Fluorine in the Earth and the solar system,where does it come from and can it be found?

This review (1) presents a summary of the distribution of fluorine in different fluid (surficial, subterranean, metamorphic, and magmatic–hydrothermal–geothermal) and solid (oceanic and continental crust, mantle, and core) domains of the Earth, and various extraterrestrial materials and bodies (meteorites, planets and moons, and the Sun); (2) it provides an estimate of the total fluorine abundance for the Earth and in its dominant reservoirs contributing to the Earth's fluorine endowment; and (3) it discusses key observations that could further improve our understanding of fluorine abundances and geochemical systematics.     

Polymer modeling: where has it been and where is it going?

The development of the area of polymer modeling often referred to as molecular modeling has been reviewed from its early beginnings to the present day. Key forces influencing the development include computational power, algorithmic advances and access to computational resources. The desire to apply modeling techniques to predict the properties of increasingly complex polymer-containing systems, taken in conjunction with a number of current limitations discussed in this brief review, is expected to define in part some essential future developments.     

Rotational flexibility in NH4+-encapsulated 18-crown-6 studied by molecular dynamics and positron annihilation: where does the positron localize?

Conformational flexibility of the ammonium complex of macrocyclic 18-crown-6 at 100 K < or = T < or = 300 K is studied using a positron microprobe in conjunction with molecular dynamics simulations. The ammonium cation, encapsulated in a three-pointed hydrogen-bonding perching arrangement, undergoes unhindered facile rotation inside the crown cavity, and the ether backbone adopts itself to the dynamic cation conformation. Preferential localization of the positron in the vicinity of the encapsulated cation results in the positron sensing changes in the local electron density distribution arising from the unrestricted rotational motion of the cation as well as from the slow motion of the crown backbone and hence sensing only an average conformation of the complex.     

Five-coordinate Fe(III)NO and Fe(II)CO porphyrinates: where are the electrons and why does it matter?

Recent years have seen dramatic growth in our understanding of the biological roles of nitric oxide (NO). Yet, the fundamental underpinnings of its reactivities with transition metal centers in proteins and enzymes, the stabilities of their structures, and the relationships between structure and reactivity remains, to a significant extent, elusive. This is especially true for the so-called ferric heme nitrosyls ([FeNO](6) in the Enemark-Feltham scheme). The Fe-CO and C-O bond strengths in the isoelectronic ferrous carbonyl complexes are widely recognized to be inversely correlated and sensitive to structural, environmental, and electronic factors. On the other hand, the Fe-NO and N-O bonds in [FeNO](6) heme complexes exhibit seemingly inconsistent behavior in response to varying structure and environment. This report contains resonance Raman and density functional theory results that suggest a new model for FeNO bonding in five-coordinate [FeNO](6) complexes. On the basis of resonance Raman and FTIR data, a direct correlation between the nu(Fe)(-)(NO) and nu(N)(-)(O) frequencies of [Fe(OEP)NO](ClO(4)) and [Fe(OEP)NO](ClO(4)).CHCl(3) (two crystal forms of the same complex) has been established. Density functional theory calculations show that the relationship between Fe-NO and N-O bond strengths is responsive to FeNO electron density in three molecular orbitals. The highest energy orbital of the three is sigma-antibonding with respect to the entire FeNO unit. The other two comprise a lower-energy, degenerate, or nearly degenerate pair that is pi-bonding with respect to Fe-NO and pi-antibonding with respect to N-O. The relative sensitivities of the electron density distributions in these orbitals are shown to be consistent with all published indicators of Fe-N-O bond strengths and angles, including the examples reported here.     

Weighted least-squares in calibration: what difference does it make?

In univariate calibration, an unknown concentration or amount x(0) is estimated from its measured response y(0) by comparison with a calibration data set obtained in the same way for known x values. The calibration function y = f(x) contains parameters obtained from a least-squares (LS) fit of the calibration data. Since minimum-variance estimation requires that the data be weighted inversely as their true variances, any other weighting leads to predictable losses of precision in the calibration parameters and in the estimation of x(0). Incorrect weighting also invalidates the apparent standard errors returned by the LS calibration fit. Both effects are studied using Monte Carlo calculations. For the strongest commonly encountered heteroscedasticity, proportional error (sigma(i) proportional, varianty(i)), neglect of weights yields as much as an order of magnitude precision loss for x(0) in the small x region, but only nominal loss in the calibration mid-range. Use of replicates gives great improvement at small x but can underperform unweighted regression in the mid-to-large x region. Variance function estimation approximates minimum-variance, even though the true variance functions are not well reproduced. A relative error test applied to the calibration data themselves is predisposed to favor 1/y(2) (or 1/x(2)) weighting, even if the data are homoscedastic. This predisposition weakens when replicate measurements are taken and disappears when the test is applied to an independent set of data. The distinction between a priori and a posteriori parameter standard errors is emphasized. Where feasible, the a priori approach permits reliable assignment of weights, application of a chi(2) test, and use of the normal distribution for confidence limits.     

Binding of amino acids to "smart" sorbents: where does hydrophobicity come into play?

Poly(N-isopropylacrylamide) (PNIPA)-based sorbents have been successfully used as sorbents in temperature-sensitive chromatography. Yet, the mechanisms controlling the binding of biochemicals to these sorbents and, therefore, the separation process are not fully understood. In the current work, the role of hydrophobic interactions in the binding of amino acids of different hydrophobicities to PNIPA microgels was studied. Binding experiments were conducted both below (25 degrees C) and above (37 degrees C) the volume-phase transition temperature of the gel. At 25 degrees C, no straightforward correlation between the partition coefficient and the hydrophobicity could be suggested for low hydrophobicity values. Contrary, at higher hydrophobicities the partition coefficient increases with increasing hydrophobicity. This correlation holds for the whole hydrophobicity range at 37 degrees C; however, the binding data suggests two different binding mechanisms of the hydrophilic amino acids and the hydrophobic ones. Isothermal titration calorimetry measurements confirmed this suggestion: The binding of hydrophobic amino acids seems to be driven by hydrophobic interactions, as evident from the positive binding enthalpy and the clear correlation between the amino acid's hydrophobicity and the binding entropy. Contrary, the binding of the hydrophilic amino acids was exothermic, implying a binding mechanism based on specific interactions, most probably hydrogen bonding.     

Metal bioavailability: how does its significance change in the time?

The significance of terms "metal bioavailability" and "bioavailable metal fraction" is evolved in the time, passing from a very simple concept to a complex concept bound to abiotic and biotic aspects. At the beginning metal toxicity was related to metal fraction present in water phase, than only free metal ion activity was considered and the free ion activity model (FIAM) was proposed. Successively, due to the exceptions observed and to the consciousness that metal bioavailability could be considered as dynamic characteristic the concept of metal bioavailability became very complex, depending on physical, chemical and biological factors.     

Communication: Where does the first water molecule go in imidazole?

Supersonic jet FTIR spectroscopy supplemented by (18)O substitution shows unambiguously that water prefers to act as an O-H···N hydrogen bond donor towards imidazole, instead of acting as a N-H···O acceptor. Previous matrix isolation, helium droplet, and aromatic substitution experiments had remained ambiguous, as are standard quantum chemical calculations. The finding is supported by a study of the analogous methanol complexes and by higher level quantum chemical calculations.     

O-alkyl resorcarenes: where are we now?

This review describes the current position in the emerging field of direct synthesis of O-alkyl resorcarenes through the use of O-protected precursors. The use of this approach for the selective synthesis of the diastereoisomers of resorc[4]arenes, the synthesis of parent resorcarenes and pyrogalloranes and the preparation of chiral partially alkylated resorc[4]arenes are highlighted. The applications of such molecules, with regard to their role as ligand platforms, for binding of cations, organic electron acceptors and in chiral discrimination are discussed.     

Ion spectroscopy: Where did it come from; where is it now; and where is it going?

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16–20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation “action spectroscopic” mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.     

Improving the intrinsic activity of electrocatalysts for sustainable energy conversion: where are we and where can we go?

As we are in the midst of a climate crisis, there is an urgent need to transition to the sustainable production of fuels and chemicals. A promising strategy towards this transition is to use renewable energy for the electrochemical conversion of abundant molecules present in the earth''s atmosphere such as H₂O, O₂, N₂ and CO₂, to synthetic fuels and chemicals. A cornerstone to this strategy is the development of earth abundant electrocatalysts with high intrinsic activity towards the desired products. In this perspective, we discuss the importance and challenges involved in the estimation of intrinsic activity both from the experimental and theoretical front. Through a thorough analysis of published data, we find that only modest improvements in intrinsic activity of electrocatalysts have been achieved in the past two decades which necessitates the need for a paradigm shift in electrocatalyst design. To this end, we highlight opportunities offered by tuning three components of the electrochemical environment: cations, buffering anions and the electrolyte pH. These components can significantly alter catalytic activity as demonstrated using several examples, and bring us a step closer towards complete system level optimization of electrochemical routes to sustainable energy conversion.

We evaluate the improvements over the past two decades in intrinsic activity of electrocatalysts for sustainable energy conversion, and highlight opportunities from tuning the electrolyte.     

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

This article discusses recent progress by a combination of spectroscopy and quantum-chemical calculations in classifying and characterizing organic mixed-valence systems in terms of their localized vs. delocalized character. A recently developed quantum-chemical protocol based on non-standard hybrid functionals and continuum solvent models is evaluated for an extended set of mixed-valence bis-triarylamine radical cations, augmented by unsymmetrical neutral triarylamine-perchlorotriphenylmethyl radicals. It turns out that the protocol is able to provide a successful assignment to class II or class III Robin-Day behavior and gives quite accurate ground- and excited-state properties for the radical cations. The limits of the protocol are probed by the anthracene-bridged system 8, where it is suspected that specific solute-solvent interactions are important and not covered by the continuum solvent model. Intervalence charge-transfer excitation energies for the neutral unsymmetrical radicals are systematically overestimated, but dipole moments and a number of other properties are obtained accurately by the protocol.     

Quality control of Photosystem II: where and how does the degradation of the D1 protein by FtsH proteases start under light stress?--Facts and hypotheses

Degradation of the reaction center-binding D1 protein of Photosystem II is central in photoinhibition of Photosystem II. In higher plant chloroplasts, Photosystem II complexes are abundant in the grana. It has been suggested that the Photosystem II complexes containing photodamaged D1 protein migrate for their repair from the grana to the non-appressed stroma thylakoids, where the photodamaged D1 protein is degraded by a specific protease(s) such as filamentation temperature sensitive H (FtsH) protease. There are several possible ways to activate the FtsH proteases. As FtsH is a membrane-bound ATP-dependent metalloprotease, it requires ATP and zinc as essential part of its catalytic mechanism. It is also suggested that a membrane protein(s) associated with FtsH is required for modulation of the FtsH activity. Here, we propose several possible mechanisms for activation of the proteases, which depend on oligomerization of the monomer subunits. In relation to the oligomerization of FtsH subunits, we also suggest unique distribution of active FtsH hexamers on the thylakoids: hexamers of the FtsH proteases are localized near the Photosystem II complexes at the grana. Degradation of the D1 protein probably takes place in the grana rather than in the stroma thylakoids to circumvent long-distance migration of both the Photosystem II complexes containing the photodamaged D1 protein and the proteases.     

PDLC shutters: where has this technology gone?

A Commentary on the paper ”Response times and voltages for PDLC shutters?, by Bao‐Gang Wu, John H. Erdman and J. William Doane. First published in Liquid Crystals, 5, 1453‐1465 (1989).     

How Does Diet Influence Our Lives? Evaluating the Relationship between Isotopic Signatures and Mortality Patterns in Italian Roman Imperial and Medieval Periods

The present research investigates the relationship between dietary habits and mortality patterns in the Roman Imperial and Medieval periods. The reconstructions of population dynamics and subsistence strategies provide a fascinating source of information for understanding our history. This is particularly true given that the changes in social, economic, political, and religious aspects related to the transition from the Roman period to the Middle Ages have been widely discussed. We analyzed the isotopic and mortality patterns of 616 individuals from 18 archeological sites (the Medieval Latium sites of Colonna, Santa Severa, Allumiere, Cencelle, and 14 Medieval and Imperial funerary contexts from Rome) to compile a survivorship analysis. A semi-parametric approach was applied, suggesting variations in mortality patterns between sexes in the Roman period. Nitrogen isotopic signatures influenced mortality in both periods, showing a quadratic and a linear effect for Roman Imperial and Medieval populations, respectively. No influence of carbon isotopic signatures has been detected for Roman Imperial populations. Conversely, increased mortality risk for rising carbon isotopic values was observed in Medieval samples.