Why are quasicrystals quasiperiodic?

In the past two decades significant progress has been made in the search for stable quasicrystals, the determination of their structures and the understanding of their physical properties. Now, quasiperiodic ordering states are not only known for intermetallic compounds, but also for mesoscopic systems such as ABC-star terpolymers, liquid crystals or different kinds of colloids. However, in spite of all these achievements fundamental questions concerning quasicrystal formation, growth and stability are still not fully answered. This tutorial review introduces the research in these fields and addresses some of the open questions concerning the origin of quasiperiodicity.     

Why do drying films crack?

Understanding the mechanism by which films fail during drying is the first step in controlling this natural process. Previous studies have examined the spacing between cracks with predictions made by assuming a balance between elastic energy released with a surface energy consumed. We introduce a new scaling for the spacing between cracks in drying dispersions. The scaling relates to the distance that solvent can flow, to relieve capillary stresses, as a film fails. The scaling collapses data for a range of evaporation rates, film thicknesses, particle sizes, and materials. This work identifies capillary pressures, induced by packed particle fronts travelling horizontally across films, as responsible for the failure in dried films.     

Science and Arts,Philosophy and Science: Why after All? Why Not?

This perspective summarizes some interdisciplinary aspects of science and the relation to philosophy, also including the basic motivations and aims as they might be discussed with young scientists starting their careers and presented also in the form of a commencement speech. The contents of this speech were repeatedly discussed also with Jack Dunitz, who showed great interest in it, given his broad interests. The speech also referred to an earlier commencement speech by Jack Dunitz in 1989. In the introduction of our essay, we mention the early common history of science and humanities under the name of philosophy. This early history can be traced back to ancient Greek philosophy and the ‘academy’ of Platon in Athens with a history of more than 1000 years until closure in 529 AD, in modern times revived as the National Greek Academy in Athens in the 19th and 20th centuries. Other ‘academies’ in Europe started in the 17th century and had publications under various names involving ‘philosophy’ with a focus on what we call science (natural science) today. After about 1800 there was increasing fragmentation of the various fields of knowledge and philosophy was considered to be part of the modern ‘humanities’ quite separate from science, and the natural sciences were fragmented into physics, chemistry, biology etc., and even finer subdivisions. The essay also describes an effort at ETH Zurich, reintegrating the various subfields of science and also stressing an education of scientists and engineers in the humanities. The essay concludes with a discussion of several global risks for mankind and a scientific imperative to maintain life on Earth. The common aspects and the foundations of all sciences as fields of knowledge aiming for an understanding of the world around us and of human beings as part of it are discussed from various perspectives.     

Why send humans to Mars?

Presidential long-term direction for the U.S. Space Program includes the goal of landing humans on Mars. Criticism of this goal includes executive commitment, NASA technical capabilities, and economics. The author suggests landing humans or robots on Mars is possible only with international cooperation on technical and economic issues.     

Why are water-hydrophobic interfaces charged?

We report ab initio molecular dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the molecular charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) molecules. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several experimental observations and explain why hydrophobic surfaces in contact with water acquire a net negative charge, a phenomenon that has important implications for biology and polymer science.     

Why do some Fischer indolizations fail?

The mechanisms of the Fischer indole synthesis and competing cleavage pathways were explored with SCS-MP2/6-31G(d) and aqueous solvation calculations. Electron-donating substituents divert the reaction pathway to heterolytic N-N bond cleavage and preclude the acid-promoted [3,3]-sigmatropic rearrangement.     

Why are Cold Molecules so Hot?

Brrrr! Cold molecule and cold atom research are juxtaposed and the challenges in cooling and trapping molecules are recounted. Both indirect and direct techniques of producing cold and slow molecules (such as buffer‐gas cooling and magnetic trapping, see picture) are described. Advanced techniques of manipulating cold or slow molecules are reviewed and ongoing work with cold molecules is outlined.

     

Why do molecules echo atomic periodicity?

Franck–Condon factors are investigated for sequences of free main‐group diatomic molecules. Theory‐based Condon loci (parabolas) and Morse‐potential loci are plotted on Deslandres tables to verify if they, indeed, follow the largest Franck–Condon factors. Then, the inclination angles of the Condon loci are determined. Thus, entire band systems are quantified by one variable, the angle. For all available isoelectronic sequences, this angle increases from a central minimum toward magic‐number molecular boundaries. The theory for the Condon locus gives the angle in terms of the ratio of the upper‐state to the lower‐state force constants. It is concluded that the periodicity is caused due to the fact that this ratio becomes larger as rare‐gas molecules are approached, a trend that probably points to the extreme cases of the rare‐gas molecules themselves. Thus, molecular periodicity echoes atomic periodicity in that data plots have extrema at molecules with magic‐number atoms, yet it does not echo the details of atomic periodicity in series between those molecules. © 2013 The Authors. International Journal of Quantum Chemistry Published by Wiley Periodicals, Inc.     

Why Does 258Fm Fission Symmetrically ?

²⁵⁸Fm fissions partly symmetrically because fission fragments with Z=50, such as ^126–132Sn, are energetically enormously favored, and because deep collective pick-up processes such as that leading to the ideally symmetric configuration ¹²⁹Sn-¹²⁹Sn can occur within a very energy-rich primordial configuration of the fissioning nucleus. The width of 8 mass units of the distribution can be justified; an even narrower mass distribution can be predicted for ²⁶⁴Fm.     

A future for nuclear analytical techniques? Why not?

The choices of universities and national research institutions in supporting scientific research are increasingly justified on the basis of, amongst others, the relevance that has to be reflected by external, preferably sustainable funding of the research programs. Many traditional fields of application such as environmental sciences do not offer a promising outlook in this respect. As a consequence, university research reactors face closure because of reallocations of university funds to more contemporary sciences such as molecular biology and nanotechnology. Therefore, laboratories operating nuclear analytical techniques (NAA, (TR)XRF, and PIXE) need to use their creativity in finding ways for participation in, for example, nanotechnology, cancer research, or genomics. This requires an open mind in terms of the opportunities, strengths, and weaknesses of the techniques, and a departure of technique-oriented research towards problem-oriented research in which other nuclear techniques can be used. The unique features of radiotracers, nuclear imaging, and nuclear beam techniques are discussed in view of the new areas mentioned above. Some examples of opportunities for nuclear analytical techniques in the above-mentioned fields are given.     

Why 11-cis-retinal? Why not 7-cis-, 9-cis-, or 13-cis-retinal in the eye?

One of the basic and unresolved puzzles in the chemistry of vision concerns the natural selection of 11-cis-retinal as the light-sensing chromophore in visual pigments. A detailed computational examination of the structure, stability, energetics, and spectroscopy of 7-cis-, 9-cis-, 11-cis-, and 13-cis-retinal isomers in vertebrate (bovine, monkey) and invertebrate (squid) visual pigments was carried out using a hybrid quantum mechanics/molecular mechanics (QM/MM) method. The results show that the electrostatic interaction between retinal and opsin dominates the natural selection of 11-cis-retinal over other cis isomers in the dark state. In all of the pigments, 9-cis-retinal was found to be only slightly higher in energy than 11-cis-retinal, which provides strong evidence for the presence of 9-cis-rhodopsin in nature. 7-cis-Retinal is suggested to be an "upside-down" version of the all-trans isomer because the structural rearrangements observed for 7-cis-rhodopsin from squid were found to be very similar to those for squid bathorhodopsin. The progressive red shift in the calculated absorption wavelength (λ(max)) (431, 456, 490, and 508 nm for the 7-cis-, 9-cis-, 11-cis-, and 13-cis-retinal isomers) is due to the decrease in bond length alternation of the retinal.     

Why are glycoproteins modified by poly-N-acetyllactosamine glyco-conjugates?

Poly-N-acetyllactosamine structures occur in mammalian glycoproteins in both N- and O-linked glycans. They represent a backbone for additional modifications by fucosyltransferases, sialyltransferases and sulfotransferases. These glycans have been suggested to be involved in biospecific interactions with selectins and other glycan-binding proteins. Moreover, the poly-N-acetyllactosamine chains in N-glycans have been found to promote tumor progression and metastasis. Poly-N-acetyllactosamine chains are synthesized by repeated alternating additions of N-acetylglucosamine and galactose, catalyzed by beta-1,3-N-acetylglucosaminyltransferases (poly-N-acetyllactosamine synthase) and beta-1,4-galactosyltransferases. This review describes the poly-N-acetyllactosamine assembling machinery and focuses on recent advances in the molecular cloning and characterization of poly-N-acetyllactosamine synthase gene families. Recent progress in revealing the biological functions of poly-N-acetyllactosamine structures by various approaches in vitro and in vivo using different model systems has also been summarized.     

Why do food analysts need quality assurance?

 Most sophisticated products require testing for compliance with specifications and safety regulations before release into many markets, and trade in many simpler commodities and products also requires supporting technical information. Test documentation has become an essential element in this trade. Food intended for human consumption certainly falls into the "sophisticated products" category. Lack of acceptance of laboratory test data across national borders may be a significant barrier to trade. In order to avoid such barriers and unnecessary duplication of laboratory tests, mutual recognition of laboratory results should be regarded as an important means of facilitating international trade in food products. It is difficult to envisage recognition of test data across borders without internationally agreed criteria for assessing the competence of testing. These criteria should, as a minimum, require that a laboratory involved in the analysis of foods operates a suitable quality system. The laboratory must create a quality system appropriate to the type, range and volume of work performed. It is necessary for the elements of this system to be documented in a quality manual which is available for use by the laboratory personnel. The quality manual must be kept up-to-date by a person or persons having responsibility for quality assurance within the laboratory. This paper describes and discusses the elements of a quality system in a food laboratory, including suitable quality assurance measures, the use of validated analytical methods and participation in proficiency testing schemes. Received: 24 February 1996 Accepted: 13 March 1996     

Why hybrid porous solids capture greenhouse gases?

Hybrid porous solids, with their tunable structures, their multifunctional properties and their numerous applications, are currently topical, particularly in the domain of adsorption and storage of greenhouse gases. Most of the data reported so far concern the performances of these solids in this domain, particularly in terms of adsorbed amounts of gas but do not explain at the atomic level why and how adsorption and storage occur. From a combination of structural, spectroscopic, thermodynamic experiments and of molecular simulations, this tutorial review proposes answers to these open questions with a special emphasis on CO(2) and CH(4) storage by some rigid and flexible hybrid porous materials.     

Why is hydrofluoric acid a weak acid?

The infrared vibrational spectra of amorphous solid water thin films doped with HF at 40 K reveal a strong continuous absorbance in the 1000-3275 cm(-1) range. This so-called Zundel continuum is the spectroscopic hallmark for aqueous protons. The extensive ionic dissociation of HF at such low temperature suggests that the reaction enthalpy remains negative down to 40 K. These observations support the interpretation that dilute HF aqueous solutions behave as weak acids largely due to the large positive reaction entropy resulting from the structure making character of the hydrated fluoride ion.     

Why is autophagy important in human diseases?

The process of macroautophagy (referred to hereafter as autophagy), is generally characterized by the prominent formation of autophagic vesicles in the cytoplasm. In the past decades, studies of autophagy have been vastly expanded. As an essential process to maintain cellular homeostasis and functions, autophagy is responsible for the lysosome-mediated degradation of damaged proteins and organelles, and thus misregulation of autophagy can result in a variety of pathological conditions in human beings. Although our understanding of regulatory pathways that control autophagy is still limited, an increasing number of studies have shed light on the importance of autophagy in a wide range of physiological processes and human diseases. The goal of the reviews in the current issue is to provide a general overview of current knowledge on autophagy. The machinery and regulation of autophagy were outlined with special attention to its role in diabetes, neurodegenerative disorders, infectious diseases and cancer.     

Why are a 3 ions rarely observed?

It has been determined experimentally that a(3) ions are generally not observed in the tandem mass spectroscopic (MS/MS) spectra of b(3) ions. This is in contrast to other b(n) ions, which often have the corresponding a(n) ion as the base peak in their MS/MS spectra. Although this might suggest a different structure for b(3) ions compared to that of other b(n) ions, theoretical calculations indicate the conventional oxazolone structure to be the lowest energy structure for the b(3) ion of AAAAR, as it is for other b(n) ions of this peptide. However, it has been determined theoretically that the a(3) ion is lower in energy than other a(n) ions, relative to the corresponding b ions. Furthermore, the a(3) --> b(2) transition structure (TS) is lower in energy than other a(n) --> b(n-1) TSs of AAAAR, compared with the corresponding b ions. Consequently, it is suggested that the b(3) ion does fragment to the a(3) ion, but that the a(3) ion then immediately fragments (to b(2) and a(3)) because of the excess internal energy arising from its relatively low energy and the facile a(3) --> b(2) reaction. That is why a(3) ions are not observed in the MS/MS spectra of b(3) ions.     

Why not using capillary electrophoresis in drug analysis?

CE and related methods are well-established techniques in the analysis of biomolecules, such as DNA and proteins. Even though CE is a rather good alternative to HPLC for the evaluation of the impurity profile and the enantiomeric purity of a drug, it is rarely applied. This might be due to the reservation of national licensing authorities and the pharmacopoeia commissions for several reasons. In this review containing some experimental data we report on several drug examples which demonstrate the superiority of CE over HPLC in special cases, i.e., in the analysis of antibiotics, amino acids and peptides, and the determination of enantiomeric purity. However, in order to make the CE techniques more suitable for pharmacopoeial purposes the general methods describing separation methods have to be complemented with the adjustment of the electrophoretic conditions being necessary to satisfy the system suitability criteria without fundamentally modifying the methods. Taken together CE should be more often applied in drug quality control.     

Why is Firefly Oxyluciferin a Notoriously Labile Substance?

The chemistry of firefly bioluminescence is important for numerous applications in biochemistry and analytical chemistry. The emitter of this bioluminescent system, firefly oxyluciferin, is difficult to handle. The cause of its lability was clarified while its synthesis was reinvestigated. A side product was identified and characterized by NMR spectroscopy and X‐ray crystallography. The reason for the lability of oxyluciferin is now ascribed to autodimerization of the coexisting enol and keto forms in a Mannich‐type reaction.