their second derivative being positive for all r. It follows that such deviations from linearity cannot be taken alone as evidence for site heterogeneity in cross-linking systems. The binding equation obtained directly is shown to be identical with that obtained with f = 2 by summation procedures involving the general expression for concentrations of complexes, mAiBj, formulated in terms of appropriate statistical factors. In this way, previous findings on precipitation and gel formation in cross-linking systems are correlated with the present development of binding theory.  相似文献   

4.
tRNACUGLeu from E. Coli was purified by column chromatography on benzoylated DEAE-cellulose, followed by hydroxyapatite prepared by an improved method. Crystals obtained by vapour diffusion gave X-ray diffraction out to 7 Å in the hk0 projection and 10 Å in h0?. The space group was P42212 with a = b = 133 A?, c = 66 A? and 8 molecules in the unit cell. Birefringence showed preferred orientation of RNA helical regions in the ab plane.  相似文献   

5.
Interactions between polynucleotides and platinum (II) complexes   总被引:1,自引:0,他引:1  
Reaction of either cis or trans Pt(NH3)2Cl2 with poly(A) in dilute aqueous solution leads to quantitative precipitation of the polymer at Pt/nucleotide ratios above 0.5. It is proposed that at ratios less than this, intramolecular binding of one Pt to two bases is favored; at higher ratios, intermolecular cross-linking becomes important and precipitation results. The absence of isomer selectivity in precipitation implies that the biological specificity of the cis form results from a process other than cross-linking of polynucleotide strands. Other observations suggest that the coordinated ammonia of nucleotide-platinum(II) ammine complexes may be unusually labile.  相似文献   

6.
Crystals of cholesteryl oleate (C45H78O2) are monoclinic, space group P21, with a = 12.65(3), b = 9.13(3), c = 18.79(5)A?, β = 93.3(3)° and have 2 molecules in the unit cell. The crystal structure has been determined by Patterson and Fourier methods at a resolution dmin = 1.1 A?, using 799 X-ray intensities (CuKα) measured by a diffractometer. Structure refinement by block-diagonal least squares gave R = 0.12. The oleate chains are almost straight except for a kink at the cis-double bond. The chains pack side by side but without a regular sub-cell structure, in a manner which might be similar to the arrangement within biological membranes. As in cholesteryl octanoate, the cholesteryl ring systems pack together with extensive overlap of anti-parallel nearest neighbours. Projecting methyl groups interlock.  相似文献   

7.
The two aspartate aminotransferase isoenzymes, AAT-P1 and AAT-P2, found in the plant cytosol fraction of lupine nodules have been separated and partially purified. Both isoenzymes showed a broad pH optimum between 7.0 and 9.5 and demonstrated high substrate specificity. Molecular weights determined by gel filtration chromatography were 105,000 and 96,000 for AAT-P1 and AAT-P2, respectively. AAT-P1 demonstrated a subunit molecular weight of 47,000 and AAT-P2, 45,000. Both isoenzymes showed oxaloacetate substrate inhibition and gave similar Km values for aspartate (2.2 and 2.6 mm) and 2-oxoglutarate (0.26 and 0.2 mm) for AAT-P1 and AAT-P2, respectively. However, AAT-P2 showed a fivefold lower Km for oxaloacetate (0.02 mm) and a 1.8-fold lower Km for glutamate (12 mm) than did AAT-P1 for these substrates. The parallel nature of the 1Vvs1[S] plots together with the product inhibition kinetics were consistent with a ping-pong-bi-bi mechanism of action. The results are discussed in terms of the possible physiological significance of these plant aspartate aminotransferases in ammonia assimilation in lupine nodules.  相似文献   

8.
The micellar properties of gangliosides in water solutions were investigated by quasielastic light scattering measurements. GM1 and GD1a gangliosides were isolated from calf brain, purified to more than 99% and dissolved in 0.025 M Tris—HCI buffer (pH 6.8) at 37°C. The average intensity of scattered light and the intensity correlation function were measured by an apparatus including a 5145 Å argon laser and a real-time digital correlator. The scattered intensity data allowed the derivation of an upper limit to the critical micelle concentration (c0) and the evaluation of the molecular weight (M) of the micelle. The intensity correlation function gave the diffusion coefficient D, and hence the hydrodynamic radius RH, and also contained information on the polydispersity of the sample. We find co < 1 × 10?6 M for both GM1 and GD1a, M = 532 000 ± 50 000 and RH = 63.9 ± 2 A? for GM1, and M = 417 000 ± 40 000 and RH = 59.5 ± 2 A? for GD1a. The mixture 3:1 of the two gangliosides gave intermediate values for all examined parameters. The presence of cations, within the physiological concentration range. and, in particular of Ca2+, did not influence significantly the values of co and the main features of the micelle.  相似文献   

9.
The effects of different neutral salts on the maximal velocity (V) and activation volume (ΔV3) of the M4-lactate dehydrogenase reaction were studied to determine the mechanistic basis of the inhibitory effects of these salts. For salting-in salts (which increase protein group solubility), increasing salt concentrations led to reductions in V and increases in ΔV3, with the order of salt effectiveness following the Hofmeister (lyotropic) series: KSCN > KI > KBr. A 50% reduction in V was associated with an approximately 17 cm3 mol?1 increase in ΔV3 for different concentrations of the same salt and for equal concentrations of different salting-in salts. Salting-out salts were also inhibitory, but no uniform correlation between changes in V and ΔV3 was observed. The strongly salting-out salt KF decreased ΔV3 at all concentrations. The weaker salting-out salt K2SO4 increased ΔV3 at concentrations below 0.1 m and decreased ΔV3 at higher concentrations. KCl increased ΔV3 as the salt concentration was raised to approximately 0.2 m; further increases in KCl concentration were without effect on ΔV3. The rate and volume effects of these neutral salts, especially the highly regular covariation in V and ΔV3 found for salting-in salts, seem difficult to explain in terms of salt-induced changes in the geometry of the active site. We propose instead that these salt effects can all be explained in terms of the energy and volume changes which accompany transfers of protein groups (amino acid side chains and peptide backbone linkages) between the hydrophobic interior of the enzyme and the enzyme-water interface during catalytic conformational changes.  相似文献   

10.
The second of the two reaction steps involved in the metabolic transformation of (?)-nicotine to (?)-cotinine (3) (i.e., the oxidation of the intermediate 2) is mediated mainly, if not solely, by the enzyme aldehyde oxidase (EC 1.2.3.1). Of the molecular species that constitute 2, nicotine Δ1′(5′) iminium ion (2a) appears to serve as the substrate. The enzyme has a strong affinity for 2a, as shown in a study on the inhibition of the oxidation of 3-(aminocarbonyl)-1-methylpyridinium chloride. This study gave a value of Ki = 6 μM; Km = 2 μM (pH 7.4). Mainly in view of this finding, “iminium oxidase” seems to be a more adequate name than “aldehyde oxidase” for this enzyme.  相似文献   

11.
Complexes of the formula cis-[Pt(HN+N)(L)Cl2], where (HN+N) are the protonated diamines including 3-aminoquinuclidine, N-aminopiperidine, piperazine, N-methylpiperazine, 1,1,4-trimethylpiperazine, and N-methyl-1,4-diazabicyclo [2,2,2] octane (N-methyl-dabco) and L = SCN?, NO2?, Br?, and F?, were synthesized from the protonated diamine complexes, [Pt(HN+N)Cl3]. The antitumor activities of the complexes were evaluated in vitro against L1210 murine leukemia cells, and ID50 values for the L-substituted complexes were compared to values of the parent complexes. In each case it was found that replacement of a chloride ion by SCN?, NO2?, Br?, or F?, either reduced or completely eliminated antitumor activity. This effect is explained in terms of the trans-directing ability of the ligand, L, compared to chloride. The NO2-substituted complex of 3- aminoquinuclidine was tested in vivo and found to exhibit little or no antitumor activity.  相似文献   

12.
The turnover rates of prolyl hydroxylase and immunologically related (cross reacting) protein were examined using labeled leucine as precursor or by measuring the decay of elevated prolyl hydroxylase and immunologically cross-reacting protein back to basal levels. Prolyl hydroxylase and immunologically cross-reacting protein were purified from neonatal rabbit skin at various times following the administration of [3H]leucine. Prolyl hydroxylase was purified by affinity chromatography. Immunologically cross-reacting protein was purified by antibody precipitation from the dialyzed 70% (NH4)SO4 supernatants and subsequent electrophoresis on 10% sodium dodecyl sulfate-polyacrylamide slab gels. The radioactivity of the species isolated, which corresponded to the two major subunits of prolyl hydroxylase, was used in the turnover studies of immunologically cross-reacting protein. The peak incorporation of label into prolyl hydroxylase was found to be 12 h while for immunologically cross-reacting protein this occured within 2 h. The loss of radioactivity from these protein pools denotes an apparent t12 for prolyl hydroxylase of 73 h and a 12 for immunologically cross-reacting protein of 53 h. From the specific activity of free skin leucine pools, the effect of reutilization could be corrected and a true t12 for prolyl hydroxylase of 45 h was determined. The t12 values of these proteins were determined by a second method in which prolyl hydroxylase and immunologically cross-reacting protein in the aorta and liver of adult male rabbits were elevated by daily epinephrine-thyroxine treatment for 12 days. The decline of prolyl hydroxylase and immunologically cross-reacting protein with termination of treatment in the aorta denotes values of 42 h for enzyme and 53 h for immunologically cross-reacting protein. Calculated enzyme κd values, by both methods, indicate that breakdown of enzyme does not account for tissue immunologically cross-reacting protein.  相似文献   

13.
Single crystals of Pseudomonas isoamylase (Mr 95,000) belong to space group P212121 with unit cell dimensions of a = 137.9 A?, b = 52.9 A?, c = 151.2 A?. A Guinier plot of the X-ray small-angle scattering of the protein solution gave the radius of gyration of the molecule as 27.5 Å.  相似文献   

14.
Levansucrase, an exocellular enzyme, has been isolated from a high producer mutant, the BS5C4 constitutive strain, of Bacillus subtilis. Three crystalline forms have been obtained, all three belonging to the orthorhombic space group P212121. The most suitable form for a three-dimensional structure investigation has cell dimensions, a = 68 A?, b = 125 A?, c = 54 A?. There is one molecule in the asymmetric unit.  相似文献   

15.
A wide range of concentrated random coil polysaccharide solutions have been assessed for textural attributes by a trained sensory panel. The only textural terms invoked to describe these model systems were ‘thickness’ and ‘stickiness’, which were shown to be highly correlated, and essentially identical numerically, using a ratio scaling technique. Viscosity (η) measurements over a wide range of shear rates (γ) for all these samples gave flow curves (log η versus log γ) of the same form. Differences in flow behaviour between samples could then be characterised completely by two parameters, the maximum viscosity at low shear rates (η0), and the shear rate (γ?0·1) at which η = solη010. A simple linear relationship was demonstrated between these two parameters and perceived thickness (T) or stickiness (S), irrespective of polysaccharide type. For Newtonian liquids, log T (or log S) varied linearly with log η. Hence the effective ‘in-mouth’ thickness of random coil polysaccharide solutions, in normal viscosity units, may be predicted directly from η0 and γ?0·1 by the simple relationship: log ηN = 1·13 log η0 + 0·45 logγ?0·1 ? 1·72 where ηN is the viscosity of a Newtonian solution which would be perceived as identical in thickness (and stickiness) to the polysaccharide solution.  相似文献   

16.
The factors that affect reliable estimations of mutation rates (μ) in cultured mammalian somatic cell populations by fluctuation analysis are studied experimentally and statistically. We analyze the differential effect of the final cell population size in each culture (Nt) and the number of parallel cultures (C) on the variation in the rate estimates (μ) inferred from the P0 method. The analysis can be made after the derivation of the variance of μ, which is a measure of variation of μ for a given combination of Nt and C in a number of repeat experiments. The variance of μ is inversely proportional to C and to the square of Nt. Nt determines the probability of occurrence of mutation in a cell culture. By influencing the size of P0, Nt also determines whether a rate estimate is obtainable from the experiment. Since Po is estimated from the fraction of cultures containing no mutation in a set of C cultures, C becomes a determining factor for the accuracy of μ. The rate estimated from P?0 is biased, but the bias is in general 2 orders of magnitude smaller than μ. By the selection of an appropriate combination of Nt and C for the experiment, this bias can be reduced even further.Based on the notion of comparing two proportions, we propose a test statistic and have applied it to experimental results for a test of equality of mutation rates in different cell lines. This development places the comparison of mutation rates on a statistical basis.  相似文献   

17.
A mathematical model of the 51Cr-release microcytotoxicity assay is utilized to find conditions under which the kinetics of this assay resemble the kinetics of a classical enzyme-substrate reaction. Assuming a steady-state approximation, that “bystander” effector cells do not bind markedly better than the cytotoxic effector cells, and that the programming of the target cells for lysis is irreversible, it is shown that the velocity of label release is v = vmaxTT/(K12+TT), where both Vmax and K12 are linear functions of the effector-cell population and TT is the initial target-cell population. Moreover, the expressions for K12 and Vmax are expressed in terms of natural kinetic parameters of the process and attributes of the noncytotoxic bystanders.  相似文献   

18.
Crystals of lysozyme from Streptomyces erythraeus are orthorhombic, space group P212121 with unit cell dimensions: a = 50.01 A?, b = 61.98 A? and c = 67.85 A?; and one molecule, of molecular weight about 18,500, per asymmetric unit.  相似文献   

19.
The crystal structure of CsReF6, together with a reinvestigation of that of BaSiF6, is reported. Both have been determined from single crystal three-dimensional X-ray diffraction data. The structure of BaSiF6 has been found to conform to the initially assigned space group R3m, contrary to the suggestions of other workers. The unit cell of BaSiF6 has the dimensiona ahex 7.189(1), chex 7.015(1) Å; Z = 3. Refinement by a least squares method gave R 0.0079 and Rw 0.0077. Crystals of CsReF6 belong to the lower symmetry rhombohedral space group R3. The unit cell has the dimensions ahex 7.853(1), chex 8.140(1) Å; Z = 3. Refinement gave R 0.031 and Rw 0.030. The lowering of symmetry is caused by rotation of the ReF6? octahedra about the 3-fold axis through each Re atom, causing CsReF6 to have the KOsF6 structure.  相似文献   

20.
The lectin from the seeds of Abrus precatorius has been crystallized and the crystals subjected to study by X-ray diffraction and electron microscopy. Three closely related crystal forms were obtained, of orthorhombic space group P212121 with a = 138 A?, b = 142 A?, and c = 173 A?, of tetragonal space group P41212 with a = b = 136 A?, c = 176 A?, and a twinned intermediate of the first two. From electron microscopy and two-dimensional spatial filtering of electron micrographs of the crystals, the molecule appears to consist of four similar domains grouped in a roughly planar diamond-shaped arrangement having a local intramolecular dyad axis. The average diameter of the Abrus lectin molecule is 50 to 60 Å and the individual domains appear to have a diameter of about 25 Å.  相似文献   

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