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1.
Hiroshi Takashima Ayako Araki Keiko Takemoto Naokazu Yoshikawa Keiichi Tsukahara 《Journal of biological inorganic chemistry》2006,11(3):316-324
In order to understand the detailed mechanism of the stereoselective photoinduced electron-transfer (ET) reactions of zinc-substituted
myoglobin (ZnMb) with optically active molecules by flash photolysis, we designed and prepared new optically active agents,
such as N,N′-dimethylcinchoninium diiodide ([MCN]I2) and N,N′-dimethylcinchonidinium diiodide ([MCD]I2). The photoexcited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by [MCN]2+ and [MCD]2+ ions to form the radical pair of ZnMb cation (ZnMb·+) and reduced [MCN]·+ and [MCD]·+, followed by a thermal back ET reaction to the ground state. The rate constants (k
q) for the ET quenching at 25 °C were obtained as k
q(MCN)=(1.9±0.1)×106 M−1 s−1 and k
q(MCD)=(3.0±0.2)×106 M−1 s−1, respectively. The ratio of k
q(MCD)/k
q(MCN)=1.6 indicates that the [MCD]2+ preferentially quenches 3(ZnMb)*. The second-order rate constants (k
b) for the thermal back ET reaction from [MCN]·+ and [MCD]·+ to ZnMb·+ at 25 °C were k
b(MCN)=(0.79±0.04)×108 M−1 s−1 and k
b(MCD)=(1.0±0.1)×108 M−1 s−1, respectively, and the selectivity was k
q(MCD)/k
q(MCN)=1.3. Both quenching and thermal back ET reactions are controlled by the ET step. In the quenching reaction, the energy
differences of ΔΔH
≠(MCD–MCN) and ΔΔS
≠(MCD–MCN) at 25 °C were obtained as −1.1 and 0 kJ mol−1, respectively. On the other hand, ΔΔH
≠(MCD–MCN)=11±2 kJ mol−1 and TΔΔS
≠(MCD–MCN)=−10±2 kJ mol−1 were given in the thermal back ET reaction. The highest stereoselectivity of 1.7 for [MCD]·+ found at low temperature (10 °C) was due to the ΔΔS
≠ value obtained in the thermal back ET reaction.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
2.
Du S Wang Y Chen LZ Shi WJ Ren FD Li YX Wang JL Cao DL 《Journal of molecular modeling》2012,18(5):2105-2115
The explosive sensitivity upon the formation of molecule-cation interaction between the nitro group of 3,4-dinitropyrazole
(DNP) and H+, Li+, Na+, Be2+ or Mg2+ has been investigated using the B3LYP and MP2(full) methods with the 6-311++G** and 6-311++G(2df,2p) basis sets. The bond
dissociation energy (BDE) of the C3–N7 trigger bond has also been discussed for the DNP monomer and the corresponding complex. The interaction between
the oxygen atom of nitro group and H+ in DNP…H+ is partly covalent in nature. The molecule-cation interaction and bond dissociation energy of the C3–N7 trigger bond follow
the order of DNP…Be2+ > DNP…Mg2+ > DNP…Li+ > DNP…Na+. Except for DNP…H+, the increment of the trigger bond dissociation energy in comparison with the DNP monomer correlates well with the molecule-cation
interaction energy, natural charge of the nitro group, electron density ρ
BCP(C3–N7), delocalization energy E
(2) and NBO charge transfer. The analyses of atoms in molecules (AIM), natural bond orbital (NBO) and electron density shifts
have shown that the electron density of the nitro group shifts toward the C3–N7 trigger bond upon the formation of the molecule-cation
interaction. Thus, the trigger bond is strengthened and the sensitivity of DNP is reduced. 相似文献
3.
A series of crown ethers containing the azobenzene moiety incorporated into crowns of various sizes [Cr(O6), Cr(O7) and Cr(O8)] and their corresponding alkali metal cation (Li+, Na+, K+, Rb+) complexes have been studied theoretically. The density functional theory (DFT) method was employed to elucidate the stereochemical
structural natures and thermodynamic properties of all of the target molecules at the B3LYP/6-31 G(d) and LANL2DZ level for
the cation Rb+. The fully optimized geometries had real frequencies, thus indicating their minimum-energy status. In addition, the bond
lengths between the metal cation and oxygen atoms, atomic torsion angles and thermodynamic energies for complexes were studied.
Natural bond orbital (NBO) analysis was used to explore the origin of the internal forces and the intermolecular interactions
for the metal complexes. The calculated results show that the most significant interaction is that between the lone pair electrons
of electron-donating oxygens in the cis-forms of azobenzene crown ethers (cis-ACEs) and the LP* (1-center valence antibond lone pair) orbitals of the alkali-metal cations (Li+, Na+, K+ and Rb+). The electronic spectra for the cis-ACEs [cis-Cr(O6), cis-Cr(O7) and cis-Cr(O8)] are obtained by the time-dependent density functional theory (TDDFT) at the B3LYP/6-31 G(d) level. The spectra of the cis-isomers show broad π → π* (S0 → S2) absorption bands at 310–340 nm but weaker n → π* (S0 → S1) bands at 480–490 nm. The calculated results are in good agreement with the experimental results. 相似文献
4.
Latosińska JN Latosińska M Tomczak MA Seliger J Zagar V 《Journal of molecular modeling》2011,17(7):1781-1800
The quinolinol derivatives clioquinol (5-chloro-7-iodo-8-quinolinol, Quinoform) and cloxiquine (5-chloro-8-quinolinol) were
studied experimentally in the solid state via 35Cl NQR, 1H-17O and 1H-14N NQDR spectroscopies, and theoretically by density functional theory (DFT). The supramolecular synthon pattern of O–H···N
hydrogen bonds linking dimers and π–π stacking interactions were described within the QTAIM (quantum theory of atoms in molecules)
/DFT (density functional theory) formalism. Both proton donor and acceptor sites in O–H···N bonds were characterized using
1H-17O and 1H-14N NQDR spectroscopies and QTAIM. The possibility of the existence of O–H···H–O dihydrogen bonds was excluded. The weak intermolecular
interactions in the crystals of clioquinol and cloxiquine were detected and examined. The results obtained in this work suggest
that considerable differences in the NQR parameters for the planar and twisted supramolecular synthons permit differentiation
between specific polymorphic forms, and indicate that the more planar supramolecular synthons are accompanied by a greater
number of weaker hydrogen bonds linking them and stronger π···π stacking interactions. 相似文献
5.
Cell adhesion molecules are important for their various roles in many cellular events and responses. In the present study,
we have analyzed the roles played by cation–π interactions in the structural stability of adhesion molecules. These interactions are mainly formed by long-range contacts.
The occurrence of arginine is higher than lysine to form cation–π interactions. The secondary structure preferences of interacting residues are independent of amino acid class. Cation–π interactions might stabilize the interface between the terminus and core in this class of proteins. The results obtained
in the present study will be useful in understanding the contribution of cation–π interactions to the overall stability of adhesion proteins. 相似文献
6.
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
7.
Potassium (K+) and chloride (Cl−) are two essential elements for plant growth and development. While it is known that plants possess specific membrane transporters
for transporting K+ and Cl−, it remains unclear if they actively use K+-coupled Cl− cotransporters (KCC), as used in animals, to transport K+ and Cl−. We have cloned an Oryza sativa cDNA encoding for a member of the cation–Cl− cotransporter (CCC) family. Phylogenetic analysis revealed that plant CCC proteins are highly conserved and that they have
greater sequence similarity to the sub-family of animal K+–Cl− cotransporters than to other cation–Cl− cotransporters. Real-time PCR revealed that the O. sativa cDNA, which was named OsCCC1, can be induced by KCl in the shoot and root and that the expression level was higher in the leaf and root tips than in any
other part of the rice plant. The OsCCC1 protein was located not only in onion plasma membrane but also in O. sativa plasma membrane. The OsCCC1 gene-silenced plants grow more slowly than wild-type (WT) plants, especially under the KCl treatment regime. After 1 month
of KCl treatment, the leaf tips of the gene-silenced lines were necrosed. In addition, seed germination, root length, and
fresh and dry weight were distinctly lower in the gene-silenced lines than in WT plants, especially after KCl treatment. Analysis
of Na+, K+, and Cl− contents of the gene-silenced lines and WT plants grown under the NaCl and KCl treatment regimes revealed that the former
accumulated relatively less K+ and Cl− than the latter but that they did not differ in terms of Na+ contents, suggesting OsCCC1 may be involved in K+ and Cl− transport. Results from different tests indicated that the OsCCC1 plays a significant role in K+ and Cl− homeostasis and rice plant development. 相似文献
8.
The H···π and X (X = F, Cl, Br, I)···π interactions between hypohalous acids and benzene are investigated at the MP2/6-311++G(2d,2p) level. Four hydrogen-bonded
and three halogen-bonded complexes were obtained. Ab initio calculations indicate that the X···π interaction between HOX and
C6H6 is mainly electrostatically driven, and there is nearly an equal contribution from both electrostatic and dispersive energies
in the case of XOH–C6H6 complexes. Natural bond orbital (NBO) analysis reveals that there exists charge transfer from benzene to hypohalous acids.
Atom in molecules (AIM) analysis locates bond critical points (BCP) linking the hydrogen or halogen atom and carbon atom in
benzene. 相似文献
9.
The microsolvation of aminoethanol (AE) with one, two, three or four water molecules was investigated using a density functional
theory (DFT) approach. Quantum theory of atoms in molecules (QTAIM) analyses were employed to elucidate the hydrogen-bonding
characteristics of AE–(H2O)
n
(n = 1–4) complexes. The results showed that AE tends to break its intramolecular OHAE···NAE hydrogen bond (H-bond) upon microsolvation and form intermolecular H-bonds with water molecules, while complexes that retain
the intramolecular OHAE···NAE H-bond show reduced stabilities. The intermolecular H-bond that forms between the nitrogen atom of AE and the hydroxyl of
a water molecule is the strongest one for the most stable AE–(H2O)
n
(n = 1–4) complexes, and as n increases from 1 to 4 they grow stronger. The partial covalent character of this H-bond was confirmed by QTAIM analyses.
Many-body interaction analysis showed that the relaxation energies and two- and three-body energies make significant contributions
to the binding energies of the complexes. 相似文献
10.
Duan-lin Cao Fu-de Ren Ya-qing Feng Sheng-nan Liu Shu-sen Chen 《Journal of molecular modeling》2010,16(3):589-598
The DFT-B3LYP and MP2 methods with 6-311G** and 6-311++G** basis sets have been applied to study the complexation energies
of the host-guest complexes between the cone calix[4]arene and Li+ or Na+ on the B3LYP optimized geometries. A comparison of the complexation energies obtained from the MP2(full) with those from
MP2(fc) method is also carried out. The result shows that it is essential to introduce the diffuse basis set into the geometry
optimizations and complexation energy calculations of the alkali-metal cation-π interaction complexes of calix[4]arene, and
the D
e values show a maximum of 21.13 kJ mol−1 (14.45% of relative error) between the MP2(full)/6-311++G** and MP2(fc)/6-311++G** method. For Li+ cation, the complexation is mainly energetically stabilized by the lower rim/cation (namely O–Li+) interaction. However, binding energies and NBO analyses confirm that Na+ cation prefers to enter the calix[4]arene cavity and the cation-π interaction is predominant, which contradicts the previous
low-level theoretical studies. Furthermore, the complexation with Li+ is preferred over that with Na+ by at least 12.70 kJ mol−1 at MP2(full)/6-311++G**//B3LYP/6-311++G** level.
相似文献
11.
Elevation of the external potassium concentration induced a two-phase inward current in freshly isolated pyramidal hippocampal
neurons. This current was voltage-dependent and demonstrated strong inward rectification. The current consisted of a leakage
current and a time-dependent current (τ=40–50 msec at 21°C); the latter was designated asI
ΔK. As was shown earlier, K+ is a major charge carrier in the development of slow potassium-activated current. The pharmacological properties ofI
ΔK were studied using a patch-clamp technique.I
ΔK was completely blocked by external 10 mM TEA or 5 mM Ba2+ (IC50=480±90mM) and exhibited low sensitivity to extracellular Cs+ (2 mM). This current was not affected by 1 mM 4-aminopyridine and was insensitive to a muscarinic agonist, carbachol (50
μM), and to 1 mM extracellular Cd2+. Elevation of external Ca2+ from 2.5 mM to 10 mM did not changeI
ΔK. Our data indicate that the pharmacological properties ofI
ΔK differ from those of other voltage-gated potassium currents, but more specific blockers must be used to make this evidence
conclusive. 相似文献
12.
The hydrogen bonding interactions between noradrenaline (NA) and DMSO were studied with density functional theory (DFT) regarding
their geometries, energies, vibrational frequencies, and topological features of the electron density. The quantum theory
of atoms in molecules (QTAIM) and the natural bond orbital (NBO) analyses were employed to elucidate the hydrogen bonding
interaction characteristics in noradrenaline-DMSO complexes. The H-bonds involving the hydroxyls hydrogen in NA and the O
atom in DMSO are dominant intermolecular H-bonds and are stronger than other H-bonds involving the methyl hydrogen of DMSO
as a H-donor. The weak H-bonds also include a π H-bond which involves the benzene ring as a H-donor or H-acceptor. QTAIM identified
the weak H-bonds formed between the methyl hydrogen of DMSO and the N atom in NA in some complexes (AB5, AB6 and AB7), which cannot be further confirmed by NBO and other methods, so there are probably no interactions between hydrogen and
nitrogen atoms among these complexes. A good linear relationship between logarithmic electron density (lnρ
b
) at the bond critical point (BCP) and structural parameter (δR
H···Y) was found. The formations of new H-bonds in some complexes are helpful to strengthen the original intramolecular H-bond,
this is attributed to the cooperativity of H-bonds in complexes and can be learned from the structure results and the NBO
and QTAIM analyses. Analysis of various physically meaningful contributions arising from the energy decomposition procedures
show that the orbital interactions of H-bond is predominant during the formation of the complex, moreover, both the hydrogen
bonding interaction and the structural deformation are responsible for the stability of the complexes. 相似文献
13.
The responses of freshly isolated hippocampal pyramidal neurons to rapid, elevations of the external potassium concentration
([K+]
out
) were investigated using the whole-cell variation of a patch-clamp technique. An elevation of [K+]
out
induced a two-phase inward current at the membrane potentials more negative than the reversal potential for K ions. This
current consisted of a leakage, current and a time-dependent current (τ=40–50 msec at 21°C), the latter designated below asI
ΔK. It displayed first-order activation kinetics that showed neither voltage, nor concentration dependence. The amplitude of
this current was determined by the external K+ concentration and increased with hyperpolarization. Voltage dependence ofI
ΔK measured within the range from −20 to −120 mV was similar to that for inward rectifier. Activation ofI
ΔK was utterly dependent on Na+; substitution of extracellular Na+ with choline chloride almost completely depressedI
ΔK.I
ΔK was absent in the cells freshly dissociated from the nodosal and dorsal root ganglia. This suggests that this earlier unrecognized
current is instrumental in preserving densely packed hippocampal pyramidal neurons from sudden increases in [K+]
out
and following spontaneous over-excitation. It prevents the neurons from responding to K+-induced depolarizations by slowing down potassium influx. 相似文献
14.
Peña-Gallego A Rodríguez-Otero J Cabaleiro-Lago EM 《Journal of molecular modeling》2012,18(2):765-770
This work is focused in three topical subjects: intermolecular interactions, metal ions, and aromaticity. A comprehensive
MP2/6-31 + G* and B3LYP/6-31 + G* study of the influence of cation-π interactions on the aromatic character of phosphole was
conducted. For this purpose, the structures of complexes were optimized at both theoretical level and different magnetic properties
were evaluated. The main conclusion is the increase of the aromatic character of the phosphole when complexes with Li+, Be2+, and Al3+ are formed. 相似文献
15.
A. Bertl J. A. Anderson C. L. Slayman H. Sentenac R. F. Gaber 《Folia microbiologica》1994,39(6):507-509
Disruption of genes encoding endogenous transport proteins inSaccharomyces cerevisiae has facilitated the recent cloning, by functional expression, of cDNAs encoding K+ channels and amino acid transporters from the plantArabidopsis thaliana [1–4]. In the present study, we demonstrate in whole-cell patch clamp experiments that the inability oftrk1Δtrk2Δ mutants ofS. cerevisiae to grow on submillimolar K+ correlates with the lack of K+ inward currents, which are present in wild-type cells, and that transformation of thetrk1Δtrk2Δ double-deletion mutant withKAT1 fromArabidopsis thaliana restores this phenotype by encoding a plasma membrane protein that allows large K+ inward currents. Similar K+ inward currents are induced by transformation of atrk1 mutant withAKT1 fromA. thaliana.
This work was supported by a grant from theForschungsgemeinschaft (A.B.), TheU.S. Department of Energy (c.L.S.), The U.S. National Science Foundation (R.F.G.) Lisboa, Portugal. 相似文献
16.
Jerzy A. Zoladz Zbigniew Szkutnik Joanna Majerczak Krzysztof Duda 《European journal of applied physiology and occupational physiology》1998,78(4):369-377
The purpose of this study was to develop a method to determine the power output at which oxygen uptake (V˙O2) during an incremental exercise test begins to rise non-linearly. A group of 26 healthy non-smoking men [mean age 22.1 (SD
1.4) years, body mass 73.6 (SD 7.4) kg, height 179.4 (SD 7.5) cm, maximal oxygen uptake (V˙O2max) 3.726 (SD 0.363) l · min−1], experienced in laboratory tests, were the subjects in this study. They performed an incremental exercise test on a cycle
ergometer at a pedalling rate of 70 rev · min−1. The test started at a power output of 30 W, followed by increases amounting to 30 W every 3 min. At 5 min prior to the first
exercise intensity, at the end of each stage of exercise protocol, blood samples (1 ml each) were taken from an antecubital
vein. The samples were analysed for plasma lactate concentration [La]pl, partial pressure of O2 and CO2 and hydrogen ion concentration [H+]b. The lactate threshold (LT) in this study was defined as the highest power output above which [La−]pl showed a sustained increase of more than 0.5 mmol · l−1 · step−1. The V˙O2 was measured breath-by-breath. In the analysis of the change point (CP) of V˙O2 during the incremental exercise test, a two-phase model was assumed for the 3rd-min-data of each step of the test: X
i
=at
i
+b+ɛ
i
for i=1,2,…,T, and E(X
i
)>at
i
+b for i =T+1,…,n, where X
1, … , X
n
are independent and ɛ
i
∼N(0,σ2). In the first phase, a linear relationship between V˙O2 and power output was assumed, whereas in the second phase an additional increase in V˙O2 above the values expected from the linear model was allowed. The power output at which the first phase ended was called the
change point in oxygen uptake (CP-V˙O2). The identification of the model consisted of two steps: testing for the existence of CP and estimating its location. Both
procedures were based on suitably normalised recursive residuals. We showed that in 25 out of 26 subjects it was possible
to determine the CP-O2 as described in our model. The power output at CP-V˙O2 amounted to 136.8 (SD 31.3) W. It was only 11 W – non significantly – higher than the power output corresponding to LT. The
V˙O2 at CP-V˙O2 amounted to 1.828 (SD 0.356) l · min−1 was [48.9 (SD 7.9)% V˙O2
max
]. The [La−]pl at CP-V˙O2, amounting to 2.57 (SD 0.69) mmol · l−1 was significantly elevated (P<0.01) above the resting level [1.85 (SD 0.46) mmol · l−1], however the [H+]b at CP-V˙O2 amounting to 45.1 (SD 3.0) nmol · l−1, was not significantly different from the values at rest which amounted to 44.14 (SD 2.79) nmol · l−1. An increase of power output of 30 W above CP-V˙O2 was accompanied by a significant increase in [H+]b above the resting level (P=0.03).
Accepted: 25 March 1998 相似文献
17.
The Ca2+-activated maxi K+ channel was found in the apical membrane of everted rabbit connecting tubule (CNT) with a patch-clamp technique. The mean
number of open channels (NP
o
) was markedly increased from 0.007 ± 0.004 to 0.189 ± 0.039 (n= 7) by stretching the patch membrane in a cell-attached configuration. This activation was suggested to be coupled with the
stretch-activation of Ca2+-permeable cation channels, because the maxi K+ channel was not stretch-activated in both the cell-attached configuration using Ca2+-free pipette and in the inside-out one in the presence of 10 mm EGTA in the cytoplasmic side. The maxi K+ channel was completely blocked by extracellular 1 μm charybdotoxin (CTX), but was not by cytoplasmic 33 μm arachidonic acid (AA). On the other hand, the low-conductance K+ channel, which was also found in the same membrane, was completely inhibited by 11 μm AA, but not by 1 μm CTX. The apical K+ conductance in the CNT was estimated by the deflection of transepithelial voltage (ΔV
t
) when luminal K+ concentration was increased from 5 to 15 mEq. When the tubule was perfused with hydraulic pressure of 0.5 KPa, the ΔV
t
was only −0.7 ± 0.4 mV. However, an increase in luminal fluid flow by increasing perfusion pressure to 1.5 KPa markedly enhanced
ΔV
t
to −9.4 ± 0.9 mV. Luminal application of 1 μm CTX reduced the ΔV
t
to −1.3 ± 0.6 mV significantly in 6 tubules, whereas no significant change of ΔV
t
was recorded by applying 33 μm AA into the lumen of 5 tubules (ΔV
t
=−7.2 ± 0.5 mV in control vs.ΔV
t
=−6.7 ± 0.6 mV in AA). These results suggest that the Ca2+-activated maxi K+ channel is responsible for flow-dependent K+ secretion by coupling with the stretch-activated Ca2+-permeable cation channel in the rabbit CNT.
Received: 21 August 1997/Revised: 20 March 1998 相似文献
18.
The complexation behavior of nine polyether type podands with a varying number of oxygen donor atoms (4–10) towards the alkali
metal cations Li+, Na+ and K+ was studied by quantum chemical methods at the DFT-B3LYP level of theory using the all-electron split-valence 6-311++G(d,p)
basis set. The optimized structures of the complexes show a regular increase in the mean cation–oxygen distance with the coordination
number. OC–CO dihedral angles of the podand arms were also found to increase with the coordination number and with the size
of the cation. Maximum values for the number of strong cation–oxygen interactions (effective coordination numbers) were found
for each cation (six for Li+, seven for Na+ and eight for K+). The calculated values for thermodynamic parameters relative to the binding of free and solvated cations to the podands
allowed the assessment of binding constants in vacuum, in water and in dichloromethane. The estimated cation extraction constants
mimic the experimental extraction trends, but their values are much larger than experimental values. Scale factors were determined
to correct the values effectively. For each podand the ratios between the calculated extraction constants of Li+ (or Na+) and the corresponding ones for K+ (seen as extraction selectivities) compare acceptably with the corresponding experimental values. 相似文献
19.
S. Benini Marco Borsari S. Ciurli Alexander Dikiy M. Lamborghini 《Journal of biological inorganic chemistry》1998,3(4):371-382
Direct cyclic voltammetry and 1H NMR spectroscopy have been combined to investigate the electrochemical and spectroscopic properties of cytochrome c
553 isolated from the alkaliphilic soil bacterium Bacillus pasteurii. A quasi-reversible diffusion-controlled redox process is exhibited by cytochrome c
553 at a pyrolitic graphite edge microelectrode. The temperature dependence of the reduction potential, measured using a non-isothermal
electrochemical cell, revealed a discontinuity at 308 K. The thermodynamic parameters determined in the low-temperature range
(275–308 K;ΔS°′=–162.7±1.2 J mol–1 K–1, ΔH°′=–53.0±0.5 kJ mol–1, ΔG°′=–4.5±0.1 kJ mol–1, E°′=+47.0±0.6 mV) indicate the presence of large enthalpic and entropic effects, leading, respectively, to stabilization and
destabilization of the reduced form of cytochrome c
553. Both effects are more accentuated in the high-temperature range (308–323 K;ΔS°′=–294.1±8.4 J mol–1 K–1, ΔH°′=–93.4±3.1 kJ mol–1, ΔG°′=–5.8±0.6 kJ mol–1, E°′=+60.3±5.8 mV), with the net result being a slight increase of the standard reduction potential. These thermodynamic parameters
are interpreted using the compensation theory of hydration of biopolymers as indicating the extrusion, upon reduction, of
water molecules from the hydration sphere of the cytochrome. The low-T and high-T conformers differ by the number of water molecules in the solvation sphere: in the high-T conformer, the number of water molecules extruded upon reduction increases, as compared to the low-T conformer. The ionic strength dependence of the reduction potential at 298 K, treated within the frame of extended Debye-Hückel
theory, yields values of E
°′
(I=0)
=–25.4±1.4 mV, z
red=–11.3, and z
ox=–10.3. The pH dependence of the reduction potential at 298 K shows a plateau in the pH range 7–10 and an increase at more
acidic pH, allowing the calculation of pK
O=5.5 and pK
R=5.7, together with the estimate of the reduction potentials of completely protonated (+71 mV) and deprotonated (+58 mV) forms
of cytochrome c
553. 1H NMR spectra of the oxidized paramagnetic cytochrome c
553 indicate the presence of a His-Met axial coordination of the low-spin (S=1/2) heme iron, which is maintained in the temperature interval 288–340 K at pH 7 and in the pH range 4.8–10.0 at 298 K.
The temperature dependence of the hyperfine-shifted signals shows both Curie-type and anti-Curie-type behavior, with marked
deviations from linearity, interpreted as indicating the presence of a fast equilibrium between the low-T and high-T conformers, having slightly different heme electronic structures resulting from the T-induced conformational change. Increasing the NaCl concentration in the range 0–0.2 M causes a slight change of the 1H NMR chemical shifts of the hyperfine-shifted signals, with no influence on their linewidth. The calculated lower limit value
of the apparent affinity constant for specific ion binding is estimated as 5.2±1.1 M–1. The pH dependence of the isotropically shifted 1H NMR signals of the oxidized cytochrome displays at least one ionization step with pK
O=5.7. The thermodynamic and spectroscopic data indicate a large solvent-derived entropic effect as the main cause for the
observed low reduction potential of B. pasteurii cytochrome c
553.
Received: 9 January 1998 / Accepted: 8 April 1998 相似文献
20.
E. Kimura Takuya Ikeda Shin Aoki Mitsuhiko Shionoya 《Journal of biological inorganic chemistry》1998,3(3):259-267
As an extension of our earlier discoveries that ZnII-cyclen complex (1) (cyclen=1,4,7,10-tetraazacyclododecane) and ZnII-acridine-pendant cyclen complex ZnII-N-(9-acridin)ylmethyl-cyclen (3) are the first compounds to selectively recognize thymidine and uridine nucleosides in aqueous solution at physiological
pH, the interaction of these and a relevant complex, bis(ZnII-cyclen) (7), has been investigated with a series of polynucleotides, single-stranded poly(U) and poly(G), and double-stranded poly(A)·poly(U),
poly(dA)·poly(dT) and poly(dG)·poly(dC). These ZnII-cyclen complexes interact with the imide-containing nucleobases in the single-stranded poly(U), unperturbed by the presence
of the anionic phosphodiester backbone. The affinity constant of 1 for each N(3)-deprotonated uracil base in poly(U) is determined to be log K= 5.1 by a kinetic measurement, which is almost the same as log K=5.2 for the interaction of 1 with uridine. Thus, they disrupt the A-U (or A-T) hydrogen bonds to unzip the duplex of poly(A)·poly(U) or poly(dA)·poly(dT),
as demonstrated by lowering of the melting temperatures (T
m) of poly(A)·poly(U) and poly(dA)·poly(dT) in 5 mM Tris-HCl buffer (pH 7.6, 10 mM NaCl) with increase in their concentrations.
The order of the denaturing efficiency is well correlated with that of the 1 : 1 affinity constants for each complex with
uracil or thymine;7>3>1. The comparison of circular dichroism (CD) spectra for poly(A)·poly(U), poly(A), and poly(U) in the presence of 3 has revealed a structural change from poly(A)·poly(U) to two single strands, poly(A) and poly(U), caused by 3 binding exclusively to uracils in poly(U). On the other hand, the acridine-pendant cyclen complex 3, which earlier was found to associate with guanine by the ZnII coordinating with guanine N(7), in addition to the π-π stacking, interacts with guanine in the double helix of poly(dG)·poly(dC)
from outside and stabilized the double-stranded structure, as indicated by higher T
m.
Received: 31 December 1997 / Accepted: 23 February 1998 相似文献