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1.
H Strzelecka-Golaszewska B Nagy J Gergely 《Archives of biochemistry and biophysics》1974,161(2):559-569
The ultraviolet difference spectra of EDTA-induced denaturation of dithiothreitoltreated actin prepared with either Ca2+, Mn2+, or Mg2+ as the strongly bound cation showed no appreciable difference, nor could any difference be found in the change of optical rotation. However, at different wavelengths the changes in the spectra have different rates and these rates do differ significantly depending on the bivalent cation bound to G-actin. The nucleotide and the cation appear to be removed simultaneously and at the fastest rate; about 50–80% is released within 1 min. The spectral changes have two phases: a fast change whose detailed kinetics have not been investigated in this paper, followed by a slower rate with first-order kinetics. The changes of optical rotation follow a single-phase first-order kinetics. The rates depend on the divalent cation, the sequence being Mn2+ > Ca2+ > Mg2+. ATP release is partially reversible upon Ca2+ addition; the reversibility is diminished as the time of incubation with EDTA is increased. On rebinding of ATP and Ca2+, the spectral and optical rotatory changes are not reversed, but no further changes occur. Such an EDTA-treated actin is polymerizable after addition of Ca2+, and the G-actin obtained after polymerization and depolymerization shows the same spectral change on a second addition of EDTA as the original actin. On the basis of these observations a scheme is suggested for the denaturation of G-actin. 相似文献
2.
Protection of Pyruvate,Pi Dikinase from Maize against Cold Lability by Compatible Solutes 总被引:6,自引:1,他引:5
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Most C4 species are chilling sensitive and certain enzymes like pyruvate,Pi dikinase of the C4 pathway are also cold labile. The ability of cations and compatible solutes to protect maize (Zea mays) dikinase against cold lability was examined. The enzyme in desalted extracts at pH 8 from preilluminated leaves could be protected against cold lability (at 0°C) by the divalent cations Mn2+, Mg2+, and Ca2+. There was substantial protection by sulfate based salts but little protection by chloride based salts of potassium or ammonium (concentration 250 millimolar). The degree of protection against cold lability under limiting MgCl2 (5 millimolar) was pH sensitive (maximum protection at pH 8), but independent of ionic strength (up to 250 millimolar by addition of KCl). In catalysis Mg2+ is required and Mn2+ could not substitute as a cofactor. Several compatible solutes reduced or prevented the cold inactivation of dikinase (in desalted extracts and the partially purified enzyme), including glycerol, proline, glycinebetaine and trimethylamine-N-oxide (TMAO). TMAO and Mg2+ had an additive effect in protecting dikinase against cold inactivation. TMAO could largely substitute for the divalent cation and addition of TMAO during cold treatment prevented further inactivation. Cold inactivation was partially reversed by incubation at room temperature; with addition of TMAO reversal was complete. The temperature dependence of inactivation at pH 8 and 3 millimolar MgCl2 was evaluated by incubation at 2 to 17°C for 45 minutes, followed by assay at room temperature. At preincubation temperatures below 11°C there was a progressive inactivation which could be prevented by TMAO (450 millimolar). The results are discussed relative to possible effects of the solutes on the quaternary structure of this enzyme, which is known to dissociate at low temperatures. 相似文献
3.
Wheat leaf RNA polymerases. II. Kinetic characterization and template specificities of nuclear, chloroplast, and soluble enzymes 总被引:1,自引:0,他引:1
Further comparisons were made of DNA-dependent RNA polymerase (nucleotide triphosphate: RNA nucleotidyl transferase, EC 2.7.7.6) activities, partially purified from purified nuclear fragments and chloroplasts and from the soluble phase of young wheat leaves. All three preparations had the same cation specificities for maximal RNA polymerase activity (Mg2+ > Mn2+ > Ca2+) and showed an absolute dependence on an added divalent cation. All three preparations showed the same thermal stabilities and pH optima, very similar pH-activity profiles, and the same type of kinetics with ATP as substrate. Enzyme activities showed negative cooperativity with respect to ATP concentration; the high and low Km values for ATP were not significantly different for the three preparations. 相似文献
4.
Glutamine synthetase of pea leaves: divalent cation effects, substrate specificity, and other properties 总被引:1,自引:0,他引:1
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Purified glutamine synthetase from pea seedlings was most active with Mg2+ as the metal activator, but Mn2+ and Co2+ were 45 to 60% and 30 to 45% as effective, respectively, when assayed at the optimal pH for each cation. The Mg2+ saturation curve was quite sigmoid, and evidence indicates that MgATP is the active ATP substance. Co2+ also gave a sigmoidal saturation curve, but when Mn2+ was varied only slightly sigmoidal kinetics were seen. Addition of Mn2+, Ca2+, or Zn2+ at low concentrations sharply inhibited the Mg2+ -dependent activity, partially by shifting the pH optimum. Addition of Co2+ did not inhibit Mg2+-dependent activity. The nucleotide triphosphate specificity changed markedly when Co2+ or Mn2+ replaced Mg2+. Using the Mg2+-dependent assay, the Michaelis constant (Km) for NH4+ was about 1.9 × 10−3 M. The Km for l-glutamate was directly proportional to ATP concentration and ranged from 3.5 to 12.4 mm with the ATP levels tested. The Km for MgATP also varied with the l-glutamate concentration, ranging from 0.14 mm to 0.65 mm. Ethylenediaminetetracetic acid activated the enzyme by up to 54%, while sulfhydryl reagents gave slight activation, occasionally up to 34%. 相似文献
5.
Partial purification of the protein system controlling the breakdown of trehalose in baker's yeast. 总被引:2,自引:0,他引:2
P van Solingen J B van der Plaat 《Biochemical and biophysical research communications》1975,62(3):553-560
The inactive form of trehalase as well as its activating protein have been partially purified from resting cells of baker's yeast using (NH4)2SO4 fractionation and subsequent DEAE- and CM-cellulose column chromatography. For its activation by cyclic 3′,5′-AMP the system appeared to be dependent on the presence of ATP and a divalent cation such as Mg2+, Mn2+ or Co2+. No sensitivity towards the pH was observed in the range 6.0 – 7.5. The amount of active trehalase formed was determined by the preincubation time and the concentration of the proteins involved. The activating protein partly lost its dependence on cyclic 3′,5′-AMP during purification. The results presented suggest that this protein may be a protein kinase and that activation of trehalase is associated with phosphorylation of the enzyme protein. 相似文献
6.
Hana Votavová Dana Kucerová Jürgen Felsberg Jaroslav Sponar 《Journal of biomolecular structure & dynamics》2013,31(3):477-489
Abstract Circular dichroism spectroscopy, absorption spectroscopy, measurements of Tm values, sedimentation analysis and electron microscopy were used to study properties of calf thymus DNA in methanol-water mixtures as a function of monovalent cation (Na+ or Cs+) concentration and also in the presence of divalent cations Ca2+, Mg2+, and Mn2+. In the absence of divalent cations only slight conformational changes occured and no condensation and/or aggregation could be detected. The Tm values depend on the amount of methanol and on the nature and concentration of cations. In methanol-water mixtures higher thermal stability was observed in solutions containing Cs+ ions. Up to 40% (v/v) methanol the addition of divalent ions leads to DNA stabilization. At methanol concentration higher than 50% the presence of divalent cations causes DNA condensation and denaturation even at room temperature. The denaturation is reversible with respect to EDTA addition indicating that no separation of complementary strands occured and the resulting form of DNA is probably similar to the P form. DNA destacking appears to be a direct consequence of stronger cation binding by the condensed DNA in methanol-water mixtures. 相似文献
7.
The activation of muscle pyruvate kinase by divalent cations was studied by steady-state kinetics. Under experimental conditions the enzyme exhibits activation by Mg2+, Co2+, Mn2+, Ni2+, and Zn2+ in descending order of maximal velocity. Combinations of cations were also studied. A synergistic activation was observed with a fixed concentration of Mg2+ and varying concentrations of Mn2+ or of Co2+. This synergism indicates at least two roles for the cations for enzymatic activation and a differential specificity among the cations for the separate functions. Synergistic activation was also observed with fixed Co2+ and varying Mn2+. These results are consistent with a cation specifically required to activate the enzyme and a cation which serves as a cation-nucleotide complex which is a substrate for the reaction. The response observed suggests that Mn2+ is a better activator of the enzyme than is Mg2+, however, MgADP is a better substrate than is MnADP. The lack of a synergistic effect by Ni2+ or Zn2+ with Mg2+ suggests that Ni2+ and Zn2+ are poor activators either because they serve one catalytic function poorly but bind to that site tightly or they serve both catalytic functions poorly in contrast to Mg2+. These studies yield the first simple kinetic evidence that muscle pyruvate kinase, under catalytic conditions of the overall reaction, has a dual divalent cation requirement for activity. 相似文献
8.
The divalent cation requirements of NOS activity in bovine retina homogenate supernatant were investigated. Supernatants were assayed under standard conditions (in mM: EDTA 0.45, Ca2+ 0.25, Mg2+ 4.0). In order to investigate the enzyme's dependence on divalent cations, the tissue homogenate was depleted of di- and trivalent cations by passing it over a cation-exchange column (Chelex 100). Surprisingly, NOS activity was 50-100% higher in this preparation. However, addition of either EDTA (33 M) or EGTA (1 mM) almost fully inhibited NOS activity, suggesting a requirement for residual divalent metal cation(s). Phenanthroline or iminodiacetic acid at low concentrations had little effect on activity, suggesting no requirement for Fe2+, Zn2+ or Cu2+. Ca2+ had a moderate stimulatory effect, with an optimum activity around 0.01 mM. Mg2+ or Mn2+ had little effect at concentrations < 0.25 mM. However, in the presence of EDTA, Mn2+ or Ca2+ markedly stimulated NOS activity with the optimum at 0.1 mM. At high concentrations (> 0.1-0.2 mM), all divalent cations tested (Ba2+, Zn2+, Co2+, Mn2+, Mg2+, Ca2+), as well as La3+, dose-dependently inhibited NOS activity. We propose that retinal NOS requires low concentrations of naturally occurring divalent metal ions, most probably Ca2+, for optimal activity and is inhibited by high di- and trivalent metal concentrations, probably by competition with Ca2+. 相似文献
9.
31P-nmr has been used to investigate the specific interaction of three divalent metal ions, Mg2+, Mn2+, and Co+2, with the phosphate groups of DNA. Mg2+ is found to have no significant effect on any of the 31P-nmr parameters (chemical shift, line-width, T1, T2, and NOE) over a concentration range extending from 20 to 160 mM. The two paramagnetic ions, Mn2+ and Co2+, on the other hand, significantly change the 31P relaxation rates even at very low levels. From an analysis of the paramagnetic contributions to the spin–lattice and spin–spin relaxation rates, the effective internuclear metal–phosphorus distances are found to be 4.5 ± 0.5 and 4.1 ± 0.5 Å for Mn2+ and Co2+, respectively, corresponding to only 15 ± 5% of the total bound Mn2+ and Co2+ being directly coordinated to the phosphate groups (inner-sphere complexes). This result is independent of any assumptions regarding the location of the remaining metal ions which may be bound either as outer-sphere complexes relative to the phosphate groups or elsewhere on the DNA, possibly to the bases. Studies of the temperature effects on the 31P relaxation rates of DNA in the absence and presence of Mn2+ and Co2+ yielded kinetic and thermodynamic parameters which characterize the association and dissociation of the metal ions from the phosphate groups. A two-step model was used in the analysis of the kinetic data. The lifetimes of the inner-sphere complexes are 3 × 10?7 and 1.4 × 10?5 s for Mn2+ and Co2+, respectively. The rates of formation of the inner-sphere complexes with the phosphate are found to be about two orders of magnitude slower than the rate of the exchange of the water of hydration of the metal ions, suggesting that expulsion of water is not the rate-determining step in the formation of the inner-sphere complexes. Competition experiments demonstrate that the binding of Mg2+ ions is 3–4 times weaker than the binding of either Mn2+ or Co2+. Since the contribution from direct phosphate coordination to the total binding strength of these metal ion complexes is small (~15%), the higher binding strength of Mn2+ and Co2+ may be attributed either to base binding or to formation of stronger outer-sphere metal–phosphate complexes. At high levels of divalent metal ions, and when the metal ion concentration exceeds the DNA–phosphate concentration, the fraction of inner-sphere phosphate binding increases. In the presence of very high levels of Mg2+ (e.g., 3.1M), the inner-sphere ? outer-sphere equilibrium is shifted toward ~100% inner-sphere binding. A comparison of our DNA results and previous results obtained with tRNA indicates that tRNA and DNA have very similar divalent metal ion binding properties. A comparison of the present results with the predictions of polyelectrolyte theories is presented. 相似文献
10.
Amarjit Kaur Sarla P. Malhotra Randhir Singh 《Journal of plant biochemistry and biotechnology.》1996,5(1):55-58
NADP-isocitrate dehydrogenase from nodules of pigeonpea (Cajanus cajan L. cv UPAS-120) was partially purified to about 57 folds and its properties were studied. The enzyme showed an absolute requirement for a divalent cation which was fulfilled either by Mn+2 or Mg+2 and to a smaller extent by Co+2. The enzyme exhibited a sigmoidal response to increasing concentrations of Mn2+ (S0.5=0.3mM). The apparent Km values for isocitrate, NADP and Mg2+ were 21, 23 and 280 μM, respectively. It had an optimum pH of 8.0–8.2. The enzyme activity was not affected by various organic acids, amino acids and amides. NADH inhibited the activity non-competitively with respect to NADP. An apparent inhibition by ATP and ADP was due to chelation of divalent cation. NADPH acted competitively against NADP and non-competitively against isocitrate. Glutamate caused uncompetitive inhibition with respect to NADP and competitive against isocitrate. Kinetic studies suggested the reaction mechanism to be probably random sequential. Possible regulation of the enzyme activity in the nodules via cellular redox state and the levels of reaction products is discussed. 相似文献
11.
The nature of the interaction between polyacrylalc ion and several divalent cations, such as Cu2+, Mn2+, Zn2+, Ba2+ and Mg2+, was investigated using Raman spectroscopy. A specific Raman band characteristic of a carboxyl group is shifted upon addition of Cu2+. Zn2+ and Mn2+ to partially neutralized poly(acrylic acid). On the other hand. no frequency shift of the specific Raman band is observed on addition of Mg2+ and Ba2+*, though the intensity of the specific Raman band decreases with concentration of MgCl2. It is concluded from these Raman data that the interaction between polyacrylatc ion and Cu2+. Zn2+ or Mn2+ includes a specific interaction with bond formation, whereas in the case of Mg2+ and Ba2+, the electrostatic interaction is dominant. 相似文献
12.
Guanylate cyclase activity was determined in a 1000g particulate fraction derived from rabbit heart homogenates using Mg2+ or Mn2+ as sole cation in the presence and absence of Triton X-100. With Mg2+, very little guanylate cyclase activity could be detected in the original particulate fraction assayed with or without Triton, or in the particulate fraction treated with varying concentrations of Triton (detergent-treated mixture) prior to enzyme assay. However, the detergent-solubilized supernatants as well as the detergent-insoluble residues (pellets) derived from detergent-treated mixtures possessed appreciable Mg2+-supported enzyme activity. With Mn2+, significant enzyme activity was detectable in the original particulate fraction assayed without Triton. Much higher activity was seen in particulate fraction assayed with Triton and in detergent-treated mixtures; the supernatants but not the pellets derived from detergent-treated mixtures possessed even greater activity. The sum of enzyme activity in pellet and supernatant fractions greatly exceeded that of the mixture. When the pellets and supernatants derived from detergenttreated mixtures were recombined, measured enzyme activities were similar to those of the original mixture. With Mg2+ or Mn2+, the specific activity of guanylate cyclase in pellet and supernatant fractions varied considerably depending on the concentration of Triton used for treatment of the particulate fraction; treatment with low concentrations of Triton (0.2–0.7 μmol/mg protein) gave supernatants showing high activity whereas treatment with relatively greater concentrations of the detergent (>0.7 μmol/mg protein) gave pellets showing high activity. The relative distribution of guanylate cyclase in pellet and supernatant fractions expressed as a function of Triton concentration during treatment (of the particulate fraction) showed that 50 to 80% of the recovered enzyme activity remained in supernatants at low detergent concentrations whereas 50 to 80% of the recovered activity resided in the pellets at higher detergent concentrations. Inclusion of excess Triton in the enzyme assay medium did not alter the specific activity profiles and the relative distribution patterns of the cyclase in pellet versus supernatant fractions. The results demonstrate the inherent potential of cardiac particulate guanylate cyclase to utilize Mg2+ in catalyzing the synthesis of cyclic GMP. However, it appears that some factor(s) endogenous to the cardiac particulate fraction severely impairs the expression of Mg2+-dependent activity; Mn2+-dependent activity is also affected by such factor(s) but apparently less severely. Further, the results suggest that previously reported activities of cardiac particulate guanylate cyclase, despite being assayed with Mn2+ and in the presence of Triton X-100, represent underestimation of what otherwise appears to be a highly active enzyme system capable of utilizing physiologically relevant divalent cation such as Mg2+. 相似文献
13.
The aminoacylation of tRNA catalysed by valyl-tRNA synthetase (EC 6.1.1.9) and isoleucyl-tRNA synthetase (EC 6.1.1.5) fromMycobacterium smegmatis is dependent on the presence of divalent metal ions. Polyamines alone, in the absence of metal ions, do not bring about aminoacylation.
In the presence of suboptimal concentrations of Mg2+, polyamines significantly stimulate the reaction. Of the cations tested, only Mn2+, Co2+ and Ca2+ can partially substitute for Mg2+ in aminoacylation, and spermine stimulates aminoacylation in the presence of these cations also. At neutral pH, spermine
deacylates nonenzymatically aminoacyl tRNA. AMP and pyrophosphate-dependent enzymatic deacylation of aminoacyl-tRNA (reverse
reaction) is also stimulated by spermine. The inhibitory effect of high concentration of KC1 on aminoacylation is counteracted,
by spermine. The low level of activity between pH 8.5–9.0 at 1.2 mM Mg2+ is restored to normal level on the addition of spermine. The inhibitory effect of high pH on aminoacylation in the presence
of low concentration of Mg2+ is also prevntedvby spemine. 相似文献
14.
Properties of partially purified NADP-malic enzyme (EC 1.1.1.40) from glumes of developing wheat grains were examined. The pH optimum for enzyme activity was influenced by malate and shifted from 7.3 to 7.6 when the concentration of malate was increased from 2 to 10 mM. The Km values, at pH 7.3, for various substrates were: malate, 0.76 mM; NADP, 20 μM and Mn2+, 0.06 mM. The requirement of Mn2+ cation for enzyme activity could be partially replaced by Mg2+ or Co2+. Mn2+ dependent enzyme activity was inhibited by Pb2+, Ni2+, Hg2+, Zn2+, Cd2+, Al3+ and Fe3+. During the reaction, substrate molecules (malate and NADP) reacted with enzyme sequentially. Activity of malic enzyme was inhibited by products of the reaction viz pyruvate, HCO3? and NADPH2. At a limiting fixed concentration of NADP, these products induced a positive cooperative response to increasing concentrations of malate. 相似文献
15.
Pyridoxine kinase purified from sheep liver was found to consist of a single polypeptide chain with a molecular weight of 60,000 as determined by gel filtration, sedimentation equilibrium ultracentrifugation, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric pH of the enzyme was 5.1, and the pH optimum was between 5.5 and 6.0. The enzyme required divalent cations for activity. At cation concentrations of 80 μm, the enzyme activity with each cation was in the order of Zn2+ > Mn2+ > Mg2+. At cation concentrations of 400 μm, the enzyme activity with each cation was in the order of Mn2+ > Zn2+ > Mg2+. Excess free divalent cation inhibited the enzyme. Pyridoxine kinase also required monovalent cations. The enzyme activation was greatest with K+, then Rb+ and NH4+, whereas the enzyme had very little activity with Na+, Li+, or Cs+. Na+ did not interfere with the activation by K+. The activation of the kinase by K+, NH4+, and Rb+ followed Michaelis-Menten kinetics, and the apparent Km values for the cations were 8.9, 3.7, and 5.3 mm, respectively. Increasing the potassium concentration lowered the apparent Km value of the enzyme for pyridoxine and had little or no effect on the Km for ZnATP2? or the V of the kinase-catalyzed reaction. 相似文献
16.
J Levilliers F Lecot J Pairault 《Biochemical and biophysical research communications》1978,84(3):727-735
Guanylate cyclase activity in Triton X-100-treated plasma membranes exhibits sigmoidal profiles as a function of MgGTP, irrespective of the excess Mg2+(1) concentration. In contrast, at low excess Mn2+ (0.2 mM) the activity vs substrate (MnGTP) concentration profile corresponds to a michaelian behaviour. In addition the enzyme does not require similar excess Mn2+ and Mg2+ for optimal activity at various substrate concentrations. Moreover, low concentrations of Ca2+ are capable of stimulating guanylate cyclase activity with Mg2+ as the major divalent cation. 相似文献
17.
The relation that exist between the Pi-PPi exchange reaction and pyrophosphate hydrolysis by the membrane-bound pyrophosphatase of chromatophores ofRhodospirillum rubrum was studied. The two reactions have a markedly different requirement for pH. The optimal pH for hydrolysis was 6.5 while the Pi-PPi exchange reaction was at 7.5; the pH affects mainly theK
m
of Mg2+ or Pi for the enzyme; Mn2+ and Co2+ support the Pi-PPi exchange reaction partially (50%), but the reaction is slower than with Mg2+; other divalent cations like Zn2+ or Ca2+ do not support the exchange reaction. In the hydrolytic reaction, Zn2+, at low concentration, substitutes for Mg2+ as substrate, and Co2+ also substitutes in limited amount (50%). Other cations (Ca2+, Cu2+, Fe2+, etc.) do not act as substrates in complex with PPi. The Zn2+ at high concentrations inhibited the hydrolytic reaction, probably due to uncomplexed free Zn2+. In the presence of high concentration of substrate for the hydrolysis (Mg-PPi) the divalent cations are inhibitory in the following order: Zn2+>Mn2+>Ca2+Co2+>Fe2+>Cu2+>Mg2+. The data in this work suggest that H+ and divalent cations in their free form induced changes in the kinetic properties of the enzyme. 相似文献
18.
The modulation of many enzymatic reactions involved in the metabolism of nucleotide phosphates such as ATP often require divalent metal ions. In the present study reverse-phase high-performance liquid chromatography (HPLC) was used to study the chelation of divalent metal ions, such as Mn2+, Mg2+, and Ca2+, by ATP. The results of our study using radiolabeled [45Ca] showed that the metal-ATP chelate formed in solution was retained longer than the metal-free ATP due to the nonpolar groups on the column packing. Recovery of the two forms of ATP showed that the [45Ca] coeluted exclusively with the ATP-metal chelate. Other experiments showed that the retention time of the chelated form of the ATP was unaffected by eluent flow rate, but was affected by eluant pH and methanol concentration. The amount of ATP in the chelated form was found to be dependent on the amount of the metal in solution and that under appropriate conditions, i.e., with 0.1 m
CaCl2 in the mobile phase, on the divalent cation as well. Thus, we found that in terms of effectiveness in chelate formation, the metal ions were Ca2+ > Mg2+ > Mn2+. Recovery of the chelate and its reanalysis by HPLC revealed that the complex had dissociated. The chelate could be reformed by restoring the metal concentration to its original value and dissociated again by the addition of EDTA. The resolution of the ATP in a metal chelated form from the ATP in an unchelated form is discussed in terms of the stability of these chelates and the role of the hydrophobic groups of the column packing used in the reverse-phase HPLC in enhancement of this stability. 相似文献
19.
(1) The structural organization of aqueous dispersions of 1,2-dioleoylphosphatidic acid has been investigated by freeze-fracture electron microscopy in relation to variations in pH, divalent cations and the local anaesthetic chlorpromazine. (2) In the pH range 4–8 in the presence of 100 mM Na+, dioleoylphosphatidic acid is organized in bilayers. (3) At pH 6 and not at pH 4 and 8.5 addition of Ca2+, Mg2+, Mn2+ and chlorpromazine results in the formation of the hexagonal HII phase. (4) Ca2+ and chlorpromazine addition to mixed phosphatidylcholine-dioleoylphosphatidic acid bilayers at pH 6 results in the formation of lipidic particles. 相似文献
20.
Y A Shin 《Biopolymers》1973,12(11):2459-2475
Changes in the conformation of poly(G), poly(C), poly(U), and poly(I) in the presence of divalent metal ions Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Cd2+, and Zn2+ have been measured by means of ORD and u.v. spectra. Mg2+ and Ca2+ ions stabilize helical structures of all the polynucleotides very effectively at concentrations several orders of magnitude lower than the effective concentration of Na+ion. Cu2+ and Cd2+ destabilize the helical structure of polynucleotides to form random coils. Zn2+, Ni2+, Co2+, and Mn2+ions do not behave in such a clear-cut manner: they selectively stabilize some ordered structures, while destabilizing others, depending on the ligand strength of the nucleotide base as well as the preferred conformation of that polynucleotide. 相似文献