A concise synthesis of β-sitosterol and other phytosterols

A convenient synthesis of sidechain-modified phytosterols is achieved via a temporary masking of the stigmasterol 5,6-alkene as an epoxide. Following performance of the desired modification, the alkene is regenerated through a mild deoxygenation. The approach is applied to the syntheses of β-sitosterol and campesterol acetate, and suggests a facile route to the (Z)-isomers of Δ22–23 phytosterols. Includes Supplementary Data

Graph-level Representation Learning with Joint-Embedding Predictive Architectures

Joint-Embedding Predictive Architectures (JEPAs) have recently emerged as a novel and powerful technique for self-supervised representation learning. They aim to learn an energy-based model by predicting the latent representation of a target signal $y$ from a context signal $x$. JEPAs bypass the need for data augmentation and negative samples, which are typically required by contrastive learning, while avoiding the overfitting issues associated with generative-based pretraining. In this paper, we show that graph-level representations can be effectively modeled using this paradigm and propose Graph-JEPA, the first JEPA for the graph domain. In particular, we employ masked modeling to learn embeddings for different subgraphs of the input graph. To endow the representations with the implicit hierarchy that is often present in graph-level concepts, we devise an alternative training objective that consists of predicting the coordinates of the encoded subgraphs on the unit hyperbola in the 2D plane. Extensive validation shows that Graph-JEPA can learn representations that are expressive and competitive in both graph classification and regression problems.Comment: Preprint. Under Revie

Dietary Plant Sterol Esters Must Be Hydrolyzed to Reduce Intestinal Cholesterol Absorption in Hamsters

Background: Elevated concentrations of LDL cholesterol are associated with the development of atherosclerosis and therefore are considered an important target for intervention to prevent cardiovascular diseases. The inhibition of cholesterol absorption in the small intestine is an attractive approach to lowering plasma cholesterol, one that is addressed by drug therapy as well as dietary supplementation with plant sterols and plant sterol esters (PSEs). Objective: This study was conducted to test the hypothesis that the cholesterol-lowering effects of PSE require hydrolysis to free sterols (FSs). Methods: Male Syrian hamsters were fed atherogenic diets (AIN-93M purified diet containing 0.12% cholesterol and 8% coconut oil) to which one of the following was added: no PSEs or ethers (control), 5% sterol stearate esters, 5% sterol palmitate esters (PEs), 5% sterol oleate esters (OEs), 5% sterol stearate ethers (STs; to mimic nonhydrolyzable PSE), or 3% FSs plus 2% sunflower oil. The treatments effectively created a spectrum of PSE hydrolysis across which cholesterol metabolism could be compared. Metabolic measurements included cholesterol absorption, plasma and liver lipid concentration, and fecal neutral sterol and bile acid excretion. Results: The STs and the PEs and SEs were poorly hydrolyzed (1.69–4.12%). In contrast,OEs were 88.3% hydrolyzed. The percent hydrolysis was negatively correlated with cholesterol absorption (r=20.85; P \u3c 0.0001) and positively correlated with fecal cholesterol excretion (r = 0.92; P \u3c 0.0001), suggesting that PSE hydrolysis plays a central role in the cholesterol-lowering properties of PSE. Conclusions: Our data on hamsters suggest that PSE hydrolysis and the presence of FSs is necessary to induce an optimum cholesterol-lowering effect and that poorly hydrolyzed PSEs may lower cholesterol through an alternative mechanism than that of competition with cholesterol for micelle incorporation

Exploring the Potential of Large Language Models (LLMs) in Learning on Graphs

Learning on Graphs has attracted immense attention due to its wide real-world applications. The most popular pipeline for learning on graphs with textual node attributes primarily relies on Graph Neural Networks (GNNs), and utilizes shallow text embedding as initial node representations, which has limitations in general knowledge and profound semantic understanding. In recent years, Large Language Models (LLMs) have been proven to possess extensive common knowledge and powerful semantic comprehension abilities that have revolutionized existing workflows to handle text data. In this paper, we aim to explore the potential of LLMs in graph machine learning, especially the node classification task, and investigate two possible pipelines: LLMs-as-Enhancers and LLMs-as-Predictors. The former leverages LLMs to enhance nodes' text attributes with their massive knowledge and then generate predictions through GNNs. The latter attempts to directly employ LLMs as standalone predictors. We conduct comprehensive and systematical studies on these two pipelines under various settings. From comprehensive empirical results, we make original observations and find new insights that open new possibilities and suggest promising directions to leverage LLMs for learning on graphs.Comment: fix some minor typos and error

Biosynthesis of HSAF, a Tetramic Acid-containing Macrolactam from \u3ci\u3eLysobacter enzymogenes\u3c/i\u3e

HSAF was isolated from Lysobacter enzymogenes, a bacterium used in the biological control of fungal diseases of plants. Structurally, it is a tetramic acid-containing macrolactam fused to a tricyclic system. HSAF exhibits a novel mode of action by disrupting sphingolipids important to the polarized growth of filamentous fungi. Here, we described the HSAF biosynthetic gene cluster which contains only a single-module polyketide synthase-nonribosomal peptide synthetase (PKS/ NRPS), although the biosynthesis of HSAF apparently requires two separate polyketide chains that are linked together by one amino acid (ornithine) via two amide bonds. Flanking the PKS/ NRPS are six genes, encoding a cascade of four tightly clustered redox enzymes on one side and a sterol desaturase/fatty acid hydroxylase and a ferredoxin reductase on the other side. The genetic data demonstrate that the four redox genes, in addition to the PKS/NRPS gene and the sterol desaturase/fatty acid hydroxylase gene, are required for HSAF production. The biochemical data show that the adenylation domain of the NRPS specifically activated L-ornithine and the fourdomain NRPS was able to catalyze the formation of a tetramic acid-containing product from acyl- S-ACP and ornithinyl-S-NRPS. These results reveal a previously unrecognized biosynthetic mechanism for hybrid PK/NRP in prokaryotic organisms

Biosynthesis of HSAF, a Tetramic Acid-containing Macrolactam from \u3ci\u3eLysobacter enzymogenes\u3c/i\u3e

HSAF was isolated from Lysobacter enzymogenes, a bacterium used in the biological control of fungal diseases of plants. Structurally, it is a tetramic acid-containing macrolactam fused to a tricyclic system. HSAF exhibits a novel mode of action by disrupting sphingolipids important to the polarized growth of filamentous fungi. Here, we described the HSAF biosynthetic gene cluster which contains only a single-module polyketide synthase-nonribosomal peptide synthetase (PKS/ NRPS), although the biosynthesis of HSAF apparently requires two separate polyketide chains that are linked together by one amino acid (ornithine) via two amide bonds. Flanking the PKS/ NRPS are six genes, encoding a cascade of four tightly clustered redox enzymes on one side and a sterol desaturase/fatty acid hydroxylase and a ferredoxin reductase on the other side. The genetic data demonstrate that the four redox genes, in addition to the PKS/NRPS gene and the sterol desaturase/fatty acid hydroxylase gene, are required for HSAF production. The biochemical data show that the adenylation domain of the NRPS specifically activated L-ornithine and the fourdomain NRPS was able to catalyze the formation of a tetramic acid-containing product from acyl- S-ACP and ornithinyl-S-NRPS. These results reveal a previously unrecognized biosynthetic mechanism for hybrid PK/NRP in prokaryotic organisms

Dietary Plant Sterol Esters Must Be Hydrolyzed to Reduce Intestinal Cholesterol Absorption in Hamsters

Background: Elevated concentrations of LDL cholesterol are associated with the development of atherosclerosis and therefore are considered an important target for intervention to prevent cardiovascular diseases. The inhibition of cholesterol absorption in the small intestine is an attractive approach to lowering plasma cholesterol, one that is addressed by drug therapy as well as dietary supplementation with plant sterols and plant sterol esters (PSEs). Objective: This study was conducted to test the hypothesis that the cholesterol-lowering effects of PSE require hydrolysis to free sterols (FSs). Methods: Male Syrian hamsters were fed atherogenic diets (AIN-93M purified diet containing 0.12% cholesterol and 8% coconut oil) to which one of the following was added: no PSEs or ethers (control), 5% sterol stearate esters, 5% sterol palmitate esters (PEs), 5% sterol oleate esters (OEs), 5% sterol stearate ethers (STs; to mimic nonhydrolyzable PSE), or 3% FSs plus 2% sunflower oil. The treatments effectively created a spectrum of PSE hydrolysis across which cholesterol metabolism could be compared. Metabolic measurements included cholesterol absorption, plasma and liver lipid concentration, and fecal neutral sterol and bile acid excretion. Results: The STs and the PEs and SEs were poorly hydrolyzed (1.69–4.12%). In contrast,OEs were 88.3% hydrolyzed. The percent hydrolysis was negatively correlated with cholesterol absorption (r=20.85; P \u3c 0.0001) and positively correlated with fecal cholesterol excretion (r = 0.92; P \u3c 0.0001), suggesting that PSE hydrolysis plays a central role in the cholesterol-lowering properties of PSE. Conclusions: Our data on hamsters suggest that PSE hydrolysis and the presence of FSs is necessary to induce an optimum cholesterol-lowering effect and that poorly hydrolyzed PSEs may lower cholesterol through an alternative mechanism than that of competition with cholesterol for micelle incorporation

I. Synthesis of β-Sitosterol and Phytosterol Esters; II. New Methodology for Singlet Oxygen Generation from 1,1-Dihydroperoxides

Phytosterols are steroid compounds structurally similar with cholesterol and vary in the nature of carbon side chain. β-Sitosterol is commercially available in preparative amount only as mixtures with other phytosterols. New semipreparative synthesis of pure β-sitosterol and sidechain-modified phytosterols is discussed in this dissertation. This new synthesis is achieved via a temporary masking of the stigmasterol 5,6-alkene as an epoxide. Following performance of the desired modification, the alkene is regenerated through a mild deoxygenation. Preparation of phytosterol esters for cholesterol metabolism study is also discussed in this dissertation. Singlet oxygen (1O2) is the lowest excited state of oxygen molecule. Due to its special properties, 1O2 has been widely used as the oxidant in chemistry, biology, and medicine. In the past decades, two major generation methods have been developed, photosensitization and chemical generation. However, most of the reported chemical generations require the water-rich media, which is associated with short 1O2 lifetime as a major drawback. Therefore, there is a need for 1O2 generation from organic solvents. The investigation of fragmentation of monoactivated derivatives of 1,1-dihydroperoxides is discussed in this dissertation. This previously unobserved fragmentation can be conducted in various organic solvents and generate high yield of 1O2. This reaction is general for a range of skeletal frameworks and activating groups, and can be applied directly to 1,1-dihydroperoxides via in situ activation. Kinetic and mechanistic investigation suggests it involving rate-limiting formation of a peroxyanion, which decompose to generate 1O2 via Grob-like process. Advisor: Patrick H. Dussaul

I. Synthesis of β-Sitosterol and Phytosterol Esters; II. New Methodology for Singlet Oxygen Generation from 1,1-Dihydroperoxides Derivatives

Phytosterols are steroid compounds structurally similar with cholesterol and vary in the nature of carbon side chain. β-Sitosterol is commercially available in preparative amount only as mixtures with other phytosterols. New semipreparative synthesis of pure β-sitosterol and sidechain-modified phytosterols is discussed in this dissertation. This new synthesis is achieved via a temporary masking of the stigmasterol 5,6-alkene as an epoxide. Following performance of the desired modification, the alkene is regenerated through a mild deoxygenation. Preparation of phytosterol esters for cholesterol metabolism study is also discussed in this dissertation. Singlet oxygen (1O2) is the lowest excited state of oxygen molecule. Due to its special properties, 1O 2 has been widely used as the oxidant in chemistry, biology, and medicine. In the past decades, two major generation methods have been developed, photosensitization and chemical generation. However, most of the reported chemical generations require the water-rich media, which is associated with short 1O 2 lifetime as a major drawback. Therefore, there is a need for 1O2 generation from organic solvents. The investigation of fragmentation of monoactivated derivatives of 1,1-dihydroperoxides is discussed in this dissertation. This previously unobserved fragmentation can be conducted in various organic solvents and generate high yield of 1O 2. This reaction is general for a range of skeletal frameworks and activating groups, and can be applied directly to 1,1-dihydroperoxides via in situ activation. Kinetic and mechanistic investigation suggests it involving rate-limiting formation of a peroxyanion, which decompose to generate 1O 2 via Grob-like process