Theoretical analysis of the porphyrin-porphyrin exciton interaction in circular dichroism spectra of dimeric tetraarylporphyrins

Chiral bis-porphyrins are currently the subject of intense interest as chiral receptors and as probes in the determination of structure and stereochemistry. To provide an improved framework for interpreting the circular dichroism (CD) spectra of bis-porphyrins, we have calculated the CD spectra of chiral bis-porphyrins from three classes: I, where porphyrins can adopt a relatively wide range of orientations relative to each other; II, porphyrins have a fixed relative orientation; III, porphyrins undergo pi-stacking. The calculations primarily utilized the classical polarizability theory of DeVoe, but were supplemented by the quantum mechanical matrix method. Class I was represented by three isomers of the diester of 5alpha-cholestane-3,17-diol with 5-(4'-carboxyphenyl)-10,15,20-triphenylporphin (2-alphabeta, 2-betaalpha, 2-betabeta). Careful analysis of the torsional degrees of freedom led to two to four minimum-energy conformers for each isomer, in each of which the phenyl-porphyrin bonds had torsional angles near 90 degrees. Libration about these bonds is relatively unrestricted over a range of +/-45 degrees. CD spectra in the Soret region were calculated as Boltzmann-weighted averages over the low-energy conformers for each isomer. Three models were used: the effective transition moment model, in which only one of the degenerate Soret components is considered, along the 5-15 direction; the circular oscillator model, in which both Soret components are given equal weight; and the hybrid model, in which the 10-20 oscillator is given half the weight of the 5-15 oscillator, to mimic the effect of extensive librational averaging about the 5-15 direction. All three models predict Soret exciton couplets with signs in agreement with experiment. Quantitatively, the best results are given by the hybrid and circular oscillator models. These results validate the widely used effective transition moment model for qualitative assignments of bis-porphyrin chirality and thus permit application of the exciton chirality model. However, for quantitative studies, the circular oscillator or hybrid models should be used. The simplified effective transition moment and hybrid models are justified by the librational averaging in the class I bis-porphyrins and should only be used with such systems. Two class II bis-porphyrins were also studied by DeVoe method calculations in the circular oscillator model, which yielded good agreement with experiment. Class III bis-porphyrins were represented by 2-alphaalpha, for which the calculations gave qualitative agreement. However, limitations in the conformational analysis with the close contacts and dynamic effects in these pi-stacked systems preclude quantitative results.     

Collection and analytical characterisation of the atmospheric particulate in the city of Bari

In this paper an application of new procedures for atmospheric particulate analysis is illustrated. PM10, PAHs (benzo[a]anthracene (BaA), benzo[b]fluoranthene (BbF), benzo[j]fluoranthene (BjF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1, 2, 3-cd]pyrene (Ip), dibenzo[a, h]anthracene (DbA)) and heavy metals (Cu, Ni, Zn, Co, Mn, Cd, Fe and Pb) were investigated. PM10 determination was performed by gravimetric method, PAHs were measured by GC-MS, and heavy metals by HPIC. An air quality monitoring campaign on the territory of Bari municipality has been organised, and its results are shown.     

Quantitative vs. qualitative approaches to the electronic structure of solids

The usefulness of qualitative and quantitative theoretical approaches in solid state chemistry is discussed by considering three different types of problems: (a) the distribution of boron and carbon atoms in MB₂C₂ (M=Ca, La, etc.) phases, (b) the band structure and Fermi surface of low-dimensional transition metal oxides and bronzes, and (c) the correlation between the crystal and electronic structure of the ternary nitride Ca₂AuN.     

Polycyclic aromatic hydrocarbons in the atmosphere: monitoring, sources, sinks and fate. II: Sinks and fate

This paper reviews the transformation processes that polycyclic aromatic hydrocarbons (PAHs) undergo in the atmosphere. These processes can take place both in the gas phase and in the particulate/aerosol one. Among the gas-phase processes, the most important ones are the daytime reaction with *OH and the nighttime reaction with *NO3. The relative importance of the two processes depends on the particular PAH molecule. For instance, gaseous naphthalene is mainly removed from the atmosphere upon reaction with *OH, while gaseous phenanthrene is mainly removed by reaction with *NO3. Oxy-, hydroxy-, and nitro-PAHs are the main transformation intermediates. Reaction with ozone and photolysis play a secondary role in the transformation of gaseous PAHs. The particle-associated processes are usually slower than the gas-phase ones, thus the gas-phase PAHs usually have shorter atmospheric lifetimes than those found on particulate. Due to the higher residence time on particulate when compared with the gas phase, direct or assisted photolysis plays a relevant role in the transformation of particle-associated PAHs. Among the other processes taking place in the condensed phase, nitration plays a very important role due to the health impact of nitro-PAHs, some of them being the most powerful mutagens found so far in atmospheric particulate extracts.     

Saffron Characterization by a Multidisciplinary Approach

Saffron is a spice obtained from the drying process of the stigmas of the flower Crocus sativus Linnaeus. It is well known that the organoleptic characteristics of this spice are closely linked to the production area and harvesting year. The present work aims to evaluate whether saffron samples produced in different years and origins present sensibly different crocin profiles. To achieve this goal, 120 saffron samples were harvested between 2016 and 2020 in four different Italian areas. The crocins were analysed, identified, and quantified by high-performance liquid chromatography–electrospray–tandem mass spectrometry (HPLC–ESI–MS/MS) in multiple reaction monitoring mode (MRM). Subsequently, ANOVA–simultaneous component analysis (ASCA) was used to evaluate whether the origin and annuity significantly affected the composition of the crocins. ASCA confirmed the relevance of these effects. Eventually, soft independent modelling by class analogy (SIMCA) models were created for each of the four different origins. Mixtures of saffron from different areas were also prepared to test the robustness of the models. SIMCA provided satisfying results; in fact, models provided 100% sensitivity for three origins (Cascia, Sardinia, and Città della Pieve) on the external test set (48 samples) and 88% (sensitivity on the external test set) for the Spoleto class.     

Optical properties of ZnS–carbon nanotubes composites

In this work we present a study of growth and characterization of ZnS/carbon nanotube (CNT) composites. The composites were prepared with a chemical mix technique and characterized by SEM analysis, Auger Electron Spectroscopy (AES) and Cathodoluminescence (CL) spectroscopy. The analyses show that the sample is formed by a direct contact between CNT and zinc sulfide. In particular, SEM images show CNTs wrapped around ZnS micrometer particles. Furthermore, AES spectroscopy shows the absence of chemical bonds between ZnS and CNT and that the sample homogeneity increases with the CNT amount in the composite.Finally CL studies show that our composites present a visible luminescence signal centered at about 500 nm, with a shift of luminescence emission peak linked to the amount of CNT in the sample.     

Experimental and theoretical investigation of the parabanic acid molecule following VUV excitation and photodissociation

Photodissociation experiments have been performed for the parabanic acid (C₃H₂N₂O₃) molecule in vapor phase using time-of-flight mass spectrometry and synchrotron radiation in the VUV photon energy range. Electron ion coincidence (PEPICO) spectra and partial ion yields have been recorded as a function of the photon energy covering the 11–21 eV valence range region. The resulting photoionization products as well as proposed fragmentation pathways leading to those species are presented and discussed. Electronic structure computations for the neutral and ionic species were also carried out at the B3LYP/aug-cc-pVTZ level of theory.     

Suppression of Liquid-Liquid Phase Separation and Aggregation of Antibodies by Modest Pressure Application

The high colloidal stability of antibody (immunoglobulin) solutions is important for pharmaceutical applications. Inert cosolutes, excipients, are generally used in therapeutic protein formulations to minimize physical instabilities, such as liquid–liquid phase separation (LLPS), aggregation and precipitation, which are often encountered during manufacturing and storage. Despite their widespread use, a detailed understanding of how excipients modulate the specific protein-protein interactions responsible for these instabilities is still lacking. In this work, we demonstrate the high sensitivity to pressure of globulin condensates as a suitable means to suppress LLPS and subsequent aggregation of concentrated antibody solutions. The addition of excipients has only a minor effect. The high pressure sensitivity observed is due to the fact that these flexible Y-shaped molecules create a considerable amount of void volume in the condensed phase, leading to an overall decrease in the volume of the system upon dissociation of the droplet phase by pressure already at a few tens of to hundred bar. Moreover, we show that immunoglobulin molecules themselves are highly resistant to unfolding under pressure, and can even sustain pressures up to about 6 kbar without conformational changes. This implies that immunoglobulins are resistant to the pressure treatment of foods, such as milk, in high-pressure food-processing technologies, thereby preserving their immunological activity.     

Critical clusters in interdependent economic sectors

In this paper we develop a data-driven hierarchical clustering methodology to group the economic sectors of a country in order to highlight strongly coupled groups that are weakly coupled with other groups. Specifically, we consider an input-output representation of the coupling among the sectors and we interpret the relation among sectors as a directed graph; then we recursively apply the spectral clustering methodology over the graph, without a priori information on the number of groups that have to be obtained. In order to do this, we resort to the eigengap criterion, where a suitable number of groups is selected automatically based on the intensity and structure of the coupling among the sectors. We validate the proposed methodology considering a case study for Italy, inspecting how the coupling among clusters and sectors changes from the year 1995 to 2011, showing that in the years the Italian structure underwent deep changes, becoming more and more interdependent, i.e., a large part of the economy has become tightly coupled.     

Catalytic Effect of Electric Fields on the Kemp Elimination Reactions with Neutral Bases

The effect of solvent reaction fields and oriented electric fields on the Kemp elimination reaction between methylamine or imidazole and 5-nitrobenzisoxazole has been theoretically studied. The Kemp reaction is the most widely used for the design of new enzymes. Our results, using the SMD continuous model for solvents, are in quite good agreement with the experimental fact that the rate of the analogous reaction with butylamine is one order of magnitude smaller in water than in acetonitrile. In the case of external electric fields, our results show that they can increase or decrease the energy barrier depending on the magnitude and orientation of the field. A duly oriented electric field may have a notable catalytic effect on the reaction. So, external electric fields and reaction fields due to the medium can contribute to the design of new enzymes. Several factors that must be taken into account to increase the catalytic effect are discussed.