Molecular Dynamics Simulation of Mg2+ and Ca2+ Ions in Water

Molecular dynamics simulations of single Mg²⁺ and Ca²⁺ ions in water have been carried out. Different ion-water potentials from the literature have been used, whereas the same water potential, a rigid simple point charged model, has been considered in all the simulations. Structural, thermodynamic, and dynamic properties have been calculated, and the results for different potentials have been compared with available experimental data. The study includes ion–water radial distribution functions, coordination numbers, solution enthalpies, hydration free energies, self-diffusion coefficients, and reorientational times of water molecules in the hydration shells.     

Ca2+、Ba2+对光敏热成像材料照相性能和稳定性的影响

以常用的硬脂酸银为银源,异位AgBr为光敏元以及亲水型PVA为粘合剂,分别考察了Ca2+和Ba2+对光敏热成像(PTG)材料照相性能以及老化稳定性的影响.实验结果表明:涂布前乳液中加入Ca2+或Ba2+后,PTG材料的感光度和灰雾均出现不同程度的降低,并且随着加入量的增加,感光度下降更快.另外,含有Ca2+或Ba2+的PTG材料随着老化时间的增加,其感光度和灰雾的变化趋势并不相同.     

Ca+ Ions Solvated in Helium Clusters

We present a combined experimental and theoretical investigation on Ca+ ions in helium droplets, HeNCa+. The clusters have been formed in the laboratory by means of electron-impact ionization of Ca-doped helium nanodroplets. Energies and structures of such complexes have been computed using various approaches such as path integral Monte Carlo, diffusion Monte Carlo and basin-hopping methods. The potential energy functions employed in these calculations consist of analytical expressions following an improved Lennard-Jones formula whose parameters are fine-tuned by exploiting ab initio estimations. Ion yields of HeNCa+ -obtained via high-resolution mass spectrometry- generally decrease with N with a more pronounced drop between N=17 and N=25, the computed quantum HeNCa+ evaporation energies resembling this behavior. The analysis of the energies and structures reveals that covering Ca+ with 17 He atoms leads to a cluster with one of the smallest energies per atom. As new atoms are added, they continue to fill the first shell at the expense of reducing its stability, until N=25, which corresponds to the maximum number of atoms in that shell. Behavior of the evaporation energies and radial densities suggests liquid-like cluster structures.     

油菜缺钙条件下钕、钙关系的研究

研究表明缺钙油菜经低浓度钕(3μmol·L-1Nd(NO3)3)处理,主根增长、根系表面积、根干重、根系阳离子交换量增加;根系氧化力提高;油菜受损程度得到了明显的改善和修复,钕对钙表现出一部分替代作用。这种作用很可能通过细胞质膜Ca2+ ATPase以信号转导途径来体现。钕在高浓度下(60μmol·L-1)则表现出毒害作用。     

Photochemical [3 + 2] cycloaddition of 2-cyanochromone to olefins

Irradiation of 2-cyanochromone in the presence of olefins gave rise to [3 + 2] cycloadducts together with the formation of normal [2 + 2] cycloadducts with the product ratio being temperature-dependent. The mechanism involving a vinyl nitrene intermediate was proposed.     

高锰酸钾-鲁米诺反应体系中碱土金属离子Mg2+,Ca2+,Sr2+,Ba2+化学发光行为研究

将被认为没有化学发光活性的第二主族(碱土金属)离子Mg^2 ,Ca^2 ,Sr^2 ,Ba^2 溶液注入到已充分反应的高锰酸钾与鲁米诺混合液中时,又发生了新的化学发光反应,并检测到强的化学发光信号．在对有关反应的动力学性质、化学发光光谱、紫外可见光谱及其它一系列实验研究的基础上,提出了可能的化学发光反应机理．同时,优化了反应条件,评价了这一反应用于Mg^2 ,Ca^2 ,Sr^2 ,Ba^2 分析的可行性．     

Photochemical and thermal (2+2)-cycloaddition of thioxanthenethione to some aliphatic pentatetraenes

Reaction of thioxanthene-9-thione 1 with the alkyl substituted pentatetraenes 2a-g gives mixtures of the thietanes 3a-g and 4a-g. The ratio $\text{3}\text{4}$ in the thermal reaction is strongly dependent on the substituents. In the case of 2a thermal rearrangement of the product leads to a stable thiete.     

pH电位滴定法测定溶液中钙、镁离子与聚天冬氨酸的生成稳定常数

pH电位滴定法测定溶液中钙、镁离子与聚天冬氨酸的生成稳定常数;pH电位滴定;聚天冬氨酸;配位化学;钙离子;镁离子     

Metal Complexes of Pharmacophore-Containing Pectin with d -Elements Ions (Cu 2+ , Co 2+ , and Mn 2+ )

Spectral methods have been used to study the complexation of apple pectin modified with organic pharmacophores (nicotinic, salicylic, 5-aminosalicylic, or anthranilic acid) with Cu2+, Co2+, and Mn2+ ions in an aqueous solution. The composition and stability series of the metal complexes, corresponding to the empirical Irving–Williams series, have been established. The standard thermodynamic parameters (ΔH°, ΔG°, and ΔS°) of the complexation process have been calculated. It has been shown by means of 13C NMR and IR spectroscopy that the interaction of pharmacophore-containing pectins with d-metal ions (Cu2+, Co2+, Mn2+) occurs via the carbonyl and hydroxyl groups of the modified polymer matrices. The viscosity and thermal properties of the metal complexes have been determined.     

Ca2+ selectivity of the sarcoplasmic reticulum Ca2+-ATPase at the enzyme-water interface and in the Ca2+ entrance channel

The sarcoplasmic reticulum (SR) Ca(2+)-ATPase, a P-type transmembrane protein, can transport Ca(2+) from the cytoplasmic to the luminal side over other cations specifically. The proposed Ca(2+) entrance channel, composed of the main-chain carbonyl oxygen and side-chain carboxyl oxygen atoms of the amino acids, opens on the enzyme surface, just above the biphospholipid layer membrane-water interface, where Trp residues are frequently found. In this work, the physicochemical nature of Ca(2+) selectivity over Mg(2+) on the surface of the SR Ca(2+)-ATPase has been investigated using the density functional theory (DFT) method. The selection process can be regarded as the first step of the specificity of the enzyme to transport Ca(2+). Subsequently, the specificity of the entrance channel to conduct Ca(2+) over other cations has also been explored. As revealed by thermodynamic analyses, either the aromatic or the aliphatic amino acid residues distributed on the surface of Ca(2+)-ATPase have a bigger affinity to Mg(2+) than to Ca(2+), resulting in a concentration decrease of free Mg(2+) in the local region. Thus, Ca(2+) can transport into the Ca(2+)-entrance channel more easily. Whereafter, for a small quantity of Mg(2+) entering this channel accompanying the Ca(2+) current, the strong electrostatic interactions between Mg(2+) and the ligands will limit the activity of this metal ion, which facilitates the weakly bonded Ca(2+) passing through the channel at a relatively high rate, as suggested by the "sticky-pore" hypothesis. Furthermore, the corresponding theoretical investigations have demonstrated that the increase of the ligand electronegativity can enhance their discrimination between these two cations effectively.     

Photochemical [2+2]-cycloaddition of cyclic α,β-enones to monosubstitüted acetylenes

The photo addition of cyclic α,β-enones to monosubstituted acetylenes gives substituted bicyclo [3,2,0]hept-6-en-2-ones and bicyclo[4,2,0]oct-7-en-2-ones. The addition of 2-cyclopentenone to different substrates of type RCCH (where R is alkyl, CH₂X, OEt, COOMe and also Ph) and addition of 2-cyclohexenones to 1-hexyne show a linear correlation between the σ-constants of substituents and the rates of cycloaddition. The reaction can be qualified as a moderately electrophilic attack of the enone triplet on the acetylenic bond (¦?¦~1.5). Temperature and solvent effects on the rates and regioselectivity were also studied. The regioselectivity of addition of these α, β,-unsaturated ketones to 1-alkynes does not correspond to what might be expected on the basis of Corey's orientation rule. The addition of 2-cyclopentenone to ethoxyacetylene 3g and to methyl propiolate3h proceeds with surprisingly low regioselectivity while phenylacetylene 3i undergoes only dimerization under photoannelation conditions.     

Effect of Na＋, K＋ and Ca2＋ Ions on Physico-chemical Properties of Thymine, Cytosine, Thymidine and Cytidine in Aqueous Urea Solution

Experimental determination of density, ultrasonic velocity and viscosity of two pyrimidine bases thymine and cytosine along with their respective nucleosides, thymidine and cytidine has been carried out in aqueous urea solutions in the presence of different concentrations of three salts, viz. NaCl, KCl and CaCl₂. The experimental data have been used for the computation of various thermodynamic parameters, viz. apparent molar volume, apparent molar compressibility, coefficients A and B of the Jones-Dole equation, internal pressure, acoustic impedance, etc. Structural studies of solutions under investigation have also been carried out by ultraviolet spectroscopy, and an attempt has been made to collaborate the findings of ultraviolet spectroscopy with results obtained thermodynamically.     

Effect of Na+ and Ca2+ Ions on a Lipid Langmuir Monolayer: An Atomistic Description by Molecular Dynamics Simulations

Studying the effect of alkali and alkaline‐earth metal cations on Langmuir monolayers is relevant from biophysical and nanotechnological points of view. In this work, the effect of Na⁺ and Ca²⁺ on a model of an anionic Langmuir lipid monolayer of dimyristoylphosphatidate (DMPA^?) is studied by molecular dynamics simulations. The influence of the type of cation on lipid structure, lipid–lipid interactions, and lipid ordering is analyzed in terms of electrostatic interactions. It is found that for a lipid monolayer in its solid phase, the effect of the cations on the properties of the lipid monolayer can be neglected. The influence of the cations is enhanced for the lipid monolayer in its gas phase, where sodium ions show a high degree of dehydration compared with calcium ions. This loss of hydration shell is partly compensated by the formation of lipid–ion–lipid bridges. This difference is ascribed to the higher charge‐to‐radius ratio q/r for Ca²⁺, which makes ion dehydration less favorable compared to Na⁺. Owing to the different dehydration behavior of sodium and calcium ions, diminished lipid–lipid coordination, lipid–ion coordination, and lipid ordering are observed for Ca²⁺ compared to Na⁺. Furthermore, for both gas and solid phases of the lipid Langmuir monolayers, lipid conformation and ion dehydration across the lipid/water interface are studied.     

Ca2+- and ba2+-ligand coordinated unilamellar, multilamellar, and oligovesicular vesicles

Ca(2+)- and Ba(2+)-coordinated vesicle phases were prepared in mixed aqueous solutions of tetradecyldimethylamine oxide (C(14)DMAO) and calcium oleate (Ca(OA)(2)) or barium oleate (Ba(OA)(2)). At the right mixing ratios, metal-ligand coordination between Ca(OA)(2) or Ba(OA)(2) and C(14)DMAO results in the formation of molecular bilayers due to the reduction in area per head group. Ca(2+) and Ba(2+) tightly associate to the head groups of surfactants and in this system the bilayer membranes are not shielded by excess salts. The structures of the birefringent samples of the Ca(OA)(2)/C(14)DMAO/H(2)O and Ba(OA)(2)/C(14)DMAO/H(2)O systems were determined by freeze-fracture transmission electron microscopy (FF-TEM), small-angle X-ray scattering (SAXS), and rheological measurements to consist of unilamellar, multilamellar, and oligovesicular vesicles. The coordination between C(14)DMAO and Ba(OA)(2) or Ca(OA)(2) plays an important role in the formation of the vesicles, which was easily confirmed by studying the phase behavior of the KOA/C(14)DMAO/H(2)O system in which only the L(1) phase forms, due to the absence of coordination between KOA and C(14)DMAO. A mechanism is proposed that accounts for the formation of these new metal-ligand coordinated vesicles.     

Influence of K+, Na+ or Ca2+ Ions on the Interaction Between AmB and Saturated Phospholipids by Langmuir Technique

Interaction between amphotericin B(AmB) and cell membrane is influenced by different metal cations. In the presence of K⁺, Na⁺ or Ca²⁺ ions, the surface pressure-area isotherms and the elastic modulus of an amphotericindipalmitoylphosphatidylcholine(AmB-DPPC) mixed monolayer were discussed. And the excess free energy and entropies of mixing were calculated according to the surface pressure-area isotherms. The phase transition of the mixed monolayer needed a higher concentration of AmB in the sequence Na⁺ > pure buffer > K⁺ > Ca²⁺. When the molar fraction of AmB(x_AmB) was 0.5, the molecular interaction changed from attraction to repulsion and the mixed monolayer turned to ordered state from disorder state under the induction of K⁺ or Ca²⁺ ions at all surface pressure in our experiment. At high surface pressure, the disorder of monolayer enhanced in the presence of Na⁺ ions at x_AmB > 0.1. At different molar ratios of AmB, the influences of these metal cations were discrepant. These cations may influence AmB molecules to form pores on the monolayer. It is helpful to understand the reduction of AmB's toxicity as theoretical reference.     

Comparison of Ca2+ and Mg2+ binding to calmodulin. An enthalpy,entropy, and gibbs free energy analysis

A consistent set of G _B , H _B , and S _B parameters have been determined from ion specific electrode, calorimetric, and spectrophotometric studies for the binding of Ca²⁺ and Mg²⁺ to bovine calmodulin at pH=7.0 and an ionic strength I of 0.113M. A non-linear least squares analysis of calcium specific ion electrode data yields, on a molar basis, four calcium dissociation constants: 10^–7 for the first site, 10^–5 for the fourth site, and two constants between these values. Both calorimetric experiments and an indicator method provide evidence that Mg²⁺ binds to calmodulin, probably at the same sites as Ca²⁺, but with affinities about 100 times smaller: 4×10^–5 for the first site and 2×10^–3 for the fourth. Calorimetric titrations on Ca²⁺ binding to calmodulin in three buffers are consistent with 0.46 protons released upon binding at all four sites and yield an average H _B per site of 5.6 and 7.9 kJ-mol^–1 for Ca²⁺ and Mg²⁺, respectively. The entropy of the system increases by 524 and 361 J-K^–1-mol^–1 when Ca²⁺ and Mg²⁺, respectively, bind to four sites on calmodulin, i.e., the selectivity of calmodulin for Ca²⁺ is primarily derived from entropy effects. Further analysis based on elimination of the entropy term for the metal ions demonstrates that calmodulin bound to Ca²⁺ has a larger entropy than the unbound calmodulin; the opposite is true for calmodulin bound to Mg²⁺. These analyses are consistent with the hypothesis that Ca²⁺ forms tight complexes at all sites on calmodulin and that release of waters of hydration upon binding is the source of the increase of entropy in the system.     

Na+-Ca2+ exchanger modulates Ca2+ content in intracellular Ca2+ stores in rat osteoblasts

Na+-Ca2+ exchanger (NCX) transports Ca2+ coupled with Na+ across the plasma membrane in a bi-directional mode. Ca2+ flux via NCX mediates osteogenic processes, such as formation of extracellular matrix proteins and bone nodules. However, it is not clearly understood how the NCX regulates cellular Ca2+ movements in osteogenic processes. In this study, the role of NCX in modulating Ca2+ content of intracellular stores ([Ca2+]ER) was investigated by measuring intracellular Ca2+ activity in isolated rat osteoblasts. Removal of extracellular Na+ elicited a transient increase of intracellular Ca2+ concentration ([Ca2+]i). Pretreatment of antisense oligodeoxynucleotide (AS) against NCX depressed this transient Ca2+ rise and raised the basal level of [Ca2+]i. In AS-pretreated cells, the expression and activity of alkaline phosphatase (ALP), an osteogenic marker, were decreased. However, the cell viability was not affected by AS-pretreatment. Suppression of NCX activity by the AS-pretreatment decreased ATP-activated Ca2+ release from intracellular stores and significantly enhanced Ca2+ influx via store operated calcium influx (SOCI), compared to those of S-pretreated or control cells. These results strongly suggest that NCX has a regulatory role in cellular Ca2+ pathways in osteoblasts by modulating intracellular Ca2+ content.