Introduction to the Special Issue on Causal Discovery

Many empirical sciences, including the social sciences and life sciences, aim to study causal relationships. Researchers in these fields need computational methods for analyzing observed data and identifying causal structures among a set of variables. Such computational methods enable researchers to draw conclusions on the basis of both their assumptions and the observed data. Moreover, these methods are useful for developing hypotheses on causal relations, designing future observational studies, and planning future experimental studies that can potentially provide stronger evidence of estimated causal relations.

The objective of this special issue is to present an up-to-date overview of causal discovery methods, which have witnessed rapid advancements in recent years. The chief editor and guest editors invited the following three survey papers on various hot topics related to causal discovery:

     

Oxidation Behavior of ZrB2–SiC Composites at Low Pressures

ZrB₂‐60 mol%SiC composite with a eutectic microstructure was oxidized at 1573 to 1873 K with reduced total pressures (P_tot) and low oxygen partial pressures (). The mass change was continuously measured by a thermobalance, and then fit with a multiple paralinear model. Oxidation scale of SiO₂/ZrO₂+SiO₂/ZrO₂/ZrB₂ was formed at  > 0.13 kPa, whereas only porous ZrO₂ remained at  < 0.13 kPa, P_tot < 1.33 kPa and higher than 1773 K. With increasing , the parabolic oxidation constant decreased, whereas the linear oxidation constant increased.     

Contents of phthalate in polyvinyl chloride toys

The presence of plasticizers in PVC toys obtained in October 1998 was investigated. Diisononyl phthalate (DINP), di-2-ethylhexyl phthalate (DEHP), di-n-butyl phthalate (DBP), dinonyl phthalate (DNP), diheptyl phthalate (DHP), and di-2-ethylhexyl adipate (DEHA) were detected. The phthalates were found in all of the 68 samples. The principal phthalate found in toys was DINP, which was present in 48 of 68 samples. The DINP content ranged from 15 mg/g to 580 mg/g, and mean content was 308 mg/g. The highest content was found in a pacifier toy. DEHP was present in 20 of 68 samples and the content ranged from 2.0 mg/g to 380 mg/g. The mean content was 162 mg/g. It was found in 60% of domestic toys.     

Organic solar cells consisting of stacked amine–thiophene copolymer and 3,4,9,10-perylenetetracarboxyl-bis-benzimidazole layers

Organic solar cells were fabricated using a new amine–bithiophene copolymer as an electron donor layer and 3,4,9,10-perylenetetracarboxyl-bis-benzimidazole (PV) as an electron acceptor layer. The amine–thiophene copolymer, poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-bis(4-tert-butylphenyl)benzidine-N,N′-bis(phenylene-4,4′-diyl)]-co-(2,2′-bithiophene-5,5′-diyl)} (PF8-TPD-T2), had a glass transition temperature (T_g) at about 77 °C, and exhibited liquid crystalline states and a high hole mobility. The rigid bithiophene units in the polymer chain are probably responsible for the formation of the liquid crystalline states and the high hole mobility. A solar cell made of the PF8-TPD-T2 copolymer and PV layers showed a photocurrent density of 0.99 mA/cm², an open-circuit voltage of 0.61 V, and an energy conversion efficiency of 0.332%. The photocurrent of the solar cells was generated at both the copolymer and PV layers, and the copolymer layer was the main contributor to photocurrent when the thickness of the polymer was about 17 nm. After annealing the solar cells at temperatures well above the glass transition temperature (T_g) of the copolymer, the photocurrent action spectra of the solar cells were broadened and the performance was improved. The changes were mostly due to the increased contribution of the PV layer to the photocurrent by the annealing.     

Delayless subband adaptive filtering using the Hadamard transform

In this correspondence, an analysis of a delayless critically decimated subband adaptive filter structure is presented. In this structure, adaptive weights in each subband are computed by the LMS algorithm and then transformed into those in fullband by the Hadamard transform. It is shown that a stationary point of the proposed algorithm corresponds to the fullband Wiener filter. Some numerical results are also presented to show the performance of this scheme     

Glycine Protects Against Strong Protein-Denaturing Activity of Oleuropein, a Phenolic Compound in Privet Leaves

In previous studies, we reported the existence of a high concentration of free glycine in the digestive juice of several Lepidoptera larvae, particularly in the digestive juice of species that feed on the privet tree, Ligustrum oblusifolium. The water extract of privet leaves showed very strong protein-denaturing activity and lysine-decreasing activity, which closely resembled activity of oxidized polyphenolics. Addition of 1% glycine to the extract could completely inhibit these activities. Free glycine may be secreted into the digestive juice by larvae as an adaptive mechanism for chemical defense against its host plants. The protein-denaturing compound in privet leaves is present in the cytosol or in the vacuoles of the leaf cells. The compound does not show protein-denaturing activity without oxidation, but when mixed with intact organelles under low osmotic conditions to give an osmotic shock, a very high protein-denaturing activity is produced. Our results suggest that the privet tree is endowed with a defense mechanism in which a stable compound in the cytosol or in the vacuoles is activated into a chemically active denaturant by an enzyme present in the organelles (including chloroplasts) after the leaves are eaten by insects and the organelles are broken by osmotic shock or by digestive mechanisms. Based upon HPLC and NMR data, we conclude that the denaturing compound is oleuropein, an o-dihydroxyphenolic compound. This compound makes up 3% of the wet weight of privet leaves. The protein-denaturing activity of purified oleuropein activated by the leaf enzyme is high enough to account for all the denaturing activity in a water extract of privet leaves. The denaturing reaction is completely inhibited by free glycine. Our results suggest that the protein-denaturing activity and lysine-decreasing activity of privet leaves are caused by oxidized polyphenolics, and that some insects secrete free glycine to counter the denaturing activity of oxidized phenolics. The chemical mechanism of counteraction by glycine is also considered. Free glycine in the midgut of insects probably protects proteins from denaturation by competing with proteins for oxidized phenolics.     

Carbon Nitride Loaded with an Ultrafine,Monodisperse, Metallic Platinum-Cluster Cocatalyst for the Photocatalytic Hydrogen-Evolution Reaction

For the realization of a next-generation energy society, further improvement in the activity of water-splitting photocatalysts is essential. Platinum (Pt) is predicted to be the most effective cocatalyst for hydrogen evolution from water. However, when the number of active sites is increased by decreasing the particle size, the Pt cocatalyst is easily oxidized and thereby loses its activity. In this study, a method to load ultrafine, monodisperse, metallic Pt nanoclusters (NCs) on graphitic carbon nitride is developed, which is a promising visible-light-driven photocatalyst. In this photocatalyst, a part of the surface of the Pt NCs is protected by sulfur atoms, preventing oxidation. Consequently, the hydrogen-evolution activity per loading weight of Pt cocatalyst is significantly improved, 53 times, compared with that of a Pt-cocatalyst loaded photocatalyst by the conventional method. The developed method is also effective to enhance the overall water-splitting activity of other advanced photocatalysts such as SrTiO₃ and BaLa₄Ti₄O₁₅.     

Challenges Based on Antiplasmodial and Antiviral Activities of 7-Chloro-4-aminoquinoline Derivatives

We report the structural functionalization of the terminal amino group of N¹-(7-chloroquinolin-4-yl) butane-1,4-diamine, leading to a series of 7-chloro-4-aminoquinoline derivatives, and their evaluation as potent anti-malarial and anti-viral agents. Some compounds exhibited promising anti-malarial effects against the Plasmodium falciparum 3D7 (chloroquine-sensitive) and Dd2 (chloroquine-resistant) strains. In addition, these compounds were assayed in vitro against influenza A virus (IAV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Compound 5 h , bearing an N-mesityl thiourea group, displayed pronounced anti-infectious effects against malaria, IAV, and SARS-CoV-2. These results provide new insights into drug discovery for the prevention or treatment of malaria and virus co-infection.