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1.
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
2.
P. V. Ershov O. V. Gnedenko A. A. Molnar A. V. Lisitsa A. S. Ivanov A. I. Archakov 《Biochemistry (Moscow) Supplemental Series B: Biomedical Chemistry》2012,6(1):94-97
The analysis of kinetic and thermodynamic parameters of binding of peptide and nonpeptide dimerization inhibitors of HIV protease
(HIVp) to the enzyme monomers immobilized on an optical chip has been studied by surface plasmon resonance. The molecular
interactions were investigated at different inhibitor concentrations (0–80 μM) and temperatures (15–35°C). Determination of
kinetic (k
on, k
off), equilibrium (K
d), and thermodynamic (ΔG, ΔH, and -TΔS) has shown that both inhibitors are characterized by similar interaction parameters and the entropic term (-TΔS) of about −20 kcal/mol is the main driving force for the HIVp complex formation with the inhibitors, while the positive value
(14 kcal/mol) of the enthalpic term (ΔH) counteracted the complex formation. 相似文献
3.
《Phytochemistry》2012
The vibrational circular dichroism (VCD) spectra of perezone and dihydroperezone measured from CDCl3 solutions were quite similar, suggesting analogous conformations for both molecules. Their absolute configurations were confirmed by comparison of the experimental VCD spectrum of each compound with curves generated from theoretical calculations using density functional theory (DFT) at the B3LYP/DGDZVP level of theory taking into account their conformational mobility. Conformational analysis of the 8-(R) enantiomer showed 19 low energy conformers in a 2.4 kcal/mol energy range, while for 8-(R), with the saturated side alkyl chain, 34 conformers were considered in the first 2 kcal/mol. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field, all MMFF94 conformers were geometrically optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by reoptimization and calculations of their vibrational frequencies at the B3LYP/DGDZVP level. Good agreement between the theoretical 8-(R) enantiomers and experimental VCD curves were observed for both. 相似文献
4.
Development of optical criteria to discriminate various types of highly turbid lake waters 总被引:1,自引:0,他引:1
Deyong Sun Yunmei Li Qiao Wang Chengfeng Le Changchun Huang Kun Shi 《Hydrobiologia》2011,669(1):83-104
Our aim was to refine the optical classification of turbid waters in order to improve the performance of water color algorithms.
Bio-optical measurements and sampling of optically active substances were performed in highly turbid lakes Taihu, Chaohu,
and Dianchi, and in Three Gorges reservoir in China. Based on strong correlations between trough depths of remote sensing
reflectance (R
rs(λ)) near 680 nm (denoted as TD680) and the ratios of inorganic suspended matter (ISM) to total suspended matter (TSM) concentrations, an empirical model was
developed for water classification. In the 400–900 nm spectral range, different correlations between R
rs(λ), TSM and chlorophyll a (Chla) concentrations indicate discrepancies among the following ISM/TSM ranges: ISM/TSM ≤ 0.5, 0.5 < ISM/TSM < 0.8, and ISM/TSM ≥ 0.8.
Corresponding findings support an important conclusion that only high ISM/TSM ratios, usually above 0.5, and using the more
sensitive and stable near infrared spectral range (730–820 nm), can assure good performances of the TSM remote sensing algorithms.
Meanwhile, the particulate absorption a
p(λ) and scattering b
p(λ) were strongly influenced by different ranges of ISM/TSM ratios. Typically the a
p(675) peaks became more and more vague as ISM/TSM increased, and the b
p(λ) values first decreased and then increased with a marked inflexion at ISM/TSM ≈ 0.5. The TD680 threshold values were derived to discriminate three types of turbid waters, i.e., Type 1 (TD680 ≥ 0.0082 sr−1), Type 2 (0.0082 sr−1 > TD680 > 0 sr−1), and Type 3 (TD680 ≤ 0 sr−1). This study provides a promising tool for identifying various types of highly turbid waters, and is significant for the
development of class-based algorithms of water color remote sensing. 相似文献
5.
Kozlowski PM Kamachi T Kumar M Yoshizawa K 《Journal of biological inorganic chemistry》2012,17(2):293-300
Density functional theory analysis was performed to elucidate the impact of one-electron reduction upon the initial step of
adenosylcobalamin-dependent enzymatic catalysis. The transition state (TS) corresponding to the Co–C bond cleavage and subsequent
hydrogen abstraction from the substrate was located. The intrinsic reaction coordinate calculations predicted that the reaction
consisting of Co–C5′ bond cleavage in [CoIII(corrin•)]–Rib (where Rib is ribosyl) and hydrogen-atom abstraction from the CH3–CH2–CHO substrate occurs in a concerted fashion. The computed activation energy barrier of the reaction (15.0 kcal/mol) was lowered
by approximately 54.5% in comparison with the reaction involving the positively charged cofactor model (Im–[CoIII(corrin)]–Rib+, where Im is imidazole; energy barrier = 33.0 kcal/mol). The Im base was detached during the TS search in the reaction involving
the one-electron-reduced analogue. Thus, to compare the energetics of the two reactions, the axial Im ligand detachment energy
for the Im–[CoIII(corrin•)]–Rib model was computed [7.6 kcal/mol (gas phase); 4.6 kcal/mol (water)]. Consequently, the effective activation energy
barrier for the reaction mediated by the Im-off [CoIII(corrin•)]–Rib was estimated to be 22.6 kcal/mol, which implied an overall 31.5% reduction in the energetic demands of the reaction.
Considering that the lengthened Co–Naxial bond has been observed in X-ray crystal structure studies of B12-dependent mutases, the catalytic impact induced by one-electron reduction of the cofactor is expected to be higher in the
presence of the enzymatic environment. 相似文献
6.
Theoretical studies of an unsymmetrical calix[4]-crown-5-N-azacrown-5 (1) in a fixed 1,3-alternate conformation and the complexes 1·K+(a), 1·K+(b), 1·K+(c) and 1·K+K+ were performed using density functional theory (DFT) at the B3LYP/6-31G* level. The fully optimized geometric structures
of the free macroligand and its 1:1 and 1:2 complexes, as obtained from DFT calculations, were used to perform natural bond
orbital (NBO) analysis. The two main types of driving force metal–ligand and cation–π interactions were investigated. NBO
analysis indicated that the stabilization interaction energies (E
2) for O…K+ and N…K+ are larger than the other intermolecular interactions in each complex. The significant increase in electron density in the
RY* or LP* orbitals of K+ results in strong host–guest interactions. In addition, the intermolecular interaction thermal energies (ΔE, ΔH, ΔG) were calculated by frequency analysis at the B3LYP/6-31G* level. For all structures, the most pronounced changes in the
geometric parameters upon interaction are observed in the calix[4]arene molecule. The results indicate that both the intermolecular
electrostatic interactions and the cation–π interactions between the metal ion and π orbitals of the two pairs that face the
inverted benzene rings play a significant role. 相似文献
7.
S. C. Barbosa E. M. Cilli Luis G. Dias Rodrigo G. Stabeli P. Ciancaglini 《Amino acids》2011,40(1):135-144
Cyclic peptides isolated from the plants of the Euphorbiaceae family have been largely studied due to their rigid conformation, which is considered significant for biologic activity.
The peptide Labaditin (L0) and its open chain analogs (L1) were synthesized by the solid-phase peptide synthesis technique (Fmoc/tBu), and purified to elucidate its interaction with
membrane models. A shift in λmax emission and Stern–Volmer constants values indicate that both tryptophans migrate to a more apolar environment, with L1 decreasing less than L0. A circular dichroism (CD) study revealed that L0 was kept unstructured in aqueous media as much as in the presence of dipalmitoilphosphatidylcholine liposomes. The thermodynamic
studies by differential calorimetry (DSC) show a ΔH increase (50 and 18 kcal/mol, for L0 and L1, respectively) with peptide concentrations, which is indicative of lipids associating with peptides, resulting in the inability
of the lipids to participate in the main transition. Therefore, all CD, DSC, and fluorescence data suggest a greater L0 membrane insertion. A probable mechanism for Labaditin interaction is based initially on the hydrophobic interaction of the
peptide with the lipid membrane, conformational change, peptide adsorption on the lipid surface, and internalization process.
Peptide’s antibacterial effect was also evaluated and revealed that only L0 showed reduction in viability in Gram-positive bacteria while no effects to the Gram-negative. 相似文献
8.
The scattering spectrum properties of highly turbid and eutrophic inland case 2 water from Taihu Lake were studied during
three cruises from 2006 to 2007. The scattering [b
p(λ)] and backscattering [b
bp(λ)] coefficients and the backscattering probability (B) for Taihu Lake were found to show a clear spectral dependence, and this dependence was well simulated by a power-law function.
This dependence, however, became weak when algae dominated the sample points. The mean values of the power-law index for b
p(λ), v, in Oct 2006, Mar 2007 and Nov 2007 were −0.6712, −0.8129 and −0.7600, respectively. To interpret the spectral characteristics
and mechanisms of b
p(λ) and b
bp(λ), water samples were collected simultaneously for the biogeochemical characterization of suspended particles. The average
values of the specific scattering coefficients for total suspended matter, inorganic suspended matter (ISPM) and organic suspended
matter (OSPM) were 0.634 (550 nm), 1.057 (532 nm), and 0.396 g m−2 (532 nm), respectively. The power-law index of b
bp(λ) (Y) was significantly related to ISPM/OSPM and b
bp(532 nm), but only weakly related to the particle size distribution index. The mean (spatial and wavelength) values of B in Oct 2006, Mar 2007, and Nov 2007 were 0.0108, 0.0138, and 0.0125, respectively. B decreases with increasing ISPM concentration because of the large contribution of ISPM to b
b(λ) and the strong restraint on b
bp(λ) caused by the multi-scattering effect under high-turbidity conditions. 相似文献
9.
Victoria J. Stokes Michael D. Morecroft James I. L. Morison 《Trees - Structure and Function》2010,24(2):297-306
The seasonal trends in water use efficiency of sun and shade leaves of mature oak (Quercus robur) and sycamore (Acer pseudoplatanus) trees were assessed in the upper canopy of an English woodland. Intrinsic water use efficiency (net CO2 assimilation rate/leaf conductance, A/g) was measured by gas exchange and inferred from C isotope discrimination (δ13C) methods. Shade leaves had consistently lower δ13C than sun leaves (by 1–2‰), the difference being larger in sycamore. Buds had distinct sun and shade isotopic signatures
before bud break and received an influx of 13C-rich C before becoming net autotrophs. After leaf full expansion, δ13C declined by 1–2‰ gradually through the season, emphasising the importance of imported carbon in the interpretation of leaf
δ13C values in perennial species. There was no significant difference between the two species in the value of intrinsic water
use efficiency for either sun or shade leaves. For sun leaves, season-long A/g calculated from δ13C (72–78 μmol CO2 [mol H2O]−1) was 10–16% higher than that obtained from gas exchange and in situ estimates of leaf boundary layer conductance. For shade
leaves, the gas exchange–derived values were low, only 10–18% of the δ13C-derived values. This is ascribed to difficulties in obtaining a comprehensive sample of gas exchange measurements in the
rapidly changing light environment. 相似文献
10.
trans -[PtCl4(NH3)(thiazole)] (1), trans-[PtCl4(cha)(NH3)] (2), cis-[PtCl4(cha)(NH3)] (3) (cha =cyclohexylamine), and cis-[PtCl4(NH3)2] (4) has been investigatedat 25 °C in a 1.0 M aqueous medium at pH 2.0–5.0 (1) and 4.5–6.8 (2–4) using stopped-flow spectrophotometry. The redox reactions follow the second-order rate law , where k is a pH-dependent rate constant and [GSH]tot the total concentration of glutathione. The reduction takes place via parallel reactions between the platinum(IV) complexes
and the various protolytic species of glutathione. The pH dependence of the redox kinetics is ascribed to displacement of
these protolytic equilibria. The thiolate species GS− is the major reductant under the reaction conditions used. The second-order rate constants for reduction of compounds 1–4 by GS− are (1.43±0.01)×107, (3.86±0.03)×106, (1.83±0.01)×106, and (1.18±0.01)×106 M−1 s−1, respectively. Rate constants for reduction of 1 by the protonated species GSH are more than five orders of magnitude smaller. The mechanism for the reductive elimination
reactions of the Pt(IV) compounds is proposed to involve an attack by glutathione on one of the mutually trans coordinated chloride ligands, leading to two-electron transfer via a chloride-bridged activated complex. The kinetics results
together with literature data indicate that platinum(IV) complexes with a trans Cl-Pt-Cl axis are reduced rapidly by glutathione as well as by ascorbate. In agreement with this observation, cytotoxicity
profiles for such complexes are very similar to those for the corresponding platinum(II) product complexes. The rapid reduction
within 1 s of the platinum(IV) compounds with a trans Cl-Pt-Cl axis to their platinum(II) analogs does not seem to support the strategy of using kinetic inertness as a parameter
to increase anticancer activity, at least for this class of compounds.
Received: 8 December 1999 / Accepted: 15 February 2000 相似文献
11.
Priya R Tadwal VS Roessle MW Gayen S Hunke C Peng WC Torres J Grüber G 《Journal of bioenergetics and biomembranes》2008,40(4):245-255
The first low resolution solution structure of the soluble domain of subunit b (b
22–156) of the Escherichia coli F1FO ATPsynthase was determined from small-angle X-ray scattering data. The dimeric protein has a boomerang-like shape with a
total length of 16.2 ± 0.3 nm. Fluorescence correlation spectroscopy (FCS) shows that the protein binds effectively to the
subunit δ, confirming their described neighborhood. Using the recombinant C-terminal domain (δ91–177) of subunit δ and the C-terminal peptides of subunit b, b
120–140 and b
140–156, FCS titration experiments were performed to assign the segments involved in δ–b assembly. These data identify the very C-terminal tail b
140–156 to interact with δ91–177. The novel 3D structure of this peptide has been determined by NMR spectroscopy. The molecule adopts a stable helix formation
in solution with a flexible tail between amino acid 140 to 145. 相似文献
12.
The interspecific variability of sunlit leaf carbon isotope composition (δ13C), an indicator of leaf intrinsic water-use efficiency (WUE, CO2 assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainforest stands
in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic
conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common
to the three stands. Mean species δ13C varied over a 6.0‰ range within each stand, corresponding to WUE varying over about a threefold range. Species occurring
in at least two stands displayed remarkably stable δ13C values, suggesting a close genetic control of species δ13C. Marked differences in species δ13C values were found with respect to: (1) the leaf phenology pattern (average δ13C=–29.7‰ and –31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance
defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic
species exhibited more negative δ13C values (average δ13C=–30.5‰) (i.e. lower WUE) than hemitolerant species (–29.3‰). However, tolerant species (–31.4‰) displayed even more negative
δ13C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The
negative relationship found between species δ13C and midday leaf water potential (Ψwm) suggests that low δ13C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination (Δ
A
) calculated from the basal area-weighed integral of the species δ13C values was similar in the three stands (average Δ
A
=23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the
three different shade-tolerance types among stands.
Received: 30 November 1999 / Accepted: 23 March 2000 相似文献
13.
The interaction between benzophenone (BP) and bovine serum albumin (BSA) was investigated by the methods of fluorescence spectroscopy
combined with UV–Vis absorption and circular dichroism (CD) measurements under simulative physiological conditions. The experiment
results showed that the fluorescence quenching of BSA by BP was resulted from the formation of a BP–BSA complex and the corresponding
association constants (K
a) between BP and BSA at four different temperatures had been determined using the modified Stern–Volmer equation. The enthalpy
change (ΔH) and entropy change (ΔS) were calculated to be –43.73 kJ mol−1 and −53.05 J mol−1 K−1, respectively, which suggested that hydrogen bond and van der Waals force played major roles in stabilizing the BP–BSA complex.
Site marker competitive experiments indicated that the binding of BP to BSA primarily took place in site I (sub-domain IIA).
The conformational investigation showed that the presence of BP decreased the α-helical content of BSA and induced the slight unfolding of the polypeptides of protein, which confirmed some micro-environmental
and conformational changes of BSA molecules. 相似文献
14.
A series of Ru(II) polypyridyl complexes [Ru(bpy)2(ptdb)](ClO4)2 (1), [Ru(bpy)2(ptda)](ClO4)2 (2) and [Ru(bpy)2(ptdp)](ClO4)2 (3) with asymmetric intercalative ligands have been synthesized and characterized by EA, mass spectra, 1H NMR and cyclic voltammetry. The crystal structure of complex 1 has been determined. The DNA-binding properties of the complexes were investigated by absorption titration, luminescence spectroscopy and viscosity measurements. The experimental results suggest that all these complexes bind to DNA in an intercalation mode. The results also show that the order of DNA-binding affinities (A) of this series of complexes is A(1) < A(2) < A(3). It is further confirmed that a ligand planarity of the complexes is a very important factor in affecting the DNA-binding behaviors of such complexes. Theoretical studies for these complexes were also carried out with the density functional theory (DFT) method. The trend in the DNA-binding affinities of this series of complexes can be reasonably explained by the synthetical considerations of the calculated planarity of intercalative ligands, some frontier molecular orbital energies of the complexes and the planarity area (S) of the intercalative ligands. 相似文献
15.
We report the first study of the relation between the wavelength of maximum absorbance (λmax) and the photoactivation energy (E
a) in invertebrate visual pigments. Two populations of the opossum shrimp Mysis relicta were compared. The two have been separated for 9,000 years and have adapted to different spectral environments (“Sea” and
“Lake”) with porphyropsins peaking at λmax=529 nm and 554 nm, respectively. The estimation of E
a was based on measurement of temperature effects on the spectral sensitivity of the eye. In accordance with theory (Stiles
in Transactions of the optical convention of the worshipful company of spectacle makers. Spectacle Makers’ Co., London, 1948), relative sensitivity to long wavelengths increased with rising temperature. The estimates calculated from this effect are
E
a,529=47.8±1.8 kcal/mol and E
a,554=41.5±0.7 kcal/mol (different at P<0.01). Thus the red-shift of λmax in the “Lake” population, correlating with the long-wavelength dominated light environment, is achieved by changes in the
opsin that decrease the energy gap between the ground state and the first excited state of the chromophore. We propose that
this will carry a cost in terms of increased thermal noise, and that evolutionary adaptation of the visual pigment to the
light environment is directed towards maximizing the signal-to-noise ratio rather than the quantum catch. 相似文献
16.
A DFT study with QST3 approach method is used to calculate kinetic, thermodynamic, spectral and structural data of tautomers
and transition state structures of some N-hydroxy amidines. All tautomers and transition states are optimized at the B3LYP/6-311++g**
and B3LYP/aug-cc-pvtz level, with good agreement in energetic result with energies obtained from CBS-QB3, a complete basis
set composite energy method. The result shows that the tautomer a (amide oxime) is more stable than the tautomer b (imino hydroxylamine) as is reported in the literature. In addition, our finding shows that, the energy difference between
two tautomers is only in about 4–10 kcal/mol but the barrier energy found in traversing each tautomer to another one is in
the range of 33–71 kcal/mol. Therefore, it is impossible to convert these two tautomers to each other at room temperature.
Additionally, transition state theory is applied to estimate the barrier energy and reaction rate constants of the hydrogen
exchange between tautomers in presence of 1–3 molecules of water. The computed activation barrier shows us that the barrier
energy of solvent assisted tautomerism is about 9–20 kcal/mol and lower than simple tautomerism and this water-assisted tautomerism
is much faster than simple tautomerism, especially with the assisting two molecules of water. 相似文献
17.
Małgorzata A. Broda Barbara Rzeszotarska 《International journal of peptide research and therapeutics》1998,5(5-6):441-443
Summary α,β-Dehydroamino acids are useful peptide modifiers. However, their stereoelectronic properties still remain insufficiently
recognized. Based on FTIR experiments in the range ofv
s(N-H), AI, AII andv
s(Cα=Cβ) and ab initio calculations with B3LYP/6–31G*, we studied the solution conformational preferences and the amide electron
density perturbation of Ac-ΔXaa-NHMe, where ΔXaa=ΔAla, (E)-ΔAbu, (Z)-ΔAbu, (Z)-ΔLeu, (Z)-ΔPhe and ΔVal. Each of these dehydroamides adopts a C5 structure, which in Ac-ΔAla-NHMe is fully extended and accompanied by the strong C5 hydrogen bond. Interaction with bond Cα=Cβ lessens the amidic resonance within the flanking amide groups. TheN-terminal C=O bond is noticeably shorter, both amide bonds are longer than the corresponding bonds in the saturated entities
and the N-terminal amide system is distorted. Ac-ΔAla-NHMe constitutes an exception. ItsC-terminal amide bond is shorter than the standard one and both amide systems are ideally planar. Ac-(E)-ΔAbu-NHMe shares stereoelectronic features with both Ac-ΔAla-NHMe and (Z)-dehydroamides. 相似文献
18.
Absorption measurements from chromophoric dissolved organic matter (CDOM) and their relationships with dissolved organic carbon
(DOC) and fluorescence were studied in Lake Taihu, a large, shallow, subtropical lake in China. Absorption spectra of lake
water samples were measured from 240 nm to 800 nm. Highest values of a(λ), DOC and F
n
(355) occurred near the river inflow to Meiliang Bay and decreased towards the central lake basin. A significant spatial difference
was found between Meiliang Bay and the central lake basin in absorption coefficient, DOC-specific absorption coefficient,
exponential slope coefficient, DOC concentration and fluorescence value. The spatial distribution of CDOM suggested that a
major part of CDOM in the lake was from river input. CDOM absorption coefficients were correlated with DOC over the wavelength
range 280–500 nm, and a(355) was also correlated with F
n
(355), which showed that CDOM absorption could be inferred from DOC and fluorescence measurement. The coefficient of variation
between a(λ) and DOC concentration decreased with increase in wavelength from 240 nm to 800 nm. Furthermore, a significant negative
linear relationship was recorded between S value and CDOM absorption coefficient, as well as DOC-specific absorption coefficient. S value and DOC-specific absorption coefficient were used as a proxy for CDOM composition and source. Accurate CDOM absorption
measurements are very useful in explaining UV attenuation and in developing, validating remote sensing model of water quality
in Lake Taihu. 相似文献
19.
The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have
been studied using a Plant Efficiency Analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an
extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven
formation of the transmembrane electric potential (ΔΨ(t)) as well as pH in the lumen (pHL(t)) and stroma (pHS(t)). PS II model parameters and numerical coefficients in ΔΨ(t), pHL(t), and pHS(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore
treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to
the elimination of transmembrane potential in the presence of ionophores, when ΔΨ = 0 and pHL(t), pHS(t) changed to small extent relative to control values in vivo, with maximum ΔΨ(t) ∼ 90 mV and ΔΨ(t) ∼ 40 mV for the stationary state at ΔpH ≅ 1.8. As the light intensity was increased from 300 to 1200 μmol m−2 s−1, the heat dissipation rate constants increased threefold for nonradiative recombination of P680+Phe− and by ∼30% for P680+QA−. The parameters ΔΨ, pHS and pHL were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of fluorescence quenching
is discussed, which is related with light-induced lumen acidification, when +QA− and P680+ recombination probability increases to regulate the QA reduction. 相似文献
20.
Using ab initio methods, we have studied conformations of the de(sidechain)de(dioxy)difluoroepothilone model to quantify the effect of stability
change between the exo and endo conformers of the epoxy ring. The DFT minimization of the macrolactone ring reveals four low energy conformers, although
MP2 predicted five stable structures. The model tested with DFT hybride functional (B3LYP/6–31+G(d,p)) exhibits the global
minimum for one of the exo forms (C), experimentally observed in the solid state, but unexpectedly with the MP2 electron correlation method for the virtual endo form (W). Using the QST3 technique, several pathways were found for the conversion of the low energy conformers to the other low
energy exo representatives, as well as within the endo analog subset. The potential energy relationships obtained for several exo forms suggest a high conformational mobility between three, experimentally observed, conformers. The high rotational barrier,
however, excludes direct equilibrium with experimental EC-derived endo form S. The highest calculated transition state for the conversion of the most stable exo
M interligand to the endo
S form is approximately a 28 kcal/mol above the energy of the former. The two-step interconversion of the exo
H conformer to the endo
S requires at least 28 kcal/mol. Surprisingly, we found that the transition state energy of the H form to the virtual endo
W has the acceptable value of about 9 kcal/mol and the next energy barrier for free interconversion of endo
W to endo
S is 13 kcal/mol.
Figure DFT Model of Epothilone Interconversions
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献