Orientational effect of aryl groups in aryl selenides: how can 1h and 13c NMR chemical shifts clarify the effect?

Two sets of delta(H) and delta(C) are proposed by employing 9-(arylselanyl)anthracenes [9-(p-YC6H4Se)Atc: 1] and 1-(arylselanyl)anthraquinones [1-(p-YC6H4Se)Atq: 2] with various Y's. Structures of 1 and 2 are (A: pl) and (B: pd), respectively, for all Y examined in chloroform-d. After elucidation of the behavior of delta(H, C: 1) and delta(H, C: 2), they are applied to determine the structures in chloroform-d solutions for 1-(arylselanyl)naphthalenes (3), 1-(arylselanyl)-2-methylnaphthalenes (4), and 1-(arylselanyl)-8-bromonaphthalenes (5). Although the structure of 4 remains in (A: pl) in the solutions for all Y examined, that of 5 is (B: pd), except for Y = CN and NO2. On the other hand, 3 is shown to equilibrate between (A: pl) and (B: pd). Although the contributions of (B: pd) and (A: pl) are predominant for Y = NMe2 and NO2, respectively, the equilibrium constants change from Y to Y in the solutions. The results are supported by the quantum chemical calculations, containing the solvent effect of chloroform. These results demonstrate that delta(H, C: 1) and delta(H, C: 2), as well as delta(Se), serve as the practical standards for pl and pd, respectively, to analyze the structures of p-YC6H4ZR (Z = Se) in solutions.     

How do ring currents affect (1)h NMR chemical shifts?

Conventional explanations of proton NMR chemical shifts need fundamental revisions. Ab initio (IGLO) analyses reveal that the downfield delta (1)H of benzene is not due to deshielding ring current effects; the shielding is less than the pi contribution to vinyl delta (1)Hs. Enhanced deshielding sigma CC influences are responsible for the more downfield delta (1)Hs of the inner protons of naphthalene and anthracene. Double pi effects shield ethynyl Hs; there is no evidence for a special "ring current influence."     

Topology prediction of helical transmembrane proteins: how far have we reached?

Transmembrane protein topology prediction methods play important roles in structural biology, because the structure determination of these types of proteins is extremely difficult by the common biophysical, biochemical and molecular biological methods. The need for accurate prediction methods is high, as the number of known membrane protein structures fall far behind the estimated number of these proteins in various genomes. The accuracy of these prediction methods appears to be higher than most prediction methods applied on globular proteins, however it decreases slightly with the increasing number of structures. Unfortunately, most prediction algorithms use common machine learning techniques, and they do not reveal why topologies are predicted with such a high success rate and which biophysical or biochemical properties are important to achieve this level of accuracy. Incorporating topology data determined so far into the prediction methods as constraints helps us to reach even higher prediction accuracy, therefore collection of such topology data is also an important issue.     

Pushing the detectability of voltammetry: how low can we go?

The coupling of adsorptive accumulation with catalytic reactions results in remarkably low (sub-picomolar) detection limits. This review assesses various strategies for attaining such dual-amplification effects, that lead to the most sensitive voltammetric technique, adsorptive-catalytic stripping voltammetry (AdCtSV).     

Oxygen-17 NMR spectroscopy: σI dependence for the substituent chemical shifts of 3-substituted pyridine 1-oxides

¹⁷O NMR spectra were measured at natural abundance in DMSO for a series of 3-substituted pyridine 1-oxides and their substituent chemical shifts were found to be correlated with σ_I substituent constants and not with ΔpK_a values.     

Resonance raman spectroscopy of conjugated polymers what can we learn?

This paper gives a review on the use of resonance Raman spectroscopy to analyse electronic and molecular structures of π-conjugated and σ-conjugated polymers. It starts with a short introduction into the present state of research in the field of conducting polymers and with a very short introduction into the principles of resonance Raman spectroscopy. Then the potentialities of the technique are demonstrated for the examples of the analysis of conjugation length in polyacetylene, for studying the thermochromic phase transition in polysilanes and in polyalkylthiophenes, for a sudy of the doping processes in polyaniline and finally for an analysis of the ground state in the narrow gap system polyisothianaphthene. Advantages and pitfalls for such analyses are outlined.     

NMR crystallography of zeolites: How far can we go without diffraction data?

Nuclear magnetic resonance (NMR) crystallography—an approach to structure determination that seeks to integrate solid-state NMR spectroscopy, diffraction, and computation methods—has emerged as an effective strategy to determine structures of difficult-to-characterize materials, including zeolites and related network materials. This paper explores how far it is possible to go in determining the structure of a zeolite framework from a minimal amount of input information derived only from solid-state NMR spectroscopy. It is shown that the framework structure of the fluoride-containing and tetramethylammonium-templated octadecasil clathrasil material can be solved from the 1D ²⁹Si NMR spectrum and a single 2D ²⁹Si NMR correlation spectrum alone, without the space group and unit cell parameters normally obtained from diffraction data. The resulting NMR-solved structure is in excellent agreement with the structures determined previously by diffraction methods. It is anticipated that NMR crystallography strategies like this will be useful for structure determination of other materials, which cannot be solved from diffraction methods alone.     

Complexation of ephedrine withβ-cyclodextrin: An NMR spectroscopy study

This study focuses on the inclusion complex of ephedrine with -CD. The association of -CD and ephedrine has been examined using¹H NMR and circular dichroism. The systematic shifts of the proton resonances of the phenyl moiety of ephedrine and that of the protons located inside the -CD cavity, provide evidence of intracavity inclusion. Two-dimensional ROESY show preferential localization of ephedrine in close proximity with protons located inside the -CD cavity. The systematic variation of circular dichroism spectra with increasing concentration of -CD is used to estimate the apparent formation constant.Emory University School of Medicine, Atlanta, GA 30322, U.S.A.     

Designer haem proteins: What can we learn from protein engineering?

Iron protoporphyrin(IX) is one of the most versatile and widespread pieces of catalytic machinery known in biology and is a key component of a multitude of proteins and enzymes. One of most challenging questions in this area has been to identify and understand the relationships that exist between different classes of haem proteins and to use protein engineering methods to rationalize the mechanisms by which the protein structure controls the specific chemical reactivity of the haem group. The application of this approach to the haem enzyme ascorbate peroxidase and the haem protein leghaemoglobin is discussed. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:501–505, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10094     

Probing the chemical reactivity of interfaces: Investigation on the interaction of dehydroindigo with Laponite by UV–vis,Raman and infrared spectroscopy

In this work, the interaction between dehydroindigo (an intermediary oxidized form of indigo) and Laponite clay was investigated. Dehydroindigo (DHI) has been detected when indigo is adsorbed by clay minerals, but it is relatively unstable and in the presence of water it turns back into indigo. It is, therefore, important to understand the factors that extend its stability and Laponite was chosen because the small aspect ratio implies in a large amount of silanol groups (SiOH) which would thus favor the DHI interaction through hydrogen bonding.A significant bathochromic shift (65 nm) of the DHI π®π* transition band in the visible region and changes in the relative intensities and position of the Raman bands at 1530, 1378 and 1167 cm⁻¹ assigned to ν(NCCN), δ(CN) and ν(CN) respectively, indicate that the interaction is stronger than expected for the van der Waals and polarization forces involved in the external surfaces interactions with the siloxane groups. It was also observed that DHI presents an enhanced photochemical stability when interacting with Laponite. These results indicate that hydrogen bonding between a DHI nitrogen atom and the −SiOH or MOH groups is mainly responsible for the behavior present in the DHI + Lap system.     

Experiments with proteins at low temperature: what do we learn on properties in their functional state?

The authors compared the spectral response of Zn-substituted horseradish peroxidase in a glycerol/water solvent to hydrostatic pressure at 2 K and ambient temperature. The low temperature experiments clearly demonstrate the presence of at least three different conformations with drastically different elastic properties. However, the main conformation, which determines the fluorescence spectrum at ambient temperature, did not show any significant difference between low and high temperature and pressure. The authors conclude that the local compressibility of the heme pocket of the protein depends only very weakly on temperature.     

Traceability of environmental chemical analyses: can theory match practice?

According to the ISO definition, the traceability concept basically implies that measurement data are linked to stated references through an unbroken chain of comparisons, all with stated uncertainties. This concept may be quite clear in theory, but we may wonder how it may be applicable to complex chemical measurements such as environmental chemical analyses in practice. This paper discusses this issue, giving some examples of drawbacks that are being faced in different environmental sectors (water, sediment, soil, biota and particulate atmospheric samples).     

35 years of palladium-catalyzed cross-coupling with Grignard reagents: how far have we come?

This tutorial review is intended to provide the reader with a timely review of major developments and the current state-of-the-art of palladium-catalyzed cross-coupling reactions with Grignard reagents. Organomagnesium reagents, the most reactive and most easily accessible nucleophiles for carbon-carbon bond forming cross-coupling reactions, were the first nucleophiles ever employed in cross-coupling reactions, but have only recently been re-discovered for highly efficient and (stereo)selective coupling reactions. This is mostly a consequence of improved catalyst systems with bulky phosphine, phosphonate or carbene ligands and new metal-halogen exchange procedures for the generation of functionalized Grignard reagents.     

AFM of biological complexes: What can we learn?

The term “biological complexes” broadly encompasses particles as diverse as multisubunit enzymes, viral capsids, transport cages, molecular nets, ribosomes, nucleosomes, biological membrane components and amyloids. The complexes represent a broad range of stability and composition. Atomic force microscopy offers a wealth of structural and functional data about such assemblies. For this review, we choose to comment on the significance of AFM to study various aspects of biology of selected non-membrane protein assemblies. Such particles are large enough to reveal many structural details under the AFM probe. Importantly, the specific advantages of the method allow for gathering dynamic information about their formation, stability or allosteric structural changes critical for their function. Some of them have already found their way to nanomedical or nanotechnological applications. Here we present examples of studies where the AFM provided pioneering information about the biology of complexes, and examples of studies where the simplicity of the method is used toward the development of potential diagnostic applications.     

Distribution of N-glycosylation sequons in proteins: how apart are they?

N-glycosylation is a common protein modification process, which affects a number of properties of proteins. Little is known about the distribution of N-glycosylation sequons, for example, the distance between glycosylated sites and their position in the protein primary sequence. Using a large set of experimentally confirmed eukaryotic N-glycoproteins we analyzed the relative position and distribution of sequons. N-Glycosylation probability was found to be lower in the termini of protein sequences compared to the mid region. N-glycosylated sequons were found much farther from C terminus compared to the N-terminus of the protein sequence and this effect was more pronounced for NXS sequons. The distribution of sequons, modeled based on balls-in-boxes classical occupancy, showed a near-maximum probability. Considerable proportion of sequons was found within a distance of ten amino acids, indicating that the steric hindrance was not a key factor in protein N-glycosylation. Interestingly, the distribution of all sequons present in N-glycoproteins showed a pattern very similar to that of glycosylated sequons. The results indicate that protein N-glycosylation chiefly follows a random design.     

Complex formation of crown ethers with α-amino acids: Estimation by NMR spectroscopy

Complex formation of 18-crown-6 and dibenzo crowns with glycine, leucine, and norleucine was studied by NMR spectroscopy. The efficiency of non-valence interactions with participation of different active centers of the host and guest molecules is determined by solvation effects, mutual arrangement of benzene rings in dibenzo crowns, and the presence of bulky aliphatic substituents in the α-amino acid. The complexation of dibenzo crowns with α-amino acids in acid medium involves a system of different non-valence interactions, the most efficient of which are NH ₃ ⁺ ... O hydrogen bond between the ammonium group in the guest molecule and ether oxygen atoms in the host molecule and dipole-dipole interaction between the guest ammonium group and host benzene ring (NH ₃ ⁺ ... Ar). The efficiency of NH ₃ ⁺ ... O hydrogen bonding decreases in going from 18-crown-6 to dibenzo crowns due to distortion of symmetry of the macroring cavity and violation of geometric complementarity of some ether oxygen atoms. The integral efficiency of non-valence interactions in the system dibenzo crown-α-amino acid was estimated on a quantitative level by ¹H NMR relaxation technique.