George C. Pimentel (1922–1989): A Retrospective Personal and Pictorial Tribute a Decade after His Death

Ever since receiving the 1993 ACS Pimentel Award in Chemical Education (Figure 1), I have been interested in the life and achievements of the Fresno-born, larger-than-life figure after whom the award was named. The year 1999 provides an especially appropriate occasion to recall Pimentels personal and professional career, for it is the tenth anniversary not only of his death but also of National Chemistry Week, the expanded form of National Chemistry Day, the popular American Chemical Society outreach program that he originated.     

Photodynamic cross-linking of proteins: IV. Nature of the His–His bond(s) formed in the rose bengal-photosensitized cross-linking of N-benzoyl-L-histidine

The photodynamic (photosensitized) cross-linking of N-benzoyl-L-histidine (Bz-His) as a model system was examined as part of a continuing study of the role of His–His intermolecular cross-links in the photosensitized cross-linking of proteins. The illumination of Bz-His in the presence of rose bengal (RB) bound to water insoluble plastic beads in 0.1 M sodium phosphate buffer of pH 7.4 resulted in the covalent cross-linking of the His derivative. The main dimeric cross-linked product (1) was isolated using a preparative silica gel 60 column and purified by preparative reverse phase HPLC. The chemical structure of the cross-link was determined using MS, 2D NMR spectral methods and other standard techniques. Product 1 was found to be a dimer of two His residues between the δ2-carbon of one residue (photo-oxidized to the carbonyl functionality at the ε1-carbon) and the ε2-nitrogen of the other residue. The formation of His–His cross-links was mediated by singlet oxygen, as would be expected with RB as the sensitizer. A mechanism for the formation of the cross-link was proposed in which the first step was the 1,4-cycloaddition of singlet oxygen to the Bz-His imidazole ring to give an unstable endoperoxide. This then underwent changes followed by nucleophilic addition and the elimination of one molecule of water to give 1.     

Thylakoid membrane–based photobioelectrochemical systems: Achievements,limitations, and perspectives

During the few past decades, intensive efforts have been focused toward optimizing ‘wiring’ of thylakoid membranes (TMs) to conductive surfaces for their further application for light-energy conversion and production of an electron flow. This review summarizes recent progress and current state of photobioelectrodes and systems based on TMs. We consider various electrode materials and surface modifications, mediators and setup designs reported until now and compare achieved current and power outputs. Finally, we discuss further potentially valuable applications and perspectives of TM-based bioelectrochemical systems.     

Rolf Huisgen,Eminent Chemist and Polymath (1920–2020): In His Own Words and In His Publication Metrics

As a compliment to several other publications that present and honor Rolf Huisgen's research achievements, the first part of this paper reveals the human side of this eminent chemist. From excerpts from many of his personal and professional writings, Huisgen's personality and philosophies of life are revealed. Also revealed is Huisgen functioning as a historian of chemistry. The second part of this paper examines the scientometrics of Huisgen's publication history. In the late 1950s and early 1960s, Huisgen's career experienced a major transition in terms of publication metrics and the influence these papers had on the organic chemistry community. This was the result of his research into 1,3-dipolar cycloadditions. Citations to his scientific contributions are well spread over many of his papers, demonstrating his constant work and the building up of a research topic, which continued after his official retirement in 1988. In fact, 17 % of his more than 600 publications appeared after 1988. The majority of Huisgen's papers were co-authored with his many graduate and postdoctoral students. Consistent with the trend of that era, Huisgen was the sole author of most of his Review articles, and not just those of his many plenary lectures, and it is those Review articles that proved to be his most cited publications. This demonstrates the power and influence of Review articles—secondary sources, in the vocabulary of historians and sociologists of science. In those Review articles, Huisgen principally described the state of the art of 1,3-dipolar cycloadditions—his golden offspring.     

Adenine–Au and adenine–uracil–Au. Non-conventional hydrogen bonds of the anions and donator–acceptor properties of the neutrals

This is a study of adenine–Au and adenine–uracil–Au (neutral, anionic and cationic), applying the B3LYP density-functional approach. In these systems, the interaction is directly related to the charge; so that as the metal atomic charge increases, the bond strength also increases. Neutral molecules are weakly bonded, the interaction in the case of cations is mainly electrostatic and in the case of the anions, the extra electron is localized on the metal atom and consequently, non-conventional hydrogen bonds are formed. In the case of adenine–Au (anion), the H dissociation energy is similar to the electron dissociation energy, and therefore both reactions may be possible. Moreover, the Au anionic atom modifies the hydrogen bonds of the uracil–adenine base pair. This may be significant in the study of point mutations that may occur in the Watson–Crick dimmer of nucleic basis. The electron-donator properties of these compounds are analyzed with the aid of the donator–acceptor map (DAM), previously described. Adenine–Au, uracil–Au and adenine–uracil–Au are more effective electron donors, but poorer electron acceptors than adenine, uracil and adenine–uracil. If the electron acceptor properties of carotenoids such as β-carotene and astaxanthin are compared, there are indications that astaxanthin may act as an oxidant instead of an antioxidant with the uracil–adenine base pair. The oxidation of nucleic acid bases by carotenoids may have important consequences, as oxidative damage of DNA and RNA appears to be linked to cancer. This is something that demands further studies and for this reason, work concerning the reactivity of carotenoids with DNA-nitrogen bases is in progress.     

Optimized structures and vibration frequencies of the ether–water complex: a DFT and FTIR study

The infrared spectrum of ether was studied using Fourier transform infrared spectroscopy in conjunction with the density functional theory (DFT). The optimized structures and vibrational frequencies of the ether·(H₂O) _n (n = 1–3) complexes were obtained at B3LYP/6-31G(d) theory levels. Compared to those of free-form ether, the C–O stretching vibrational frequencies of the ether–water complexes are found to shift to red by up to 39 cm^?1 with an increase in the C–O length of 0.016 Å. Meanwhile, the frequency of the O–H stretching modes of water in the complexes appears significantly redshifted to a varying degree. The DFT calculations suggest that these shifts are caused by the hydrogen bonding between ether and water.     

Brightening,Blinking, Bluing and Bleaching in the Life of a Quantum Dot: Friend or Foe?

Semiconductor nanocrystals or quantum dots (QDs) are highly photoluminescent materials with unique optical attributes that are being exploited in an ever‐increasing array of applications. However, the complex surface chemistry of these finite‐sized fluorophores gives rise to a number of photophysical phenomena that can complicate their use in imaging applications. Fluorescence intermittency (FI), photoluminescence enhancement (PLE) and spectral bluing are properties of QD emission that would appear, at first sight, detrimental to quantitative measurement. Fortunately, developments in rational QD synthesis and surface modification are promising to minimize the effects of these fluorescence instabilities, while applications that exploit them are now coming to the fore. We review recent experimental and theoretical studies of FI, PLE and bluing, highlighting the benefits, as well as complications, they bring to key applications.     

Polymorphism of Inclusion Complexes and Unsolvated Hosts. V. Crystal Structure of the α-Dimorph of the 1 : 2 Dianilinegossypol–DMSO Complex and a 1 : 1 : 1 Inclusion Complex Based on a Mixed Dianilinegossypol/1,4-Dioxane Host Matrix and 1,2-Dichloroethane Guest

Dianilinegossypol forms a 1 : 2 host-guest complex with DMSO:monoclinic, space group P2₁/n, a = 8.522(3), b = 18.034(4), c= 28.462(6) , = 94.14(2)°, V = 4362³, Z = 4, D x = 1.26 g cm^-3, T = 295 K.Final R value is 0.102 for 1793 observed reflections. A 1 : 1 : 1 adduct ofdianilinegossypol with 1,4-dioxane and 1,2-dichloroethane is found to beisostructural with the dianilinegossypol complex with DMSO: monoclinic,space group P2₁/n, a = 8.281(2), b = 19.245(3), c = 27.970(7), = 95.18°, V = 4439 ³, Z = 4, D x =1.28 g cm^-3, T = 295 K. Final R value is 0.114 for 2458observed reflections.The host molecules are associated by O(4)—H ...O(3) H-bonds toinfinite chains running in the direction of the c-axis The chains areincorporated into layers through 1,4-dioxane or DMSO molecules havingH-bonds with dianilinegossypol molecules. Another DMSO or 1,2-dichloroethanemolecule is included as a guest in the channels formed between the layers.At 60 °C a cryptate-type inclusion complex of dianilinegossypol isformed with DMSO or 1,4-dioxane. It is isostructural with the acetonecomplex reported in Part IV of the present series.     

M. S. Tswett and the discovery of chromatography I: Early work (1899–1903)

Summary On the occasion of the 90th anniversary of the Warsaw lecture of M. S. Tswett representing the first report on the chromatographic technique, this two-part article outlines the evolution of the method. This part deals with Tswett's early work in 1899–1901 and his research in 1901–1903 leading to the lecture at Warsaw. The second part will deal with the fundamental twin papers of 1906, with his book of 1910, summarizing all his activities related to the investigation of plant pigments and to the development of chromatography, and with a few special questions related to Tswett's scientific activities in this field.Part II of this article will be published in the next month's issue ofChromatographia.     

Relativistic Spin–Orbit Electronegativity and the Chemical Bond Between a Heavy Atom and a Light Atom

Relativistic effects are known to alter the chemical bonds and spectroscopic properties of heavy-element compounds. In this work, we introduce the concept of spin–orbit (SO) electronegativity of a heavy atom, as reflected by an SO-induced change in the interatomic distance between the heavy atom (HA) and a neighboring light atom (LA). We provide a transparent interpretation of these SO effects by using the concept of spin–orbit electron deformation density (SO-EDD). Spin–orbit coupling at the HA induces rearrangement of the electron density for the scalar-relativistically optimized geometry that, in turn, exerts a new force on the LA. The resulting expansion or contraction of the HA−LA bond depends on the nature and electron configuration of the HA. In addition, we quantify the change in atomic electronegativity induced by SO coupling for a series of hydrides, thereby complementing the SO-EDD picture. The trends in the SO-induced electronegativity and the HA−LA bond length across the periodic table of elements are demonstrated and interpreted, and also linked, intuitively, with the SO-induced NMR shielding at the LA.     

M. S. Tswett and the discovery of chromatography II: Completion of the development of chromatography (1903–1910)

Summary In this, the second part of our review we continue the discussion of the activities of M. S. Tswett related to the development of chromatography. We shall deal here with his activities in Warsaw from 1903 onwards, resulting in his fundamental twin papers of 1906 and his book of 1910, summarizing all his activities related to the investigation of plant pigments and to the development of chromatography. Finally, a few special questions related to Tswett's work shall be discussed.For Part I, seeChromatographia 35, 223 (1993). References are numbered through.     

An Investigation of the Positive Column of a Cd–Ne Glow Discharge: II. Afterglow

The afterglow of a Cd–Ne glow discharge is investigated theoretically and experimentally. The time-dependent electron Boltzmann equation and the balance equations for the Cd excited states and charged particles have been self-consistently solved. The temporal relaxation of the electron energy distribution function, electron, and cadmium excited states densities is reported and the influence of neon and cadmium vapor pressure on the plasma characteristics in the afterglow is studied. The modeling compares favorably with experimental results.