Conditions for Reversible Na Intercalation in Graphite: Theoretical Studies on the Interplay Among Guest Ions,Solvent, and Graphite Host

Graphite is the most widely used anode material for Li‐ion batteries and is also considered a promising anode for K‐ion batteries. However, Na⁺, a similar alkali ion to Li⁺ or K⁺, is incapable of being intercalated into graphite and thus, graphite is not considered a potential electrode for Na‐ion batteries. This atypical behavior of Na has drawn considerable attention; however, a clear explanation of its origin has not yet been provided. Herein, through a systematic investigation of alkali metal graphite intercalation compounds (AM‐GICs, AM = Li, Na, K, Rb, Cs) in various solvent environments, it is demonstrated that the unfavorable local Na‐graphene interaction primarily leads to the instability of Na‐GIC formation but can be effectively modulated by screening Na ions with solvent molecules. Moreover, it is shown that the reversible Na intercalation into graphite is possible only for specific conditions of electrolytes with respect to the Na‐solvent solvation energy and the lowest unoccupied molecular orbital level of the complexes. It is believed that these conditions are applicable to other electrochemical systems involving guest ions and an intercalation host and hint at a general strategy to tailor the electrochemical intercalation between pure guest ion intercalation and cointercalation.     

Consequences of Sequential Ca2+ Occupancy for the Structure and Dynamics of CalbindinD9K: Computational Simulations and Comparison to Experimental Determinations in Solution

Abstract

Molecular dynamics (MD) simulations of the structures of calbindin_D9K (CAB) with different occupancies of the two Ca²⁺ binding sites were carried out to gain insight into structural and energetic consequences of sequential Ca²⁺ binding. The aim of the study is to identify effects of Ca-binding site occupancy that relate to the properties and functions of Ca-binding proteins containing EF-hand motifs. Two different models of solvation were employed, one defined by a linear, distance dependent dielectric permittivity (ε = r) and inclusion only of the 36 crystallographically observed water molecules, and the other with the protein immersed in a 9Å shell of explicit waters and ε = 1. Experimental results from x-ray crystallography, and insights from NMR and from measurements of hydrogen exchange rates in these systems served as tests and guides for assessing the quality, validity and mechanistic interpretation of the results from the computational study. The results of the MD simulations agree very well with earlier experimental observations that the structure of calbindin_D9k is rather insensitive to removal of Ca²⁺, and indicate that this insensitivity is not dependent on the order in which the ions are removed. The calculated values of the electrostatic potentials at the Ca²⁺ binding sites are very similar, in agreement with the small differences in the measured microscopic binding constants. Details of the dynamic mechanisms of molecular flexibility revealed by the MD simulations are also in good agreement with experimental findings, including the local properties identified from comparisons of hydrogen exchange rates in various parts of the structures of sequentially occupied forms of CAB. Estimation of the changes in configurational entropy from the rms fluctuations in the structures of CAB at various levels of Ca²⁺ occupancy in the EF-hands, supports earlier suggestions relating the dynamic properties of the protein to the observed cooperativity in the binding of Ca²⁺. The computational approaches and the results of the MD simulations are evaluated in relation to the study of effects of Ca²⁺ occupancy in calmodulin and troponin C where ion binding determines function and is known to trigger significant changes in structural and dynamic properties.     

Cu2‐xS Surface Phases and Their Impact on the Electronic Structure of CuInS2 Thin Films – A Hidden Parameter in Solar Cell Optimization

The surface properties of CuInS₂ (CIS) thin‐film solar cell absorbers are investigated by a combination of electron and soft X‐ray spectroscopies. Spatially separated regions of varying colors are observed and identified to be dominated by either CuS or Cu₂S surface phases. After their removal by KCN etching, the samples cannot be distinguished by eye and the CIS surface is found to be Cu‐deficient in both regions. However, a significantly more pronounced off‐stoichiometry in the region initially covered by Cu₂S can be identified. In this region, the resulting surface band gap is also significantly larger than the E_g^Surf of the initially CuS‐terminated region. Such variations may represent a hidden parameter which, if overlooked, induces irreproducibility and thus prevents systematic optimization efforts.     

Studies on the regulatory complex of rabbit skeletal muscle: Contributions of troponin subunits and tropomyosin in the presence and absence of Mg2+ to the acto-S1 ATPase activity

The regulatory roles of the components of the troponin-tropomyosin complex in the presence and absence of Mg²⁺ on the acto-S1 ATPase have been examined. The effect of free Mg²⁺ on the inhibition of the acto-S1 ATPase by rabbit skeletal troponin (Tn) was studied at S1 to actin ratios ranging from 0.17:1 to 2.5:1. These studies were performed using two Mg²⁺ concentrations: 2.5 mM Mg²⁺-2.5 mM ATP, conditions considered to have low free Mg²⁺; and 5.0 mM Mg²⁺-2.5 mM ATP, conditions providing a high free Mg²⁺ concentration of 2.5 mM. In the presence of high free Mg²⁺ (2.5 mM ATP-5.0 mM MgCl₂) the Tn inhibition of acto-S1-TM ATPase increased by approximately 40–50% over a range of S1 to actin ratios of 0.17:1 to 2.5:1. The effect of free Mg²⁺ on increasing quantities of Tn in the absence or presence of tropomyosin was studied independently at two S1 to actin ratios (1:1 and 2:1). In the absence of TM, at 5 mM Mg²⁺ there is an additional 38% (1:1 S1 to actin) or 37% (2:1) decrease in the ATPase activity by Tn compared to 2.5 mM Mg²⁺. Similarly, in the presence of TM and Tn, Mg²⁺ exerts its effect at both S1 to actin ratios. Significantly, the inhibition by the IT complex in the presence of TM is unaffected by free Mg²⁺. Furthermore, ultracentrifugation binding studies using¹⁴C-iodoacetamide-labeled Tn and TM established that the Tn-TM regulatory complex was firmly bound to F-actin at both Mg²⁺ concentrations, indicating that faciliation of binding to F-actin by Mg²⁺ is not responsible for the increased inhibition. Hence, it is concluded from the data that Mg²⁺ binding and by analogy Ca²⁺ binding to the Ca²⁺-Mg²⁺ sites of TnC promotes muscle relaxation by inducing inhibition of the actomyosin ATPase, whereas Ca²⁺ binding to the Ca²⁺-specific sites promotes contraction by potentiating the ATPase. The inhibition of the acto-S1-TM ATPase by TnT has also been further examined. The data indicate that TnT exerts the same level of inhibition upon the ATPase as TnI or Tn. The inhibitory activity requires TM, and occurs to the same extent under conditions where TM alone would have either a potentiating (2:1 S1 to actin) or an inhibitory (1:1 S1 to actin) effect upon the ATPase. In the presence of TM the IT complex is a more effective inhibitor than either TnI, TnT, or Tn. The inhibitory activity of the IT complex is partially released by TnC in the absence of Ca²⁺. These observations, in conjunction with those by Chong, Asselbergs, and Hodges, which showed that the inhibition by TnT is partially released by TnC plus Ca²⁺, indicate that the role of TnT involves more than anchoring Tn to the thin filament.     

Influence of Na+, K+, and Ca2+ on Glutamine Synthesis and Distribution in Rat Brain Cortex Slices: A Possible Linkage of Glutamine Synthetase with Cerebral Transport Processes and Energetics in the Astrocytes

The ouabain-induced suppression of glutamine synthesis and retention in incubating rat brain cortex slices was found to be mimicked by changes in the cationic content of the incubation medium, which cause an increase in the intracellular [Na+] and a decrease in the intracellular [K+]. The suppression of glutamine synthesis (and fixation of ammonia) was also found to take place when Ca2+ was omitted from the incubation medium. This occurred whether endogenous or exogenous glutamate was the substrate for glutamine synthesis. The suppressions cannot be due solely to an effect on glutamate uptake, because the uptake is not markedly affected by these conditions. The results show that Na+, K+, and Ca2+ influence the synthesis and distribution of glutamine in the brain. They suggest that Ca2+ and the Na+, K+ pump may serve a role in regulating the activity of ATP-dependent glutamine synthetase, a key enzyme of the glutamate-glutamine cycle, located in the astrocytes. This may be mediated via a direct effect on the enzyme or through an effect on the production of ATP.