Influence of metal cations on the intramolecular hydrogen-bonding network and pKa in phosphorylated compounds

The binding of the most common metal cations of cytoplasm (Li+, Na+, K+, Mg2+ and Ca2+) to a model molecule having an intramolecular hydrogen-bonding network, myo-inositol-2-monophosphate, was studied using first principles. A strong correlation between the conformation of metal inositol phosphate complexes with the type of metal cation, degree of deprotonation state, and the surrounding environment has been observed. On the basis of the hydrogen-bonding network analysis of the cation-phosphate complexes (Mn+-Ins(2)P1), the alkali cations show little effect on the conformational preference while the conformational preference for the binding of the alkaline earth cations is pH-dependent and solvent-dependent. For example, these calculations predict that Mg2+-Ins(2)P1(0) and Mg2+-Ins(2)P1(2-) favor the 1a/5e form while Mg2+-Ins(2)P1(1-) favors the 5a/1e conformation. The Ca2+-Ins(2)P1(2-) complex prefers the 1a/5e conformation in the gas phase and in a nonpolar protein environment, but inverts to the 5a/1e conformation upon entering the polar aqueous phase. The binding affinities of the cations and the pK(a) values for the cation-phosphate complexes are derived from thermodynamical analysis.     

Study on the reaction of triple helix poly(A:2I) with Cu~(2+)

The effect of Cu2+ on the triple-stranded helical structure of poly(A:2I) was studied by means of circular dichroism spectral method with the help of ultraviolet spectral and ethidium bromide fluorescence probe methods. It was found that Cu2+ destabilizes the structure of the triple helix poly(A:2I) and induces its structural transformations, meanwhile, the transformations can be partially reversed by a higher NaCl concentration. The structural transformations may be expressed by the following scheme: poly(A:2I) - poly(A:I) + poly(I)- poly(A) + 2poly(I)     

Cu~（2＋）对三螺旋poly（A：2I）解旋的结构分析

通过圆二色谱（ＣＤ）和吸收光谱研究了Ｃｕ（２＋）对三螺旋ｐｏｌｙ（Ａ：２Ｉ）的解旋行为，并从结构的角度分析了其解旋机理。结果表明：Ｃｕ（２＋）通过与ｐｏｌｙ（Ａ：２Ｉ）中的碱基的结合，破坏了Ｈｏｏｇｓｔｅｅｎ碱基对和Ｗａｔｓｏｎ－Ｃｒｉｃｋ碱基对中的氢键，从而导致了三螺旋ｐｏｌｙ（Ａ：２Ｉ）不可复性的解旋。     

Magnetic resonance and kinetic studies of the mechanism of membrane-bound sodium and potassium ion- activated adenosine triphosphatase.

EPR and water proton relaxation rate (1/T1) studies of partially (40%) and "fully" (90%) purified preparations of membrane-bound (Na+ + K+) activated ATPase from sheep kidney indicate one tight binding site for Mn2+ per enzyme dimer, with a dissociation constant (KD = 0.88 muM) in agreement with the kinetically determined activator constant, identifying this Mn2+-binding site as the active site of the ATPase. Competition studies indicate that Mg2+ binds at this site with a dissociation constant of 1 mM in agreement with its activator constant. Inorganic phosphate and methylphosphonate bind to the enzyme-Mn2+ complex with similar high affinities and decrease 1/T1 of water protons due to a decrease from four to three in the number of rapidly exchanging water protons in the coordination sphere of enzyme-bound Mn2+. The relative effectiveness of Na+ and K+ in facilitating ternary complex formation with HPO2-4 and CH3PO2-3 as a function of pH indicates that Na+ induces the phosphate monoanion to interact with enzyme-bound Mn2+. Thus protonation of an enzyme-bound phosphoryl group would convert a K+-binding site to a Na+-binding site. Dissociation constants for K+ and Na+, estimated from NMR titrations, agreed with kinetically determined activator constants of these ions consistent with binding to the active site. Parallel 32Pi-binding studies show negligible formation (less than 7%) of a covalent E-P complex under these conditions, indicating that the NMR method has detected an additional noncovalent intermediate in ion transport. Ouabain, which increases the extent of phosphorylation of the enzyme to 24% at pH 7.8 and to 106% at pH 6.1, produced further decreases in 1/T1 of water protons. Preliminary 31P- relaxation studies of CH3PO2-3 in the presence of ATPase and Mn2+ yield an Mn to P distance (6.9 +/- 0.5 A) suggesting a second sphere enzyme-Mn-ligand-CH3PO2-3 complex. Previous kinetic studies have shown that T1+ substitutes for K+ in the activation of the enzyme but competes with Na+ at higher levels. From the paramagnetic effect of Mn2+ at the active site on the enzyme on I/T1 of 205T1 bound at the Na+ site, a Mn2+ to T1+ distance of 4.0 +/- 0.1 A is calculated, suggesting the sharing of a common ligand atomy by Mn2+ and T1+ on the ATPase. Addition of Pi increases this distance to 5.4 A consistent with the insertion of P between Mn2+ and T1+. These results are consistent with a mechanism for the (Na+ + K+)-ATPase and for ion transport in which the ionization state of Pi at a single enzyme active site controls the binding and transport of Na+ and K+, and indicate that the transport site for monovalent cations is very near the catalytic site of the ATPase. Our mechanism also accounts for the order of magnitude weaker binding of Na+ compared to K+.     

Role of Mg2+ in hammerhead ribozyme catalysis from molecular simulation

Molecular dynamics simulations have been performed to investigate the role of Mg2+ in the full-length hammerhead ribozyme cleavage reaction. In particular, the aim of this work is to characterize the binding mode and conformational events that give rise to catalytically active conformations and stabilization of the transition state. Toward this end, a series of eight 12 ns molecular dynamics simulations have been performed with different divalent metal binding occupations for the reactant, early and late transition state using recently developed force field parameters for metal ions and reactive intermediates in RNA catalysis. In addition, hybrid QM/MM calculations of the early and late transition state were performed to study the proton-transfer step in general acid catalysis that is facilitated by the catalytic Mg2+ ion. The simulations suggest that Mg2+ is profoundly involved in the hammerhead ribozyme mechanism both at structural and catalytic levels. Binding of Mg2+ in the active site plays a key structural role in the stabilization of stem I and II and to facilitate formation of near attack conformations and interactions between the nucleophile and G12, the implicated general base catalyst. In the transition state, Mg2+ binds in a bridging position where it stabilizes the accumulated charge of the leaving group while interacting with the 2'OH of G8, the implicated general acid catalyst. The QM/MM simulations provide support that, in the late transition state, the 2'OH of G8 can transfer a proton to the leaving group while directly coordinating the bridging Mg2+ ion. The present study provides evidence for the role of Mg2+ in hammerhead ribozyme catalysis. The proposed simulation model reconciles the interpretation of available experimental structural and biochemical data, and provides a starting point for more detailed investigation of the chemical reaction path with combined QM/MM methods.     

LIF studies of iodine oxide chemistry. Part 3. Reactions IO + NO3 --> OIO + NO2, I + NO3 --> IO + NO2, and CH2I + O2 --> (products): implications for the chemistry of the marine atmosphere at night

The technique of pulsed laser photolysis coupled to LIF detection of IO was used to study IO + NO(3) --> OIO + NO(2); I + NO(3) --> (products); CH(2)I + O(2) --> (products); and O((3)P) + CH(2)I(2) --> IO + CH(2)I, at ambient temperature. was observed for the first time in the laboratory and a rate coefficient of k(1 a) = (9 +/- 4) x 10(-12) cm(3) molecule(-1) s(-1) obtained. For , a value of k(2) (298 K) = (1.0 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1) was obtained, and a IO product yield close to unity determined. IO was also formed in a close-to-unity yield in , whereas in an upper limit of alpha(3)(IO) < 0.12 was derived. The implications of these results for the nighttime chemistry of the atmosphere were discussed. Box model calculations showed that efficient OIO formation in was necessary to explain field observations of large OIO/IO ratios.     

Selective 226Ra2+ ionophores provided by self-assembly of guanosine and isoguanosine derivatives

The self-assembled guanosine (G 1)-based hexadecamers and isoguanosine (isoG 2)-based decamers are excellent 226Ra2+ selective ionophores even in the presence of excess alkali (Na+, K+, Rb+, and Cs+) and alkaline earth (Mg2+, Ca2+, Sr2+, and Ba2+) cations over the pH range 3-11. G 1 requires additional picrate anions to provide a neutral assembly, whereas the isoG 2 assembly extracts 226Ra2+ cations without any such additives. Both G 1-picrate and isoG 2 assemblies show 226Ra2+ extraction even at a 0.35 x 10(6) fold excess of Na+, K+, Rb+, Cs+, Mg2+, or Ca2+ (10(-2) M) to 226Ra2+ (2.9 x 10(-8) M) and at a 100-fold salt to ionophore excess. In the case of the G 1-picrate assembly, more competition was observed from Sr2+ and Ba2+, as extraction of 226Ra2+ ceased at an M2+/226Ra2+ ratio of 10(6) and 10(4), respectively. With the isoG 2 assembly, 226Ra2+ extraction also occurred at a Sr2+/226Ra2+ ratio of 10(6) but ceased at a 10(6) excess of Ba2+. The results clearly demonstrate the power of molecular self-assembly for the construction of highly selective ionophores.     

Quantitative analysis of the effect of salt concentration on enzymatic catalysis.

Like pH, salt concentration can have a dramatic effect on enzymatic catalysis. Here, a general equation is derived for the quantitative analysis of salt-rate profiles: k(cat)/K(M) = (k(cat)/K(M))(MAX)/[1+([Na+]/K[Na+])(n')], where (k(cat)/K(M))(MAX) is the physical limit of k(cat)/K(M), K(Na+) is the salt concentration at which k(cat)/K(M) = (k(cat)/K(M))(MAX)/2, and -n' is the slope of the linear region in a plot of log(k(cat)/K(M)) versus log [Na+]. The value of n' is of special utility, as it reflects the contribution of Coulombic interactions to the uniform binding of the bound states. This equation was used to analyze salt effects on catalysis by ribonuclease A (RNase A), which is a cationic enzyme that catalyzes the cleavage of an anionic substrate, RNA, with k(cat)/K(M) values that can exceed 10(9) M(-1) s(-1). Lys7, Arg10, and Lys66 comprise enzymic subsites that are remote from the active site. Replacing Lys7, Arg10, and Lys66 with alanine decreases the charge on the enzyme as well as the value of n'. Likewise, decreasing the number of phosphoryl groups in the substrate decreases the value of n'. Replacing Lys41, a key active-site residue, with arginine creates a catalyst that is limited by the chemical conversion of substrate to product. This change increases the value of n', as expected for a catalyst that is more sensitive to changes in the binding of the chemical transition state. Hence, the quantitative analysis of salt-rate profiles can provide valuable insight into the role of Coulombic interactions in enzymatic catalysis.     

Factors governing the substitution of La3+ for Ca2+ and Mg2+ in metalloproteins: a DFT/CDM study

Trivalent lanthanide cations are extensively being used in biochemical experiments to probe various dication-binding sites in proteins; however, the factors governing the binding specificity of lanthanide cations for these binding sites remain unclear. Hence, we have performed systematic studies to evaluate the interactions between La3+ and model Ca2+ - and Mg2+ -binding sites using density functional theory combined with continuum dielectric methods. The calculations reveal the key factors and corresponding physical bases favoring the substitution of trivalent lanthanides for divalent Ca2+ and Mg2+ in holoproteins. Replacing Ca2+ or Mg2+ with La3+ is facilitated by (1) minimizing the solvent exposure and the flexibility of the metal-binding cavity, (2) freeing both carboxylate oxygen atoms of Asp/Glu side chains in the metal-binding site so that they could bind bidentately to La3+, (3) maximizing the number of metal-bound carboxylate groups in buried sites, but minimizing the number of metal-bound carboxylate groups in solvent-exposed sites, and (4) including an Asn/Gln side chain for sites lined with four Asp/Glu side chains. In proteins bound to both Mg2+ and Ca2+, La3+ would prefer to replace Ca2+, as compared to Mg2+. A second Mg2+-binding site with a net positive charge would hamper the Mg2+ --> La3+ exchange, as compared to the respective mononuclear site, although the La3+ substitution of the first native metal is more favorable than the second one. The findings of this work are in accord with available experimental data.     

Effect of Mg2+ Ions on the Stability of PolyA/2PolyU Three-Stranded Helices in Aqueous Solutions

The influence of Mg²⁺, Na⁺ and temperature on the conformational state of three-stranded helical polyA/2polyU (A2U) has been studied by the thermal denaturation method. At Na⁺ concentrations of 0.01–0.1 M , on heating the transition A2U→AU+U (the 3→2 transition) and then AU→A+U transition (the 2→1 transition) are observed. (AU is double helix polyA/polyU; A and U are single-stranded polyA and polyU, respectively.) With 0.01 M and 0.03 M Na⁺ these transitions occur at Mg²⁺ concentrations within (0 ÷ 0.003) M . At these ionic concentrations, there is a narrow temperature region (3 ÷ 5°C) at which double-helical AU formed by the 3→2 transition is resistant to heating. In 0.1 M Na⁺, a rise in the Mg²⁺ concentration leads to a continuous decrease in the temperature range of this region, and above a critical concentration of Mg²⁺ (ca. 3.6×10^–5 M )_cr there is only one transition (the 3→1 transition) instead of the successive transitions 3→2→1. The constants of Mg²⁺ ion association with polyU, polyA and A2U were calculated using equilibrium binding theory. The data obtained helped explain the reasons for the different phase diagrams for A2U + Mg²⁺ complexes in solution at high and low Na⁺ concentrations.     

Formation of metal complex ions from amino acid in the presence of Li+, Na+ and K+ by electrospray ionization: metal replacement of hydrogen in the ligands

Alkali metal cations easily form complexes with proteins in biological systems; understanding amino acid clusters with these cations can provide useful insight into their behaviors at the molecular level including diagnosis and therapy of related diseases. For the purpose of characterization of basic interaction between amino acids and alkali metal, each of the 20 naturally occurring amino acids were ionized in the presence of lithium, sodium and potassium cations by electrospray ionization, and the resulting product ions were analyzed. We focus our attention on the gas phase alkali metal ion-proton exchanged complexes in current study, specifically complexes with serine, threonine, asparagine and glutamine, which share characteristic pattern unlike other amino acids. All amino acids generated [M + H](+) and [M + Na](+) ions, where M stands for the neutral amino acid. Serine, threonine, asparagine and glutamine generated cluster ions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) , where n = 1-7. While the (M - H + Li) and (M - H + K) species were not observed, the neutral (M - H + Na) species formed by proton-sodium cation exchange had a highly stable cyclic structure with ketone and amine ligand sites, suggesting that (M - H + Na) serves as a building block in cluster ion formation. Cluster ion intensity distributions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) showed a magic number at n = 3 and 4, respectively. Extensive B3LYP-DFT quantum mechanical calculations were carried out to elucidate the geometry and energy of the cluster ions, and they provided a reasonable explanation for the stability and structure of the cluster ions.     

Comprehensive study on the solvation of mono- and divalent metal cations: Li+, Na+, K+, Be2+, Mg2+ and Ca2+

Hydration of mono- and divalent metal ions (Li(+), Na(+), K(+), Be(2+), Mg(2+) and Ca(2+)) has been studied using the DFT (B3LYP), second-order M?ller-Plesset (MP2) and CCSD(T) perturbation theory as well as the G3 quantum chemical methods. Double-zeta and triple-zeta basis sets containing both (multiple) polarization and diffuse functions were applied. Total and sequential binding energies are evaluated for all metal-water clusters containing 1-6 water molecules. Total binding energies predicted at lower levels of theory are compared with those from the high level G3 calculations, whereas the sequential binding energies are compared with available experimental values. An increase in the quality of the basis set from double-zeta to triple-zeta has a significant effect on the sequential binding energies, irrespective of the geometries used. Within the same group (I or II), the sequential binding energy predictions at the MP2 and B3LYP vary appreciably. We noticed that, for each addition of a water molecule, the change of the M-O distance in metal-water clusters is higher at the B3LYP than at the MP2 level. The charge of the metal ion decreases monotonically as the number of water molecules increase in the complex.     

Conformational change of poly(L-lysine) and poly(L-ornithine) and cooperative binding of sodium alkanesulfonate surfactants with different chain length

Conformations of poly(L-lysine) (PLL) and poly(L-ornithine) (PLO) were examined in aqueous solutions of sodium alkanesulfontates (C_nSO₃Na, n=9, 10, 11, 12) in the presence of 0.02 M NaCl by circular dichroism (CD) spectroscopy. These surfactants induce the-structure for PLL and the-helix for PLO. The binding of surfactants on the polypeptides was measured potentiometrically with a surfactant ion electrode and was found to be highly cooperative. The cooperativity increases with increasing chain length of surfactant. The behavior accompanying the surfactant binding and the conformational change indicated that the conformational change requires a certain amount of bound surfactants in the case of C₉SO₃Na and starts immediately on binding of surfactant in the case of C_{1 2}SO₃Na. The clustering of bound surfactants due to the cooperative binding as well as neutralization of polypeptides contributes to their conformational change. A slow conformational change of PLO was found in the time scale of hours, sometimes days, for C₉- and C₁₀SO₃Na at low concentrations, but the binding process reached the equilibrium quickly. This slow mode might occur due to the slow interaction between surfactant/polypeptide complexes.     

Multiple charging of poly(propylene glycol) by binary mixtures of cations in electrospray

Single, double and triple charging of poly(propylene glycol) (PPG) (Mn = 1900 g/mol) in the presence of binary mixtures of cations (Li+, Na+, K+, Cs+, and NH4+) under electrospray ionization (ESI) conditions were investigated. For these studies, sodium ion was selected as the reference cation, and the resulting ion-intensities were evaluated as a function of the [Na+]/[C+] ratio (where C+ is the other cation, i.e., Li+, K+, Cs+ and NH4+). A linear relationship was found between INa+/IC+)and [Na+]/[C+] (INa+ and IC+ stand for the intensity of the singly charged PPG molecules cationized with Na+ and C+ ions, respectively). The slope of the INa+/IC+--[Na+]/[C+] plot (alpha) indicates the binding selectivity of Na+ ions to PPG chains with respect to cation C+. In the case of the doubly charged PPG chains, the INaNa2+/INaC2+ and INaC2+/ICC2+ versus [Na+]/[C+] ratio also yield straight lines with slopes of approximately alpha/2 and 2alpha, respectively (INaNa2+, INaC2+ and ICC2+ are the intensity of the doubly charged PPG chains cationized with two Na+ ions, Na+ and C+ ions, and two C+ ions, respectively). Similarly, linear dependences with the [Na+]/[C+] ratio for the corresponding intensity ratios of the triply charged PPG were found. Based on the value of alpha, the selectivity of the cations was found to increase in the order of Li+ < Cs+ approximately Na+ < K+ approximately NH4+. The observed relative ion intensities are interpreted on the basis of the solution state equilibrium between PPG and the cations. In addition, the investigations showed that the abundances of the doubly and triply charged PPG-containing mixed cations can be optimized in a simple way using the value of alpha.     

Gas-phase conformations of cationized poly(styrene) oligomers

The gas-phase conformations of poly(styrene) oligomers cationized by Li+, Na+, Cu+, and Ag+ (M+ PSn) were examined using ion mobility experiments and molecular mechanics/ dynamics calculations. M+PSn ions were formed by MALDI and their ion-He collision cross-sections were measured by ion mobility methods. The experimental collision cross-sections of each M+PS n-mer were similar for all four metal cations and increased linearly with n. Molecular modeling of selected M+ PS oligomers cationized by Li+ and Na+ yielded quasi-linear structures with the metal cation sandwiched between two phenyl groups. The relative energies of the structures were approximately 2-3 kcal/mol more stable when the metal cation was sandwiched near the middle of the oligomer chain than when it was near the ends of the oligomer. The cross-sections of these theoretical structures agree well with the experimental values with deviations typically around 1-2%. The calculations also show that the metal cation tends to align the phenyl groups on the same side of the -CH2-CH- backbone. Calculations on neutral poly(styrene), on the other hand, showed structures in which the phenyl groups were more randomly positioned about the oligomer backbone. The conformations and metal-oligomer binding energies of M+PS are also used to help explain CID product distributions and fragmentation mechanisms of cationized PS oligomers. etry     

Diazacoronand linked beta-cyclodextrin dimer complexes of Brilliant Yellow tetraanion and their sodium(I) analogues

Complexation of the Brilliant Yellow tetraanion, 3(4-), by two new diazacoronand linked beta-cyclodextrin (beta CD) dimers 4,13-bis(2-(6A-deoxy-beta-cyclodextrin-6A-yl)aminooctylamidomethyl- and 4,13-bis(8-(6A-deoxy-beta-cyclodextrin-6A-yl)aminooctylamidomethyl)-4,13- diaza-1,7,10-trioxacyclopentadecane, 1 and 2, respectively, has been studied in aqueous solution. UV-visible spectrophotometric studies at 298.2 K, pH 10.0 and I = 0.10 mol dm-3 (NEt4ClO4) yielded complexation constants for the complexes 1 x 3(4-) and 2 x 3(4-), K1 = (1.08 +/- 0.01) x 10(5) and (6.21 +/- 0.08) x 10(3) dm3 mol-1, respectively. Similar studies at 298.2 K, pH 10.0 and I = 0.10 mol dm-3 (NaClO4) yielded K3 = (4.63 +/- 0.09) x 10(5) and (3.38 +/- 0.05) x 10(4) dm3 mol-1 for the complexation of 3(4-) by Na+ x 1 and Na+ x 2 to give Na+ x 1 x 3(4-) and Na+ x 2 x 3(4-), respectively. Potentiometric studies of the complexation of Na+ by 1 and 2 by the diazacoronand component of the linkers to give Na+ x 1 and Na+ x 2 yielded K2 = (2.00 +/- 0.05) x 10(3) and (1.8 +/- 0.05) x 10(3) dm3 mol-1, respectively, at 298.2 K and I = 0.10 mol dm-3(NEt4ClO4). For complexation of Na+ by 1 x 3(4-) and 2 x 3(4-) to give Na+ x 1 x 3(4-) and Na+ x 2 x 3(4-) K2K3/K1 = K4 = 8.6 x 10(2) and 9.8 x 10(3) dm3 mol-1, respectively. The pKaS of 1H4(4+) are 7.63 +/- 0.01, 6.84 +/- 0.02, 5.51 +/- 0.04 and 4.98 +/- 0.03, and those of 2H4(4+) are 8.67 +/- 0.02, 8.11 +/- 0.02, 6.06 +/- 0.02 and 5.14 +/- 0.05. The larger magnitude of K1 for 1 by comparison with K1 for 2 is attributed to the octamethylene linkers of 2 competing with 3(4-) for occupancy of the annuli of the beta CD entities while the competitive ability of the dimethylene linkers of 1 is less. A similar argument applies to the relative magnitudes of K3 for Na+ x 1 and Na+ x 2. Increased electrostatic attraction probably accounts for K3 > K1 for Na+ x 1 x 3(4-) and 1 x 3(4-) and for Na+ x 2 x 3(4-) and 2 x 3(4-). The lesser magnitudes of K2 and K4 for Na+ x 1 and Na+ x 1 x 3(4-) compared with those for Na+ x 2 and Na+ x 2 x 3(4-) are attributed to the octamethylene linkers of 2 producing a more hydrophobic environment for the diazacoronand than that produced by the dimethylene linkers of 1. 1H NMR spectroscopic studies and the syntheses of 1 and 2 are described.     

A PGSE diffusion and electrophoretic NMR study of Cs+ and Na+ dynamics in aqueous crown ether systems

Multinuclear pulsed gradient spin-echo (PGSE) NMR diffusion and linewidth measurements were used to probe binding and transport in aqueous Na+-15-crown-5, Na+-18-crown-6, Cs+-15-crown-5 and Cs+-18-crown-6 systems. Since direct PGSE observation of many alkali cations is precluded by either low inherent sensitivity or rapid relaxation (or both), the feasibility of proton-detected electrophoretic NMR (ENMR) measurements to complement PGSE data was investigated. ENMR measurements were performed on aqueous Cs+-, Li+-, Na+-, K+-, and Rb+- 18-crown-6 systems. The data analysis is based on a two-site binding model and its corresponding association constants. Cs+ was found to bind considerably more tightly to 18-crown-6 (K=8 M-1) than to 15-crown-5 (K approximately 2 M-1), whereas Na+ had almost equal affinity (K approximately 4.5 M-1) for 15-crown-5 and 18-crown-6. The difficulties encountered in analysing the NMR parameters, methodological limitations and the implied need for more complicated binding models are discussed.     

Cyclohexane as a Li+ selective ionophore

The M+[cyclohexane][Ar] (M = Li, Na, and K) cluster ions were investigated using infrared photodissociation spectroscopy in the C-H stretching region. The alkali metal cation binds to the cyclohexane ring above the ring on the S6 axis via eta3 coordination. The C-H stretching modes are perturbed due to binding of the metal cation and display a significant spread in frequency. The shifts are greatest for the Li+ and decrease for Na+ and K+ with increasing ionic radius. It has been observed that cyclohexane displays greater selectivity for Li+ over Na+ than the cyclic ether, 12-crown-4. The charge transfer interaction between Li+ and cyclohexane is believed to be responsible for the selectivity of Li+ over other alkali metal ions.     

RNA GG x UU motif binds K+ but not Mg2+

A hairpin model of the group I intron P5b loop was synthesized with [8-13C-7-15N]-guanosine in the GG.UU metal binding site, [7-15N]-guanosine at a nonbinding site, and [3-15N]-uridine. 15N NMR showed specific binding for Co(NH3)63+ and K+, but not for Zn2+, Cd2+, or Mg2+.     

磷酸二甲酯阴离子(DMP~-)与一价、二价金属离子 相互作用的理论研究

在B3LYP、HF和MP2水平上运用全电子从头算(AE)和相对论有效实势(RECP)及6-311+G**和LanL2DZ基组计算Ⅰa、Ⅰb、Ⅱa和Ⅱb族金属离子与磷酸二甲酯阴离子(DMP-)的相互作用。 RECP用于除Li+、Be2+外所有的金属离子。 对Na+、K+、Cu+、Mg2+、Ca2+、Zn2+用AE和RECP 2种方法处理。 结果表明:RECP能可靠地用于重金属离子络合物; 二价金属离子络合物(DMP-—M2+)比一价金属离子络合物 (DMP-—M+)稳定;二价金属离子(M2+)可能比一价金属离子(M+)更易使多核苷酸折叠。