共查询到20条相似文献,搜索用时 31 毫秒
1.
J. Monaselidze G. Majagaladze Sh. Barbakadze D. Khachidze M. Gorgoshidze Y. Kalandadze 《Journal of biomolecular structure & dynamics》2013,31(4):419-424
Abstract Thermodynamic parameters of melting process (δHm, Tm, δTm) of calf thymus DNA, poly(dA)poly(dT) and poly(d(A-C))·poly(d(G-T)) were determined in the presence of various concentrations of TOEPyP(4) and its Zn complex. The investigated porphyrins caused serious stabilization of calf thymus DNA and poly poly(dA)poly(dT), but not poly(d(A-C))poly(d(G-T)). It was shown that TOEpyp(4) revealed GC specificity, it increased Tm of satellite fraction by 24°C, but ZnTOEpyp(4), on the contrary, predominately bound with AT-rich sites and increased DNA main stage Tm by 18°C, and Tm of poly(dA)poly(dT) increased by 40 °C, in comparison with the same polymers without porphyrin. ZnTOEpyp(4) binds with DNA and poly(dA)poly(dT) in two modes—strong and weak ones. In the range of r from 0.005 to 0.08 both modes were fulfilled, and in the range of r from 0.165 to 0.25 only one mode—strong binding—took place. The weak binding is characterized with shifting of Tm by some grades, and for the strong binding Tm shifts by ~ 30–40°C. Invariability of ΔHm of DNA and poly(dA)poly(dT), and sharp increase of Tm in the range of r from 0.08 to 0.25 for thymus DNA and 0.01–0.2 for poly(dA)poly(dT) we interpret as entropic character of these complexes melting. It was suggested that this entropic character of melting is connected with forcing out of H2O molecules from AT sites by ZnTOEpyp(4) and with formation of outside stacking at the sites of binding. Four-fold decrease of calf thymus DNA melting range width ΔTm caused by increase of added ZnTO- Epyp(4) concentration is explained by rapprochement of AT and GC pairs thermal stability, and it is in agreement with a well-known dependence, according to which ΔT~TGC-TAT for DNA obtained from higher organisms (L. V. Berestetskaya, M. D. Frank-Kamenetskii, and Yu. S. Lazurkin. Biopolymers 13, 193–205 (1974)). Poly (d(A-C))poly(d(G-T)) in the presence of ZnTOEpyp(4) gives only one mode of weak binding. The conclusion is that binding of ZnTOEpyp(4) with DNA depends on its nucleotide sequence. 相似文献
2.
Interaction of poly-L-lysine with nucleic acids. II. Poly(A+U), poly(A+2U), and rice dwarf virus RNA 总被引:1,自引:0,他引:1
The optical density–temperature profile of double-stranded poly(A + U), triple stranded poly(A + 2U), and double-stranded RNA from rice dwarf virus in solutions with and without poly-L -lysine has been examined. When poly-L -lysine is added, more than one melting temperature Tm is observed for poly(A + U) and poly(A + 2U). One of them is considered to correspond to the melting of the polynucleotide molecule free from poly-L -lysine, and another to the melting of a polynucleotide–poly-L -lysine complex. For rice dwarf virus RNA, the Tm assignable to the complex is not found to be lower than 99°C. In every case, however, the hyperchromicity observed at the Tm of the free poly-nucleotide molecule is lowered linearly as the amount of poly-L -lysine added to the solution increases. This fact is taken as indicating that there is a stoichiometric complex formed. The stoichiometric ratio lysine/nucleotide in each complex is determined by examining the relation between the amount of poly-L -lysine added to the solution and the percentage of hyperchromicity remaining at Tm of the free polynucleotide molecule. The ratio is found to be 2/3 for all of the three complexes. A discussion is given on the molecular conformations of four types of polynucleotide–polylysine complex hitherto found: (A) double-stranded DNA plus poly-L -lysine in which the lyslne/nucleotide ratio is 1, (B) three-stranded RNA [poly(A + 2U)] plus poly-L -lysine in which the ratio is 2/3, (C) double-stranded RNA [poly (A + U) or rice dwarf virus RNA] plus poly-L -lysine in which the ratio is 2/3, and (D) double-stranded RNA [poly(I + C)] plus poly-L -lysine in which the ratio is 1/2. 相似文献
3.
The interaction of the fluorinated antimalarial drug fluoroquine [7-fluoro-4-(diethyl-amino-1-methylbutylamino)quinoline] with DNA, tRNA, and poly(A) has been investigated by optical absorption, fluorescence, and 19F-nmr chemical-shift and relaxation methods. Optical absorption and fluorescence experiments indicate that fluoroquine binds to nucleic acids in a similar manner to that of its well-known analog chloroquine. At low drug-to-base pair ratios, binding of both drugs appears to be random. Fluoroquine and chloroquine also elevate the melting temperature (Tm) of DNA to a comparable extent. Binding of fluoroquine to DNA, tRNA, or poly(A) results in a downfield shift of about 1.5 ppm for the 19F-nmr resonance. The chemical shift of free fluoroquine depends on the isotopic composition of the solvent (D2O vs H2O). The solvent isotope shift is virtually eliminated by fluoroquine binding to any one of the nucleic acids. 19F-nmr relaxation experiments were carried out to measure the spin-lattice relaxation time (T1), 19F{1H} nuclear Overhauser effect (NOE), off-resonance intensity ratio (R), off-resonance rotating-frame spin-lattice relaxation time (T), and linewidth for fluoroquine in the nucleic acid complexes. By accounting for intramolecular proton-fluorine dipolar and chemical-shift anisotropy contributions to the fluorine relaxation, all of the relaxation parameters for the fluoroquine–DNA complex can be well described by a motional model incorporating long-range DNA bending on the order of a microsecond and an internal motion of the drug on the order of a nanosecond. Selective NOE experiments indicate that the fluorine in the drug is near the ribose protons in the RNA complexes, but not in the DNA complex. Details of the binding evidently differ for the two types of nucleic acids. This study provides the foundation for an investigation of fluoroquine in intact cells. 相似文献
4.
高原鼠兔干扰虽然能够改变高寒草甸植物多样性与土壤养分含量,但植物多样性与土壤养分间的关系对高原鼠兔干扰的响应尚不清晰。利用高原鼠兔有效洞口密度将高原鼠兔干扰程度划分为T_1(7个/625 m~2)、T_2(12个/625 m~2)、T_3(22个/625m~2)、T_4(38个/625 m~2)4个水平,运用RDA冗余分析法研究了高原鼠兔不同干扰程度下高寒草甸植物多样性与土壤养分间的关系。结果表明:随着高原鼠兔干扰水平的增加,优势种高山嵩草(Kobresia pygmaea)的重要值先增加后降低,而伴生种小花草玉梅(Anemone rivularis var.flore-minors)和莓叶委陵菜(Potentilla fragarioides)的重要值先降低后增加;当高原鼠兔干扰水平从T_1到T_2时植物多样性指数变化不显著,而高原鼠兔干扰程度超过T_2时则植物多样性指数具有降低趋势;土壤全氮和硝态氮含量随高原鼠兔干扰水平增加而降低,而土壤铵态氮含量则降低后增加,土壤有机碳和全磷先增加后降低;多样性指数与0—10cm土壤深度硝态氮、10—20cm土壤深度全钾间的相关性从T_1到T_3时为正相关,而到T_4时则变为负相关,而与0—10cm土壤深度全氮的相关性则表现T_1到T_3时为负相关,T_4时为正相关,与铵态氮间相关性只有T_1时为负相关,这说明高原鼠兔干扰改变了植物多样性与土壤养分间的关系,其变化阈值介于T_2和T_3。 相似文献
5.
P. O. Vardevanyan A. P. Antonyan G. A. Manukyan A. T. Karapetyan A. K. Shchyolkina O. F. Borisova 《Molecular Biology》2000,34(2):272-276
Isotherms of the EtBr adsorption on native and denatured poly(dA)poly(dT) in the temperature interval 20–70°C were obtained.
The EtBr binding constants and the number of binding sites were determined. The thermodynamic parameters of the EtBr intercalation
complex upon changes of solution temperature 20–48°C were calculated: 1.0·106 M−1≤K≤1.4·106 M−1, free energy ΔG
o=−8.7±0.3 kcal/mol, enthalpy ΔH
o≅0, and entropy ΔS
o=28±0.5 cal/(mol deg). UV melting has shown that the melting temperature (T
m) of EtBr-poly(dA)poly(dT) complexes (μ=0.022,4.16·10−5 M EtBr) increased by 17°C as compared with the ΔT
m of free homopolymer, whereas the half-width of the transition (T
m) is not changed. It was shown for the first time that EtBr forms complexes of two types on single-stranded regions of poly(dA)poly(dT)
denatured at 70°C: strong (K
1=1.7·105 M−1; ΔG
o=−8.10±0.03 kcal/mol) and weak (K
2=2.9·103 M−1; ΔG
o=−6.0±0.3 kcal/mol).The ΔG
o of the strong and weak complexes was independent of the solution ionic strength, 0.0022≤μ≤0.022. A model of EtBr binding
with single-stranded regions of poly(dA)poly(dT) is discussed. 相似文献
6.
Ali Akbar Khandar Christine Cardin John McGrady Seyed Amir Zarei 《Inorganica chimica acta》2010,363(14):4080-4087
A series of hexadentate ligands, H2Lm (m = 1−4), [1H-pyrrol-2-ylmethylene]{2-[2-(2-{[1H-pyrrol-2-ylmethylene]amino}phenoxy)ethoxy]phenyl}amine (H2L1), [1H-pyrrol-2-ylmethylene]{2-[4-(2-{[1H-pyrrol-2-ylmethylene]amino}phenoxy)butoxy]phenyl}amine (H2L2), [1H-pyrrol-2-ylmethylene][2-({2-[(2-{[1H-pyrrol-2-ylmethylene]amino}phenyl)thio]ethyl}thio)phenyl]amine (H2L3) and [1H-pyrrol-2-ylmethylene][2-({4-[(2-{[1H-pyrrol-2-lmethylene]amino}phenyl)thio]butyl}thio) phenyl]amine (H2L4) were prepared by condensation reaction of pyrrol-2-carboxaldehyde with {2-[2-(2-aminophenoxy)ethoxy]phenyl}amine, {2-[4-(2-aminophenoxy)butoxy]phenyl}amine, [2-({2-[(2-aminophenyl)thio]ethyl}thio)phenyl]amine and [2-({4-[(2-aminophenyl)thio]butyl}thio)phenyl]amine respectively. Reaction of these ligands with nickel(II) and copper(II) acetate gave complexes of the form MLm (m = 1−4), and the synthesized ligands and their complexes have been characterized by a variety of physico-chemical techniques. The solid and solution states investigations show that the complexes are neutral. The molecular structures of NiL3 and CuL2, which have been determined by single crystal X-ray diffraction, indicate that the NiL3 complex has a distorted octahedral coordination environment around the metal while the CuL2 complex has a seesaw coordination geometry. DFT calculations were used to analyse the electronic structure and simulation of the electronic absorption spectrum of the CuL2 complex using TDDFT gives results that are consistent with the measured spectroscopic behavior of the complex. Cyclic voltammetry indicates that all copper complexes are electrochemically inactive but the nickel complexes with softer thioethers are more easily oxidized than their oxygen analogs. 相似文献
7.
Interaction of poly(X) with poly(U) to form a regular, ordered structure has been investigated by uv, CD, and ir spectroscopy. The XU complex exhibits a marked dependence of Tm on the nature of the alkali metal ion present in solution. Stability of the complex is surprisingly high and increases with ionization of X residues at N3. The ir evidence shows the U residues are present as the diketo form and X residues as the 6-keto-2-enolate anion and indicates that the X and U carbonyl groups are vibrationally coupled. A mixing curve exhibits a single minimum, at 1:1, but we conclude that the actual combining ratio is twice this value and that the formula of the complex is X2U2. We propose a model based on a four-standard helix with size-specific alkali metal complexing to X and U carbonyl oxygens in the axial channel of the helix. This model and previously suggested two-stranded structures are evaluated in terms of the physical and chemical properties of the complex. 相似文献
8.
Dominique Hermann G. Victor Fazakerley Claude Houssier Wilhelm Guschlbauer 《Biopolymers》1984,23(5):945-960
The covalent binding of trans-Pt (NH3)2Cl2 to the double-stranded poly(I)·poly(C) follows three types of reactions, depending on rb and the concentration of polynucleotide in the reaction mixture. At rb ? 0.1, the principal reaction is coordination to poly(I), giving rise to some destabilization of the double strand, as shown by uv and CD spectra, and a decrease in Tm values, giving rise to free loops of poly(C). At higher rb and low polynucleotide concentration, the free cytidine bases react with platinum bound on the complementary strand to form intramolecular (interstrand) crosslinks that restabilize the double-stranded structure. At high rb and high polynucleotide concentration, while the above reaction still occurs, the predominant one is the formation of intermolecular crosslinks. Under no conditions has strand separation been observed. 相似文献
9.
Joo Carlos M. Carvalho Fernando R. Francisco Kleber A. Almeida Sunao Sato Attilio Converti 《Journal of phycology》2004,40(3):589-597
Arthrospira (Spirulina) platensis (Nordstedt) Gomont was cultivated under light‐limited conditions in 5‐L open tanks by daily supplying NH4Cl as nitrogen source. Exponentially increasing feeding rates were adopted to prevent ammonia toxicity. The total feeding time (T) was varied between 12 and 20 days, and the starting (m0) and total (mT) quantities of the nitrogen source per unit reactor volume were varied in the ranges 0.19–1.7 mM and 2.3–23.1 mM, respectively. This intermittent addition of the nitrogen source prevented ammonia from reaching inhibitory levels and ensured final cell concentrations (Xm) and cell productivities (Px) comparable with those of batch runs with KNO3. Moreover, the lower nitrogen addition due to the use of NH4Cl rather than KNO3 allowed for higher nitrogen‐to‐cell conversions (Yx/n). These results were evaluated using three‐factor, five‐level, central composite experimental planning, combined with the response surface methodology, selecting T, m0, and mT as the independent variables and Xm, Px, and Yx/n as the response variables. This approach allowed us to identify, through the simultaneous optimization of the variables, T=16 days, m0=1.7 mM, and mT=21.5 mM as the best conditions for A. platensis cultivation at 72 μmol photons·m?2·s?1. Under these conditions, a maximum cell concentration of 1239 mg ·L?1 was obtained, which is a value comparable with that obtained using KNO3 as nitrogen source and nearly coincident with the theoretical one estimated by the response surface methodology. 相似文献
10.
Studies on poly(L-lysine50, L-tyrosine50)-DNA interaction 总被引:3,自引:0,他引:3
Interaction between poly(Lys50, Tyr50) and DNA has been studied by absorption, circular dichroism (CD), and fluorescence spectroscopy and thermal denaturation in 0.001M Tris, pH 6.8. The binding of this copolypeptide to DNA results in an absorbance enhancement and fluorescence quenching on tyrosine. There is also an increase in the tyrosine CD at 230 nm. The CD of DNA above 250 nm is slightly shifted to the longer wavelength which is qualitatively similar to, but quantitatively much smaller than, that induced by polylysine binding. At physiological pH the poly(Lys50, Tyr50)–DNA complex is soluble until there is one lysine and one tyrosine per nucleotide in the complex. The same ratio of amino acid residues to nucleotide has also been observed in copolypeptide-bound regions of the complex. The addition of more poly(Lys50, Tyr50) to DNA yields a constant melting temperature, Tm′, for bound base pairs at 90°C which is close to that of polylysine-bound DNA under the same condition. The melting temperature, Tm, of free base pairs at about 60°C on the other hand, is increased by 10°C as more copolypeptide is bound to DNA. As the temperature is raised, both absorption and CD spectra of the complexes with high coverage are changed, suggesting structural alteration, perhaps deprotonation, on bound tyrosine. The results in this report also suggest that intercalation of tyrosine in DNA is unlikely to be the mode of binding. 相似文献
11.
Polynucleotides. XI. Thermodynamics of (A) N -2(U) infinity from the dependence of T m N on oligomer length 总被引:3,自引:0,他引:3
The variation in the helix-coil transition temperature, TmN, with oligomer length, N, for the system ((I)) has been examined. The results for N = 4-13, measured in 0.2M Na+, have been analyzed in terms of the expression of Blake (1972): ((II)) where cm is the free oligomer concentration at TmN, and Vrf is the thermodynamic free volume available to a helical base-triplet residue. The correlation coefficient for the fit to expression (II) of data obtained over a 50° temperature range is 0.997 when ΔHr = ?12.6 kcal/mole of base-triplets (independent of oligomer length (N ? 4) or temperature), the value previously obtained from both calorimetry of (A)∞·2(U)∞ and (A)4 concentration dependence of Tm. It is found that Vrf = 8.0 × 10?4 1/mole (± 30%) or 1.33 Å3 per helical base-triplet, and is constant with temperature. A maximum value for Vrf of 21.0 × 10?4 1/M (± 1.3%), equivalent to 3.54 Å3 per helical basetriplet is obtained by the same treatment of the helix-coil transition data for the three-stranded helix formed by adenosine (N = 1) and 2(U)∞ obtained by Davies and Davidson (1971). 相似文献
12.
The physicochemical properties of a high-molecular-weight spin-labeled nucleic acid, (RUGT,U)n, synthesized by enzymatic copolymerization, were evaluated by uv and ESR spectroscopy. It was shown earlier that spin labeling of nucleic acids by chemical modification to an extent which gives a nitroxide-to-nucleotide ratio greater than 0.002 can cause noticeable lattice perturbations (A. M. Bobst, A. Hakam, P. W. Langemeier, and S. Kouidou (1979), Arch. Biochem. Biophys. 187, 339–345). The presence of RUGT, a 5-nitroxide-labeled uridine residue, in a (U)n lattice at a ratio of 0.01 is shown here not to affect the complexation with (A)n, since the uv melting temperature (T0OD) of the 2 → 1 transition and the hypochromicity changes were the same for (RUGT,U)n· (A)n and (U)n·(A)n. ESR measurements indicated that the nitroxide radical reflects the transition accurately within the error limit, although a slight destabilization of the spinlabeled segment could not be excluded. Computer simulations showed conclusively that the spin melting temperature (Tmsp) corresponds to the temperature at which half of the spin-labeled segments are no longer complexed, for the ESR spectrum at Tmspcan be simulated with equal contributions from the line shapes of ESR spectra taken before and after the transition. Arrhenius plots obtained by using two different approaches for computing correlation times were qualitatively the same. Computer analysis also revealed that the formation of a (RUGT,U)n·(A)n complex can be described by a two-state model, in contrast to results obtained with chemically spin-labeled (U)n. Thus, using (RUGT,U)n over chemically spin-labeled (U)n can offer distinct advantages. 相似文献
13.
Thermodynamics of the B to Z transition in poly(dGdC) 总被引:1,自引:0,他引:1
The thermodynamics of the B to Z transition in poly(dGdC) was examined by differential scanning calorimetry, temperature-dependent absorbance spectroscopy, and CD spectroscopy. In a buffer containing 1 mM Na cacodylate, 1 mM MgCl2, pH 6.3, the B to Z transition is centered at 76.4°C, and is characterized by ΔHcal = 2.02 kcal (mol base pair)?1 and a cooperative unit of 150 base pairs (bp). The tm of this transition is independent of both polynucleotide and Mg2+ concentrations. A second transition, with ΔHcal = 2.90 cal (mol bp)?1, follows the B to Z conversion, the tm of which is dependent upon both the polynucleotide and the Mg2+ concentrations. Turbidity changes are concomitant with the second transition, indicative of DNA aggregation. CD spectra recorded at a temperature above the second transition are similar to those reported for ψ(–)-DNA. Both the B to Z transition and the aggregation reaction are fully and rapidly reversible in calorimetric experiments. The helix to coil transition under these solution conditions is centered at 126°C, and is characterized by ΔHcal = 12.4 kcal (mol bp)?1 and a cooperative unit of 290 bp. In 5 mM MgCl2, a single transition is seen centered at 75.5°C, characterized by ΔHcal = 2.82 kcal (mol bp)?1 and a cooperative unit of 430 bp. This transition is not readily reversible in calorimetric experiments. Changes in turbidity are coincident with the transition, and CD spectra at a temperature just above the transition are characteristic of ψ(–)-DNA. A transition at 124.9°C is seen under these solution conditions, with ΔHcal = 10.0 kcal (mol bp)?1 and which requires a complex three-step reaction mechanism to approximate the experimental excess heat capacity curve. Our results provide a direct measure of the thermodynamics of the B to Z transition, and indicate that Z-DNA is an intermediate in the formation of the ψ-(–) aggregate under these solution conditions. 相似文献
14.
Poly(Lys48, His52), a random copolypeptide of L -lysine (48%) and L -histidine (52%), was used as a model protein for investigating the effects of protonation on the imidazole group of histidines on protein binding to DNA. The complexes formed between poly(Lys48, His52) and DNA were examined using absorbance, circular dichroism (CD), and thermal denaturation. Although increasing pH reduces the charges on histidine side chains in the model protein, the protein still binds the DNA with approximately one positive charge per negative charge in protein-bound regions. Nevertheless, CD and melting properties of poly(Lys48, His52)-DNA complexes still depend upon the solution pH which determines the protonation state of imidazole group of histidine side chains. At pH 7.0, the complexes show two characteristic melting bands with a tm (46–51°C) for free base pairs and a t′m (94°C) for protein-bound base pairs. The t′m of the complexes is reduced to 90°C at pH 9.2, although at this pH there is still one lysine per phosphate in protein-bound regions. Presumably, the presence of deprotonated histidine residues destabilizes the native structure of protein-bound DNA. The binding of this model protein to DNA causes a red shift of the crossover point and both a red shift and a reduction of the positive CD band of DNA near 275 nm. This phenomenon is similar to that caused by polylysine binding. These effects, however, are greatly diminished when histidine side chains in the model protein are deprotonated. The structure of already formed poly(Lys48, His52)·DNA complexes can be perturbed by changing the solution pH. However, the results suggest a readjustment of the complex to accommodate charge interactions rather than a full dissociation of the complex followed by reassociation between the model protein and DNA. 相似文献
15.
The unfolding of B. subtilis 5S RNA is examined by direct calorimetric measurement in the presence of various concentrations of Na+ and Mg2+. The composite differential scanning calorimetry (DSC) curve is analyzed into 3–5 individual two-state melting transitions. In the absence of added Na+ or Mg2+, the 5S RNA segments melt together at Tm = 40°C. Addition of Na+ stabilizes the molecular structure (Tm = 56°C) and widens the melting temperature range, so that up to five component transitions are observed. Addition of Mg2+ alone produces a very stable structure (Tm = 75°C) with highly cooperative melting. Finally, addition of both Na+ and Mg2+ produces the highest stability (Tm = 76°C). The results are interpreted according to hypothetical secondary and tertiary base-pairing schemes. The conformational changes demonstrated here may facilitate the movement of the protein synthesis machinery during RNA translation. 相似文献
16.
Leung YK Ho JW 《Comparative biochemistry and physiology. Part B, Biochemistry & molecular biology》1999,124(4):69-456
An in vitro study of effects of vitamin C-palmitate on the metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in rat microsomes was performed. A sensitive assay method has been developed for the detection of metabolites of NNK in microsomes. Only the reduced metabolite of NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-butanol (NNAL), was detected and measured in a time-course study. Vitamin C-palmitate enhanced the reduction of NNK in a concentration-dependent manner. The results indicate a significant increase in Vmax and Km in the presence of vitamin C. However, the rate of formation of NNAL at low substrate concentration varied. The ratio of Vmax to Km decreases. The results suggest that the kinetics are accounted for best by an uncompetitive activator binding model at low concentration of vitamin C. The uncompetitive binding model becomes sketchy at higher concentration of vitamin C. These observations infer that vitamin C loosely binds to the substrate-enzyme complex. Furthermore, the nature of the binding would facilitate the modulation of NNK biotransformation leading to the formation of NNAL. The results also show that vitamin C-palmitate is a potent activator of NNK reduction in rat liver microsomes. Thus, vitamin C-palmitate would mediate the metabolism of NNK through reduction. The resulting NNAL-glucuronide is more readily eliminated in urine. 相似文献
17.
Poly(Nε-trimethyl-L -lysine), [Lys(Me3)]n, and poly(Nδ-trimethyl-L -ornithine), [Orn(Me3)]n, in sodium dodecylsulfate do not assume the β-structure or α-helix, respectively, of their parent polymers. In 0.5M Ca(ClO4)2 both [Lys(Me3)]n and [Orn(Me3)]n are aggregated and display CD spectra indicative of a regular, perhaps helical, structure. For [Lys]n and [Lys(Me3)]n, the T1 of the α-hydrogens are 0.379 and 0.230 sec, respectively, indicating greater rigidity for [Lys(Me3)]n. The CD spectrum of [Lys(Me3)]n at pH 8 is more heat resistant than that of [Lys]n. It is suggested that apolar interactions are more important in the methylated polymers than in the parent polymers. 相似文献
18.
The conformation and dynamic structure of single-stranded poly(inosinic acid), poly(I), in aqueous solution at neutral pH have been investigated by nmr of four nuclei at different frequencies: 1H (90 and 250 MHz), 2H (13.8 MHz), 13C (75.4 MHz), and 31P (36.4 and 111.6 MHz). Measurements of the proton-proton coupling constants and of the 1H and 13C chemical shifts versus temperature show that the ribose is flexible and that base-base stacking is not very significant for concentrations varying from 0.04 to 0.10M in the monomer unit. On the other hand, the proton T1 ratios between the sugar protons, T1 (H1′)/T1 (H3′), indicate a predominance of the anti orientation of the base around the glycosidic bond. The local motions of the ribose and the base were studied at different temperatures by measurements of nuclear Overhauser enhancement (NOE) of protonated carbons, the ratio of the proton relaxation times measured at two frequencies (90 and 250 MHz), and the deuterium quadrupolar transverse relaxation time T2. For a given temperature between 22 and 62°C, the 13C-{1H} NOE value is practically the same for seven protonated carbons (C2, C8, C1′, C2′, C3′, C4′, C5′). This is also true for the T1 ratio of the corresponding protons. Thus, the motion of the ribose–base unit can be considered as isotropic and characterized by a single correlation time, τc, for all protons and carbons. The τc values determined from either the 13C-{1H} NOE or proton T1 ratios, T1(90 MHz)/T1(250 MHz), and/or deuterium transverse relaxation time T2 agree well. The molecular motion of the sugar-phosphate backbone (O-P-O) and the chemical-shift anisotropy (CSA) were deduced from T1 (31P) and 31P-{1H} NOE measurements at two frequencies. The CSA contribution to the phosphorus relaxation is about 12% at 36.4 MHz and 72% at 111.6 MHz, corresponding to a value of 118 ppm for the CSA (σ = σ∥ ? σ?). Activation energies of 2–6 kcal/mol for the motion of the ribose–base unit and the sugarphosphate backbone were evaluated from the proton and phosphorus relaxation data. 相似文献
19.
Melting parameters of 2U:1A complexes formed by polyuridylic acid [poly(U)] and three adenine dinucleotides, diribonucleoside monophosphonate ApA and diastereoisomers of dideoxyribonucleoside methyl phosphonate [(dApA)1 and (dApA)2], in 1M NaCl and at a number of dinucleotide concentrations were obtained from differential scanning microcalorimetric data and interpreted in terms of the theory of helix–coil equilibrium in oligonucleotide–polynucleotide systems. The apparent binding constant, 1/cm, at 39°C and melting temperatures, Tm, at 1 × 10?3 M dinucleotide concentration indicate the following order of thermodynamic stability of the complexes: 2 poly(U) · (dApA)2 (2.27 × 103M?1, 44.2°C) > 2 poly(U) · (dApA)1 (9.9 × 102M1, 39.2°C) > 2 poly(U) · (ApA) (5.9 × 102M?1, 35.8°C). Corresponding calorimetric enthalpies of melting, ΔHm: 13.5, 12.7, and 12.8 kcal/mol (UUA base triplets) were found to be considerably lower than the van't Hoff enthalpies, ΔHapp: 29.4, 16.2, and 16.2 kcal/mol, respectively, evaluated from the dependence of the melting temperatures on dinucleotide concentration. Self-association of dinucleotides and their simultaneous binding as monomers, dimers, and higher-order associated species is suggested as the most probable cause of the differences between ΔHm and ΔHapp values. The differences in thermodynamic properties of the complexes formed by (dApA)1 and (dApA)2 diastereoisomers are discussed in connection with their known conformational properties. The higher and essentially enthalpic stability of the 2 poly(U) · (dApA)2 complex correlates with a lower degree of intramolecular stacking of the (dApA)2 isomer. The hydrophobically enhanced strong self-association of the latter greatly influences the thermodynamics of its complex formation with poly(U) and results in ΔHapp/ΔHm = 2.3. 相似文献
20.
R. J. RITCHIE 《Plant, cell & environment》1996,19(11):1307-1316
Synechococcus R-2 (PCC 1942) actively accumulates sulphate in the light and dark. Intracellular sulphate was 1.35 ± 0.23 mol m?3 (light) and 0.894 ± 0.152 mol m?3 (dark) under control conditions (BG-11 media: pHo, 7.5; [SO42?]o, 0.304 mol m?3). The sulphate transporter is different from that found in higher plants: it appears to be an ATP-driven pump transporting one SO42?/ATP [ΔμSO42?i,o=+ 27.7 ± 0.24 kJ mol?1 (light) and + 24 ± 0.34 kj mol?1 (dark)]. The rate of metabolism of SO42?at pHo, 7.5 was 150 ± 28 pmol m?2 s?1 (n = 185) in the light but only 12.8 ± 3.6 pmol m?2 s?1 (n = 61) in the dark. Light-driven sulphate uptake is partially inhibited by DCMU and chloramphenicol. Sulphate uptake is not linked to potassium, proton, sodium or chloride transport. The alga has a constitutive over-capacity for sulphate uptake [light (n= 105): Km= 0.3 ± 0.1 mmol m?3, Vmax, = 1.8 ± 0.6 nmol m?2 s?1; dark (n= 56): Km= 1.4 ± 0.4 mmol m?3, Vmax= 41 ± 22 pmol m?2 s?1]. Sulphite (SO32?) was a competitive inhibitor of sulphate uptake. Selenate (SeO42?) was an uncompetitive inhibitor. 相似文献