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1.
Glenn L. Butterfoss Eugene F. DeRose Scott A. Gabel Lalith Perera Joseph M. Krahn Geoffrey A. Mueller Xunhai Zheng Robert E. London 《Journal of biomolecular NMR》2010,48(1):31-47
Methionine residues fulfill a broad range of roles in protein function related to conformational plasticity, ligand binding,
and sensing/mediating the effects of oxidative stress. A high degree of internal mobility, intrinsic detection sensitivity
of the methyl group, and low copy number have made methionine labeling a popular approach for NMR investigation of selectively
labeled protein macromolecules. However, selective labeling approaches are subject to more limited information content. In
order to optimize the information available from such studies, we have performed DFT calculations on model systems to evaluate
the conformational dependence of 3
J
CSCC, 3
J
CSCH, and the isotropic shielding, σiso. Results have been compared with experimental data reported in the literature, as well as data obtained on [methyl-13C]methionine and on model compounds. These studies indicate that relative to oxygen, the presence of the sulfur atom in the
coupling pathway results in a significantly smaller coupling constant, 3
J
CSCC/3
J
COCC ~ 0.7. It is further demonstrated that the 3
J
CSCH coupling constant depends primarily on the subtended CSCH dihedral angle, and secondarily on the CSCC dihedral angle. Comparison
of theoretical shielding calculations with the experimental shift range of the methyl group for methionine residues in proteins
supports the conclusion that the intra-residue conformationally-dependent shift perturbation is the dominant determinant of
δ13Cε. Analysis of calmodulin data based on these calculations indicates that several residues adopt non-standard rotamers characterized
by very large ~100° χ3 values. The utility of the δ13Cε as a basis for estimating the gauche/trans ratio for χ3 is evaluated, and physical and technical factors that limit the accuracy of both the NMR and crystallographic analyses are
discussed. 相似文献
2.
Two-dimensional 1H-nmr methods are described to obtain information on the sidechain conformation of valyl residues of the lac repressor headpiece and to assign the resonances of their methyl groups stereospecifically. The spin–spin coupling constants (Jαβ) between Cαand Cβ protons are obtained from two-dimensional correlated spectroscopy experiments. Large values for Jαβ(10–12 Hz) corresponding to trans orientations for these protons (g+ conformation) are found for all valyl residues in α-helical segments. For these valyl residues, the distance between one methyl group (γ1)and the valyl amide proton is much shorter than for the other methyl group, so that stereospecific resonance assignments follow from relative intensities of the corresponding cross peaks in a two-dimensional nuclear Overhauser enhancement spectrum. Thus, streospecific assignments could be made for the methyl groups of Val 9, 20, 23, and 38 (of a total of eight valyl residues). 相似文献
3.
In order to find the susceptibility of the amino-Claisen rearrangement and the next proton shift reaction of N-allyl-N-arylamine to the substituent effects in the para position, the kinetic and thermodynamic parameters were calculated at the B3LYP level using the 6-31G** basis set. The calculated activation energies for the rearrangements and proton shift reactions are close to 44.4 and 49.5 kcal mol? 1, respectively. The transition states of the rearrangement with electron-donor substituents are more stable than those with electron-withdrawing substituent groups, but for the proton shift reaction, this situation is reversed (with the exception of fluorine atom for the rearrangement and fluorine and chlorine atoms for the proton shift reaction). Negative values for the activation entropy confirm the concerted mechanism for the amino-Claisen rearrangement and proton shift reaction. The Hammett ρ values of ? 2.4172 and ? 1.7791 are obtained for σp and σ? (enhanced sigma) in the amino-Claisen rearrangement, respectively. The correlation between log(k X/k H) and σp is weaker than that with σ? (enhanced sigma). A negative Hammett ρ value indicates that the electron-donating groups slightly increase the rate of amino-Claisen rearrangement. A positive Hammett ρ value (2.4921) for the proton shift reaction indicates that electron-withdrawing groups increase the rate of reaction. 相似文献
4.
C, N CP MAS and high resolution multinuclear NMR study of methyl
Four new derivatives of methyl
were studied by 1H, 13C, 15N NMR in CDCl3 solutions and by 13C, 15N NMR in the solid state. The replacement of one aryl substituent by another has no influence on the proton and carbon chemical shifts within the sugar moiety, in solution. The differences in 13C chemical shifts Δ = δliquid - δsolid are significant for C-3 (deshielding of -3.4 to -3.8 ppm), C-5 and OMe but not observed for C-2, where the ureido substituent is linked, thus indicating that this fragment of the structure is rigid. The values of Δ in 15N chemical shifts of N-3′ are -2.3 to -2.8 ppm (increase of shielding in the solids); the effect of replacement of substituent at aromatic ring is larger than the contribution of intermolecular H-bond interaction. The values of 15.5–16.1 Hz for 1JC-1′-N and 21.2–21.5 Hz for 1JCO-N indicate that the two C---N-3′ bonds are of significant double bond character. 相似文献
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5.
Dagmara Baraniak Daniel Baranowski Piotr Ruszkowski Jerzy Boryski 《Nucleosides, nucleotides & nucleic acids》2013,32(11):807-835
AbstractThe fluorinated nucleoside dimers with a 1,2,3-triazole linkage are novel compounds within the field of bioorganic chemistry. We report on the synthesis and properties of two groups of nucleoside dimers analogs possessing a different arrangement of the 1,4-disubstituted 1,2,3-triazole linkage. Based on analysis of the 3JHH, 3JH1′C2, and 3JH1′C6 we estimated conformational preferences of sugar part and orientation around glycosidic bond. These compounds show moderate anticancer activity, with cytostatic studies in three different cancer cell lines. 相似文献
6.
Heike Rudnick Sven Hendrich Ulrich Pilatus Karl-Heinz Blotevogel 《Archives of microbiology》1990,154(6):584-588
Methanosarcina frisia accumulates phosphate up to 14% of its dry weight. The phosphate is stored as long-chain polyphosphates as shown by 31P-NMR investigations. Further results show that the accumulation of phosphates is substrate-dependent. In the presence of H2 and CO2 as the only carbon and energy source 180 mg of PO
inf4
sup3-
/g protein were accumulated, whereas 260 mg PO
inf4
sup3-
/g protein were accumulated in the presence of methanol. This is far more than necessary for the maintenance of essential metabolic pathways. In addition, the 31P-NMR studies show the occurrence of cyclic 2,3-diphosphoglycerate in Methanosarcina frisia. The role of the phosphate metabolites in cell metabolism are discussed.Abbreviations
M.
Methanosarcina
- CCP
cyclic 2,3-diphosphoglycerate 相似文献
7.
P Gupta-Bhaya 《Biopolymers》1975,14(6):1143-1160
The electron-mediated spin–spin coupling constant J between the amide NH and the α-CH protons in the dipeptide fragment Cα? CO(NH? CαH)R? C′ONH? Cα is dependent on the dihedral angle of rotation (Φ) around the N? C bond. Measurement of J in a series of zwitterionic dipeptides H3N+? CHR1? CONH? CHR2? CO2? (which is conformationally similar to the dipeptide fragment) in TFA solution shows that J is independent of R1, but dependent on the steric bulk of R2. The data are interpreted in terms of a model that assumes that what we measure is an average value of J? a thermal average over all the possible rotamers. The groups R1 and R2 are, in most cases, sterically kept apart by the trans and planar amide bonds, and hence the independence of J of R1. This model is consistent with the theoretical calculations done on the dipeptide fragment. The effect of the structural characteristics of the side chains (e.g., the effect of lengthening and branching the side chains) on the J values in dipeptides is discussed in the light of the existing results of theoretical calculations. Study of 〈J〉 values in tripeptides (C6H5CH2OCONH? CHR1? CONH? CHR2? CO2CH3, essentially three linked peptide units) shows that electrostatic interaction between the two amide bonds modifies the potential energy surface and the 〈J〉 value of a dipeptide subunit in the tripeptides. Also in some cases, direct steric interaction between the two side chains in the two adjacent dipeptide subunits in the tripeptide affects the potential energy surfaces of the individual dipeptide subunits and hence the 〈J〉 values. The influence of the structural characteristics of the side chains of individual amino acids on structure formation at or beyond the dipeptide level is discussed at various points. The J(NH? αCH) values of CH3CONH? CHR? CONH2 and CH3CONH? CHR? CO2CH3 with the same R are quite different for R = valine, leucine, phenylalanine, methionine, but equal for R = glycine. This, coupled with the fact that one of the carboxamide NH resonances has a chemical shift different from its counterpart in simple amides like CH3CONH2 and the other carboxamide NH has the same chemical shift as its counterpart in CH3CONH2, suggest the presence of a hydrogen bond in dipeptide CH3CONH? CHR? CONH2 with carboxamide NH as the donor. Theoretical evidence for two seven-membered hydrogen-bonded rings with the carboxamide NH as donor and the acetyl oxygen as acceptor is summarized. Our data cannot suggest the number of such hydrogen-bonded rings, nor can they conclude the relative proportion of these rings in a particular dipeptide. A discussion of the difficulty of interpretation is presented and the data are discussed under certain simplifying assumptions. 相似文献
8.
Wenhui Zhang Hongqiu Zhao Ian Carmichael Anthony S. Serianni 《Carbohydrate research》2009,344(12):1582-9509
Methyl α-cellobioside (methyl β-d-glucopyranosyl-(1→4)-α-d-glucopyranoside) was labeled with 13C at C4′ for use in NMR studies in DMSO-d6 solvent to attempt the detection of a trans-H-bond J-coupling (3hJCCOH) between C4′ and OH3. Analysis of the OH3 signal at 600 MHz revealed only the presence of two homonuclear J-couplings: 3JH3,OH3 and a smaller, longer range JHH. No evidence for 3hJC4′,OH3 was found. The longer range JHH was traced to 4JH4,OH3 based on 2D 1H–1H COSY data and inspection of the H2 and H4 signal lineshapes. A limited set of DFT calculations was performed on a methyl cellobioside mimic to evaluate the structural dependencies of 4JH2,O3H and 4JH4,O3H on the H3–C3–O3–H torsion angle. Computed couplings range from about −0.7 to about +1.1 Hz, with maximal values observed when the C–H and O–H bonds are roughly diaxial. 相似文献
9.
NMR coupling constants, both direct one‐bond (1J) and geminal two‐bond (2J), are employed to analyze the protein secondary structure of human oxidized ERp18. Coupling constants collected and evaluated for the 18 kDa protein comprise 1268 values of 1JCαHα, 1JCαCβ, 1JCαC′, 1JC′N′, 1JN′Cα, 1JN′HN, 2JCαN′, 2JHNCα, 2JC′HN, and 2JHαC′. Comparison with 1J and 2J data from reference proteins and pattern analysis on a per‐residue basis permitted main‐chain φ,ψ torsion‐angle combinations of many of the 149 amino‐acid residues in ERp18 to be narrowed to particular secondary‐structure motifs. J‐coupling indexing is here being developed on statistical criteria and used to devise a ternary grid for interpreting patterns of relative values of J. To account for the influence of the varying substituent pattern in different amino‐acid sidechains, a table of residue‐type specific threshold values was compiled for discriminating small, medium, and large categories of J. For the 15‐residue insertion that distinguishes the ERp18 fold from that of thioredoxin, the J‐coupling data hint at a succession of five isolated Type‐I β turns at progressively shorter sequence intervals, in agreement with the crystal structure. Proteins 2011. © 2010 Wiley‐Liss, Inc. 相似文献
10.
Teiki Iwaoka Harumitsu Kuwano Shigeki Muramatsu Rokuro Endo Yasuo Nakada Junya Ide 《Bioscience, biotechnology, and biochemistry》2013,77(6):1381-1387
New types of stable chrysanthemic acids and esters were synthesized, and their 13C-NMR were examined and fully analyzed. The configurations of the cyclopropanecarboxylic acid and halomethylvinyl group were reflected on the spin-lattice relaxation time of the substituted methyl carbon involved in their structure. The long-range spin-spin coupling constants (3JCH) correlated well to the NOE and T1 measurements, which can generally be used to distinguish the geometry of the substituted double bond. 相似文献
11.
Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σr) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σr assumed values of the order of about 0·2–0·4. The σr values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m?2 s?1; relative humidity 20–40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σr= 1. Further increase of the light intensity to about 400 μmol m?2 s?1 resulted in a slight but significant decrease of the σr values to about 0·8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906–912) that, in intact transpiring plants at low light intensities of about 10 μmol m?2 s?1 and at relative humidities of 20–40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σr values 0·3–0·5). In contrast to wheat and barley, very high light intensities (about 700 μmol m?2 s?1) were needed to shift the radial reflection coefficients of maize roots to values of about 0·9. Osmotically induced xylem pressure changes were apparently linked to changes in turgor pressure in the root cortical parenchyma cells, as shown by simultaneous measurements of xylem and cell turgor pressure. In analogy to the σr values of the respective glycophytes, the σc values of the root cortical cells of wheat and barley were close to unity, whereas σc for maize was significantly smaller (about 0·7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m?2 s?1 the cellular reflection coefficient of maize roots increased to about 0·95. In contrast to the σr values, the σc values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21–31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed. 相似文献
12.
Thomas U. L. Biber Thomas C. Walker Terry L. Mullen 《The Journal of membrane biology》1980,56(1):81-92
Summary The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (J
13
Cl
) and efflux (J
31
Cl
), short-circuit current, transepithelial potential, and conductance (G
m).J
13
Cl
as well asJ
31
Cl
were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component ofJ
31
Cl
appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute toG
m. The saturating component ofJ
31
Cl
is only 5% of totalJ
31
Cl
with 115mm Cl in the serosal medium. Replacement of 115mm Cl– in external medium by SO
4
=
, NO
3
–
, HCO
3
–
or I– results in 87–97% reduction ofJ
31
Cl
, whereas replacement with Br– has no effect. As external Cl concentration is raised in steps from 2 to 115mm,J
13
Cl
andJ
31
Cl
increase by the same amount butJ
13
Cl
is persistently 0.15 eq/cm2 hr larger thanJ
31
Cl
. These results indicate that at least 3/4 of linear components ofJ
13
Cl
andJ
31
Cl
proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component ofJ
13
Cl
is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na.Reprinted from The Journal of Membrane Biology, Vol. 54, No. 3, pages 191–202. Our apologies for deleting the author's names on the original version. 相似文献
13.
A cohesion/tension model for the gating of aquaporins allows estimation of water channel pore volumes in Chara 总被引:2,自引:2,他引:0
The water permeability (hydraulic conductivity; Lp) of turgid, intact internodes of Chara corallina decreased exponentially as the concentration of osmolytes applied in the medium increased. Membranes were permeable to osmolytes and therefore they could be applied on both sides of the plasma membrane at concentrations of up to 2.0 m (5.0 MPa of osmotic pressure). Organic solutes of different molecular size (molecular weight, MW) and reflection coefficients (σs) were used [heavy water HDO, MW: 19, σs: 0.004; acetone, MW: 58, σs: 0.15; dimethyl formamide (DMF), MW: 73, σs: 0.76; ethylene glycol monomethyl ether (EGMME), MW: 76, σs: 0.59; diethylene glycol monomethyl ether (DEGMME), MW: 120, σs: 0.78 and triethylene glycol monoethyl ether (TEGMEE), MW: 178, σs: 0.80]. The larger the molecular size of the osmolyte, the more efficient it was in reducing cell Lp at a given concentration. The residual cell Lp decreased with increasing size of osmolytes. The findings are in agreement with a cohesion/tension model of the osmotic dehydration of water channels (aquaporins; AQPs), which predicts both reversible exponential dehydration curves and the dependence on the size of osmolytes which are more or less excluded from AQPs (Ye, Wiera & Steudle, Journal of Experimental Botany 55, 449–461, 2004). In the presence of big osmolytes, dehydration curves were best described by the sum of two exponentials (as predicted from the theory in the presence of two different types of AQPs with differing pore diameters and volumes). AQPs with big diameters could not be closed in the presence of osmolytes of small molecular size, even at very high concentrations. The cohesion/tension theory allowed pore volumes of AQPs to be evaluated, which was 2.3 ± 0.2 nm3 for the narrow pore and between 5.5 ± 0.8 and 6.1 ± 0.8 nm3 for the wider pores. The existence of different types of pores was also evident from differences in the residual Lp. Alternatively, pore volumes were estimated from ratios between osmotic (Pf) and diffusional (Pd) water flow, yielding the number of water molecules (N) in the pores. N-values ranged between 35 and 60, which referred to volumes of 0.51 and 0.88 nm3/pore. Values of pore volumes obtained by either method were bigger than those reported in the literature for other AQPs. Absolute values of pore volumes and differences obtained by the two methods are discussed in terms of an inclusion of mouth parts of AQPs during osmotic dehydration. It is concluded that the mouth part contributed to the absolute values of pore volumes depending on the size of osmolytes. However, this can not explain the finding of the existence of two different types or groups of AQPs in the plasma membrane of Chara. 相似文献
14.
Summary The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (J
13
Cl
) and efflux (J
31
Cl
), short-circuit current, transepithelial potential, and conductance (G
m).J
13
Cl
as well asJ
31
Cl
were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component ofJ
31
Cl
appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute toG
m. The saturating component ofJ
31
Cl
is only 5% of totalJ
31
Cl
with 115mm Cl in the serosal medium. Replacement of 115mm Cl– in external medium by SO
4
=
, NO
3
–
, HCO
3
–
or I– results in 87–97% reduction ofJ
31
Cl
, whereas replacement with Br– has no effect. As external Cl concentration is raised in steps from 2 to 115mm,J
13
Cl
andJ
31
Cl
increase by the same amount butJ
13
Cl
is persistently 0.15 eq/cm2 hr larger thanJ
31
Cl
. These results indicate that at least 3/4 of linear components ofJ
13
Cl
andJ
31
Cl
proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component ofJ
13
Cl
is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na. 相似文献
15.
Alternate promoters direct stress-induced transcription of the Bacillus subtilis clpC operon 总被引:1,自引:0,他引:1
clpC ofBacillus subtilis is part of an operon containing six genes. Northern blot analysis suggested that all genes are co-transcribed and encode stress-inducible proteins. Two promoters (PA and PB) were mapped upstream of the first gene. PA resembles promoters recognized by the vegetative RNA polymerase EσA. The other promoter (PB) was shown to be dependent on σB, the general stress σ factor in B. subtilis, suggesting that clpC, a potential chaperone, is expressed in a σB-dependent manner. This is the first evidence that σB in B, subtilis is involved in controlling the expression of a gene whose counterpart, clpB, is subject to regulation by σ32 in Escherichia coli, indicating a new function of σB-dependent general stress proteins. PB deviated from the consensus sequence of σB promoters and was only slightly induced by starvation conditions. Nevertheless, strong induction by heat, ethanol, and salt stress occurred at the σB-dependent promoter, whereas the vegetative promoter was only weakly induced under these conditions. However, in a sigB mutant, the σA-like promoter became inducible by heat and ethanol stress, completely compensating for sigB deficiency. Only the downstream σA-like promoter was induced by certain stress conditions such as hydrogen peroxide or puromycin. These results suggest that novel stress-induction mechanisms are acting at a vegetative promoter. Involvement of additional elements in this mode of induction are discussed. 相似文献
16.
Geminal two‐bond couplings (2J) in proteins were analyzed in terms of correlation with protein secondary structure. NMR coupling constants measured and evaluated for a total six proteins comprise 3999 values of 2JCαN′, 2JC′HN, 2JHNCα, 2JC′Cα, 2JHαC′, 2JHαCα, 2JCβC′, 2JN′Hα, 2JN′Cβ, and 2JN′C′, encompassing an aggregate 969 amino‐acid residues. A seamless chain of pattern comparisons across the spectrum datasets recorded allowed the absolute signs of all 2J coupling constants studied to be retrieved. Grouped by their mediating nucleus, C′, N′ or Cα, 2J couplings related to C′ and N′ depend significantly on ?,ψ torsion‐angle combinations. β turn types I, I′, II and II′, especially, can be distinguished on the basis of relative‐value patterns of 2JCαN′, 2JHNCα, 2JC′HN, and 2JHαC′. These coupling types also depend on planar or tetrahedral bond angles, whereas such dependences seem insignificant for other types. 2JHαCβ appears to depend on amino‐acid type only, showing negligible correlation with torsion‐angle geometry. Owing to its unusual properties, 2JCαN′ can be considered a “one‐bond” rather than two‐bond interaction, the allylic analog of 1JN′Cα, as it were. Of all protein J coupling types, 2JCαN′ exhibits the strongest dependence on molecular conformation, and among the 2J types, 2JHNCα comes second in terms of significance, yet was hitherto barely attended to in protein structure work. Proteins 2010. © 2009 Wiley‐Liss, Inc. 相似文献
17.
Daniel Baranowski Bozenna Golankiewicz Wojciech Folkman Mariusz Popenda 《Nucleosides, nucleotides & nucleic acids》2013,32(10):707-719
GRAPHICAL ABSTRACT We synthesized a new 2-methyl derivative of wyosine using a multistep procedure starting from guanosine. We examined different synthetic paths and optimized the conditions for each step. Based on MD calculations and analysis of the 3 J HH and J C1′H1′ of the ribose moiety, we discovered that the sugar part adopted conformation specific for the East region rarely occurring in solution. This unusual conformational preference is probably due to steric repulsions between the methyl group at position 2 and the 5′-CH2OH group. We observed that N-glycosidic bond stability weakened 14-fold upon the introduction of the methyl group in position 2 compared with wyosine. 相似文献
18.
Thomas U. L. Biber Terry L. Mullen John A. DeSimone 《The Journal of membrane biology》1980,52(2):133-139
Summary The effect of addition of FeCl3 to the media bathing the isolated skin ofRana pipiens was studied by measuring short-circuit current, transepithelial potential, and resistance, and by determining the influx and efflux of sodium (J
13
Na
andJ
31
Na
, respectively) and the influx and efflux of chloride (J
13
Cl
andJ
31
Cl
, respectively) across the epithelium. With normal Ringer's solution on both sides of the skin, addition of 10–3
m FeCl3 to the external medium resulted in nearly complete inhibition of active Na transport (J
13
Na
decreased from 1.30±0.14 to 0.10±0.04 eq/cm2 hr (N=8)) and in appearance of active chloride transport in outward direction due to an 80% increase inJ
31
Cl
. Average (J
31
Cl
–J
13
Cl
) obtained from means of 8 skins in 6 consecutive control and last 3 experimental periods was –0.17±0.04 and 0.38±0.05 eq/cm2 hr, respectively. FeCl3 added to external medium also induced substantial net chloride movement in outward direction when external medium contained Na-free choline chloride Ringer's or low ionic strength solution. Under the latter condition net Na movement was virtually eliminated by external FeCl3. After addition of FeCl3 to serosal medium there was delayed inhibition ofJ
13
Na
but no change in chloride fluxes. Immediate and profound changes in Na and Cl transport systems seen after external application of FeCl3 indicate charge effects of Fe3+ on surface of apical cell membranes, possibly close to or in ion channels. 相似文献
19.
Water and solute relations of young roots of Phaseolus coccineus have been measured using the root pressure probe. Biphasic root pressure relaxations were obtained when roots were treated with solutions containing different osmotic test solutes. From the relaxations, the hydraulic conductivity (Lpr), the permeability coefficient (Psr), and the reflection coefficient (σsr) of the roots could be evaluated. Lpr was 1.8 to 8.4 . 10?8 m . s?1 . MPa?1 and Psr (in 10?10 m . s?1): methanol, 27–62; ethanol, 44–73; urea, 5–11; mannitol, 1.5; KCl, 7.1–9.2; NaCl, 2.1; NaNO3, 3.7. The hydraulic conductivity was similar when using osmotic and hydrostatic pressure gradients as driving forces. The hydraulic conductivity of individual root cortex cells (Lp) was by two orders of magnitude larger than Lpr (Lp = 0.3 to 4.7 . 10?6 m . s?1 . MPa?1) which indicated a predominant cell-to-cell rather than an apoplasmic transport of water in the Phaseolus root. Except for distances shorter than 20 mm from the root apex, the hydraulic resistance of the roots was limited by the radial movement of water across the root cylinder and not by the hydraulic resistance within the xylem. Reflection coefficients were low: methanol: 0.16–0.34; ethanol: 0.15–0.47; urea: 0.41–0.51; mannitol: 0.68; KCl: 0.43–0.54; NaCl: 0.59; NaNO3: 0.54. The transport coefficients (Lpr, Psr, σsr) have been critically examined for influences of unstirred layers and active transport. The low σsr suggests that the common treatment of the root as a rather perfect osmometer (σsr = 1) analogous to plant cells should be treated cautiously. The reasons for the low σsr and the possible implications of the absolute values of the transport parameters for the absorption of water and nutrients are discussed. 相似文献
20.
Wayne J. P. Blonski Frank E. Hruska Krishan L. Sadana Peter C. Loewen 《Biopolymers》1983,22(2):605-616
The geminal and vicinal 13C-31P coupling constants have been monitored, as a function of pH, for a series of uracil and cytosine 3′- and 5′-nucleotides with a ribose, arabinose, or 2′-deoxyribose sugar. Data were also obtained for two 3′,5′-diphosphates in the ribose and arabinose series. The geminal J(C5′-P5′) and J(C3′-P3′) couplings show only a small dependence on the ionization state of the phosphate, decreasing by < 0.5 Hz in the pH 5–7 range. For the ribose and arabinose 3′-nucleotides, the vicinal J(C4′-P3′) increase (up to 1.5 Hz) on secondary phosphate ionization in the pH 5–7 range, whereas their J(C2′-P3′) couplings decrease (up to 1.5 Hz) over the same pH range. In contrast for the 2′-deoxyribose molecules, both couplings decrease (~0.5 Hz) on phosphate ionization. The titration curves provide information about the influence of the sugar on the conformation about the C3′? O3′ bond. Some conformational trends could be rationalized by consideration of the sugar-puckerdependent contact interactions between the 3′-phosphate and the substituents on the furanose ring. 相似文献