Tag: 200-300

Interactions between Leu. mesenteroides and L. plantarum in Chinese northeast sauerkraut was written by Yang, Xiaozhe;Hu, Wenzhong;Xiu, Zhilong;Ji, Yaru;Guan, Yuge. And the article was included in LWT–Food Science and Technology in 2022.Name: (2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(isopropylthio)tetrahydro-2H-pyran-3,4,5-triol The following contents are mentioned in the article:

LAB are mostly used microorganisms for traditional fermented foods, e.g. Chinese northeast sauerkraut. However, the interactions between these strains are little known in mixed culture. This study aimed to explore the interaction between Leu. mesenteroides and L. plantarum based on metabolomics and transcriptomics during single- and mix-cultured fermentation of Chinese northeast sauerkraut. Metabolomics anal. revealed that the mixed culture showed different metabolite profiles in comparison with single culture. Higher levels of 4-vinylphenol (4.51%), 2,6-diaminopimelic acid (4.47%), Et dodecanoate (5.53%) were observed in mix-cultured samples than that with Leu. mesenteroides. In addition, the levels of 4-isopropylbenzoic acid (3.54%), 2,3-butanediol (7.23%), 1-octadecene (5.15%), Et dodecanoate (5.53%) were higher than that with L. plantarum. The most influenced pathway was carbohydrate metabolism, which was mostly related to 116 genes considered as DEGs in C_30 vs. D_30 groups (CD), according to transcriptomics anal. Addnl., 69 genes were considered as significantly DEGs in C_30 vs. A_30 groups (CA) which were considerably mapped to amino acid metabolism These results provided integrated views into the adaptive responses of the two strains to mix-cultured fermentation, which was useful for the rational development of mixed cultures in sauerkraut industry. This study involved multiple reactions and reactants, such as (2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(isopropylthio)tetrahydro-2H-pyran-3,4,5-triol (cas: 367-93-1Name: (2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(isopropylthio)tetrahydro-2H-pyran-3,4,5-triol).

(2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(isopropylthio)tetrahydro-2H-pyran-3,4,5-triol (cas: 367-93-1) belongs to alcohols. Alcohols are among the most common organic compounds. They are used as sweeteners and in making perfumes, are valuable intermediates in the synthesis of other compounds, and are among the most abundantly produced organic chemicals in industry. Under carefully controlled conditions, simple alcohols can undergo intermolecular dehydration to give ethers. This reaction is effective only with methanol, ethanol, and other simple primary alcohols.Name: (2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(isopropylthio)tetrahydro-2H-pyran-3,4,5-triol

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

Formation and nature of non-extractable residues of emerging organic contaminants in humic acids catalyzed by laccase was written by Zhou, Yue;Sun, Feifei;Wu, Xuan;Cao, Siqi;Guo, Xiaoran;Wang, Qilin;Wang, Yongfeng;Ji, Rong. And the article was included in Science of the Total Environment in 2022.Synthetic Route of C13H12O2 The following contents are mentioned in the article:

Formation of non-extractable residues (NERs) is the major fate of most environmental organic contaminants in soil, however, there is no direct evidence yet to support the assumed phys. entrapment of NERs (i.e., type I NERs) inside soil humic substances. Here, we used ¹⁴C-radiotracer and silylation techniques to analyze NERs of six emerging and traditional organic contaminants formed in a suspension of humic acids (HA) under catalysis of the oxidative enzyme laccase. Laccase induced formation of both type I and covalently bound NERs (i.e., type II NERs) of bisphenol A, bisphenol F, and tetrabromobisphenol A to a large extent, and of bisphenol S (BPS) and sulfamethoxazole (SMX) to a less extent, while no induction for phenanthrene. The type I NERs were formed supposedly owing to laccase-induced alteration of primary (active groups) and secondary (conformation) structure of humic supramols., contributing surprisingly to large extents (23.5%-65.7%) to the total NERs, particularly for BPS and SMX, which both were otherwise not transformed by laccase catalysis. Electron-withdrawing sulfonyl group and bromine substitution significantly decreased amount and kinetics of NER formation, resp. This study provides the first direct evidence for the formation of type I NERs in humic substances and implies a “Trojan horse” effect of such NERs in the environment. This study involved multiple reactions and reactants, such as 4,4′-Methylenediphenol (cas: 620-92-8Synthetic Route of C13H12O2).

4,4′-Methylenediphenol (cas: 620-92-8) belongs to alcohols. The oxygen atom of the strongly polarized O―H bond of an alcohol pulls electron density away from the hydrogen atom. This polarized hydrogen, which bears a partial positive charge, can form a hydrogen bond with a pair of nonbonding electrons on another oxygen atom. Alcohols may be oxidized to give ketones, aldehydes, and carboxylic acids. These functional groups are useful for further reactions. Oxidation of organic compounds generally increases the number of bonds from carbon to oxygen (or another electronegative element, such as a halogen), and it may decrease the number of bonds to hydrogen.Synthetic Route of C13H12O2

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

Exploring the therapeutic potential of Cu(II)-complexes with ligands derived from pyridoxal was written by Nunes, Patrique;Marques, Fernanda;Cavaco, Isabel;Costa Pessoa, Joao;Correia, Isabel. And the article was included in Inorganica Chimica Acta in 2020.SDS of cas: 65-22-5 The following contents are mentioned in the article:

Three new copper(II) complexes formulated as [Cu(L)(X)], where X = H₂O or Cl and H₂L is a Schiff base (H₂L^1,2) or its reduced version (H₃L³Cl) derived from pyridoxal, are prepared, as well as two ternary complexes [Cu(L)(phen)] also containing 1,10-phenanthroline. All compounds are characterized by the usual techniques: elemental analyses, ESI mass spectrometry, UV-Vis absorption, FTIR and EPR spectroscopies. The ligands coordinate the Cu(II) center forming complexes with square-planar based geometries. Their antioxidant properties are evaluated with a radical scavenging activity assay, with one of the ligand precursors showing activity higher than the pos. control, ascorbic acid. The antiproliferative activity of all compounds is evaluated against two cancer cell lines: ovarian (A2780) and breast (MCF7). All complexes show moderate to excellent activity with the ternary Cu-complexes showing IC₅₀ values between 0.7 and 9.3μM after 24 h of incubation, values much lower than those reported for cisplatin, the reference drug. The hydrolytic stability of the complexes and their ability to bind albumin and DNA are evaluated by spectroscopic techniques, showing that the compounds bind bovine serum albumin. The [Cu(L)(phen)] complexes show ability to target DNA via intercalation. This study involved multiple reactions and reactants, such as 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride (cas: 65-22-5SDS of cas: 65-22-5).

3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride (cas: 65-22-5) belongs to alcohols. A strong base can deprotonate an alcohol to yield an alkoxide ion (R―O−). For example, sodamide (NaNH2), a very strong base, abstracts the hydrogen atom of an alcohol. Alcohols may be oxidized to give ketones, aldehydes, and carboxylic acids. These functional groups are useful for further reactions. Oxidation of organic compounds generally increases the number of bonds from carbon to oxygen (or another electronegative element, such as a halogen), and it may decrease the number of bonds to hydrogen.SDS of cas: 65-22-5

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

New method for the determination of endocrine disrupting chemicals in Mediterranean mussel (Mytilus galloprovincialis) using ultra-high performance liquid chromatography-tandem mass spectrometry was written by Garcia-Fernandez, L.;Garcia-Corcoles, M. T.;Navalon, A.;Martin-Pozo, L.;Hidalgo, F.;Zafra-Gomez, A.. And the article was included in Microchemical Journal in 2022.Recommanded Product: 4,4′-Methylenediphenol The following contents are mentioned in the article:

There are numerous types of contaminants that pose a health risk to aquatic organisms and consequently also to humans through consumption. Endocrine disrupting compounds are found in daily-use products and have the potential to mimic natural hormones. The main objective of this work is to optimize and validate a method for the determination of bisphenols, parabens and triclocarban in natural samples of Mediterranean mussel (Mytilus galloprovincialis). The procedure involves ultrasound-assisted extraction (UAE), and a subsequent clean-up of the extracts using dispersive solid phase extraction (d-SPE) with C18 adsorbent, and anal. by ultrahigh performance liquid chromatog.-tandem mass spectrometry (UHPLC-MS/MS). Sensitivity, accuracy (trueness and precision), linearity and selectivity of the method were studied. The limits of detection ranged from 0.2 ng g-1 to 1.5 ng g-1 dry weight The trueness of the method (estimation of recovery) was between 90 % for TCC (triclocarban) and 109.6 % for BPP (bisphenol P), with an estimated precision lower than 12.6 % for all the investigated analytes. The application of the method was to specimens of Mytilus galloprovincialis collected along the Mediterranean coast of Granada (South Spain), where the species is abundant. The study conducted in different sample sites revealed EDCs presence in this aquatic species. This study involved multiple reactions and reactants, such as 4,4′-Methylenediphenol (cas: 620-92-8Recommanded Product: 4,4′-Methylenediphenol).

4,4′-Methylenediphenol (cas: 620-92-8) belongs to alcohols. The oxygen atom of the strongly polarized O―H bond of an alcohol pulls electron density away from the hydrogen atom. This polarized hydrogen, which bears a partial positive charge, can form a hydrogen bond with a pair of nonbonding electrons on another oxygen atom. Converting an alcohol to an alkene requires removal of the hydroxyl group and a hydrogen atom on the neighbouring carbon atom. Dehydrations are most commonly carried out by warming the alcohol in the presence of a strong dehydrating acid, such as concentrated sulfuric acid.Recommanded Product: 4,4′-Methylenediphenol

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

Two photosensitive chalcone-based benzoxazine monomers and their high-performance polymers from renewable sources was written by Feng, Zijian;Zeng, Ming;Tan, Dengru;Lu, Xiang;Shen, Yufang;Xu, Qingyu;Meng, Dawei. And the article was included in Journal of Materials Science in 2022.COA of Formula: C13H12O2 The following contents are mentioned in the article:

Two novel photosensitive bio-based benzoxazines are firstly synthesized from chalcone derivates and furfurylamine. The thermal and photo-thermal curing methods are applied to polymerize benzoxazine monomers. Both thermal and photo-thermal cured benzoxazine resins exhibit high glass transition temperatures and good thermal stability properties, resulting from the curing of double bond of chalcone moiety and substitution reaction of furan ring for the thermal cured resins, and photo-dimerization of chalcone structure and grafting reaction of furan ring for the photo-thermal cured samples. It is especially noteworthy that the photo-thermal curing method is good for the formation of cyclobutane ring upon irradiation, resulting in the improvement of crosslinking d. and the reduction in the mol. polarity. Hence, the photo-thermal cured resins present higher glass transition temperatures and lower superhigh-frequency dielec. constants than those of the corresponding thermal cured samples. Therefore, this work not only provides an effective strategy for the design and preparation of high-heat-resistance bio-based benzoxzine resins with superhigh-frequency low dielec. constants, but also provides insight into the effects of photosensitive chalcone moiety and reactive furan ring as well as curing methods on the curing behavior, and thermal and superhigh-frequency dielec. properties of bio-based benzoxazine resins. This study involved multiple reactions and reactants, such as 4,4′-Methylenediphenol (cas: 620-92-8COA of Formula: C13H12O2).

4,4′-Methylenediphenol (cas: 620-92-8) belongs to alcohols. Similar to water, an alcohol can be pictured as having an sp3 hybridized tetrahedral oxygen atom with nonbonding pairs of electrons occupying two of the four sp3 hybrid orbitals. A multistep synthesis may use Grignard-like reactions to form an alcohol with the desired carbon structure, followed by reactions to convert the hydroxyl group of the alcohol to the desired functionality.COA of Formula: C13H12O2

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

SmdA is a novel cell morphology determinant in Staphylococcus aureus was written by Myrbraaten, Ine Storaker;Stamsaas, Gro Anita;Chan, Helena;Angeles, Danae Morales;Knutsen, Tiril Mathiesen;Salehian, Zhian;Shapaval, Volha;Straume, Daniel;Kjos, Morten. And the article was included in mBio in 2022.Related Products of 367-93-1 The following contents are mentioned in the article:

Cell division and cell wall synthesis in staphylococci need to be precisely coordinated and controlled to allow the cell to multiply while maintaining its nearly spherical shape. The mechanisms ensuring correct placement of the division plane and synthesis of new cell wall have been studied intensively. However, hitherto unknown factors and proteins are likely to play key roles in this complex interplay. Here, we identified and investigated a protein with a major influence on cell morphol. in Staphylococcus aureus. The protein, named SmdA (for staphylococcal morphol. determinant A), is a membrane protein with septum-enriched localization. By CRISPRi knockdown and overexpression combined with different microscopy techniques, we demonstrated that proper levels of SmdA were necessary for cell division, including septum formation and cell splitting. We also identified conserved residues in SmdA that were critical for its functionality. Pulldown and bacterial two-hybrid interaction experiments showed that SmdA interacted with several known cell division and cell wall synthesis proteins, including penicillin-binding proteins (PBPs) and EzrA. Notably, SmdA also affected susceptibility to cell wall targeting antibiotics, particularly in methicillin-resistant S. aureus (MRSA). Together, our results showed that S. aureus was dependent on balanced amounts of membrane attached SmdA to carry out proper cell division. This study involved multiple reactions and reactants, such as (2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(isopropylthio)tetrahydro-2H-pyran-3,4,5-triol (cas: 367-93-1Related Products of 367-93-1).

(2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(isopropylthio)tetrahydro-2H-pyran-3,4,5-triol (cas: 367-93-1) belongs to alcohols. Because alcohols are easily synthesized and easily transformed into other compounds, they serve as important intermediates in organic synthesis. Grignard and organolithium reagents are powerful tools for organic synthesis, and the most common products of their reactions are alcohols.Related Products of 367-93-1

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

Cardanol and bisphenol-F based benzoxazines with zirconium phosphate reinforced composites coating for protecting the mild steel surface from corrosion was written by Latha, Govindraj;Hariharan, Arumugam;Balaji, Krishnasamy;Kumaravel, Ammasai;Alagar, Muthukaruppan. And the article was included in Journal of Macromolecular Science.Application of 620-92-8 The following contents are mentioned in the article:

The present work is to develop and characterize the benzoxazines from cardanol and bisphenol-F with imdazole core based amine in order to utilize them in the form of (50/50 wt%) blended composites coating for corrosion resistance application. The benzoxazine blends were reinforced with varying weight percentages of (0.5, 1, 1.5, 2.0 and 2.5 wt%) of 3-glycidoxypropyltrimethoxysilane (GPTMS) functionalized zirconium phosphate (ZrP) to obtain resp. composites blended coatings. The thermal stability of composites was studied by thermogravimetric anal. (TGA). In order to ascertain their hydrophobic behavior the water contact angle studies were carried out and the values obtained for 0.5, 1, 1.5, 2.0 and 2.5 wt% GPTMS functionalized ZrP reinforced (50/50 wt% of C-ima/BF-ima) benzoxazines blended composites are 114,116,117,119,120 and 123° resp. The UV shielding behavior of ZrP reinforced polybenzoxazine blended composites material was also studied by UV-Vis spectroscopic technique and the results obtained infer that these materials possess good UV shielding behavior. ZrP reinforced benzoxazine blended composites were coated on the mild steel specimens and their corrosion resisting behavior was studied. of Nyquist plot and Tafel plot, ascertain that among the coated specimens, the specimen coated with 2.5 wt% ZrP reinforced poly(C-ima/BF-ima) blended composites exhibits highest corrosion resisting efficiency. This study involved multiple reactions and reactants, such as 4,4′-Methylenediphenol (cas: 620-92-8Application of 620-92-8).

4,4′-Methylenediphenol (cas: 620-92-8) belongs to alcohols. Similar to water, an alcohol can be pictured as having an sp3 hybridized tetrahedral oxygen atom with nonbonding pairs of electrons occupying two of the four sp3 hybrid orbitals. A multistep synthesis may use Grignard-like reactions to form an alcohol with the desired carbon structure, followed by reactions to convert the hydroxyl group of the alcohol to the desired functionality.Application of 620-92-8

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

Effect of temperature on saccharification and oligosaccharide production efficiency in koji amazake was written by Oguro, Yoshifumi;Nakamura, Ayana;Kurahashi, Atsushi. And the article was included in Journal of Bioscience and Bioengineering in 2019.Category: alcohols-buliding-blocks The following contents are mentioned in the article:

Koji amazake, prepared from rice koji, is a traditional Japanese sweet beverage. The main source of sweetness is glucose derived from rice starch following digestion by enzymes of Aspergillus oryzae during saccharification. The temperature of this process was empirically determined as 45°C-60°C, but no studies have systematically investigated the effect of temperature on saccharification efficiency. We addressed this in the present study by evaluating saccharification efficiency at various temperatures We found that glucose content was the highest at 50°C (100%) and was reduced at temperatures of 40°C (66.4%), 60°C (91.9%), and 70°C (76.6%). We previously reported that 12 types of oligosaccharides are present in koji amazake; the levels of eight of these, namely nigerose, kojibiose, trehalose, isomaltose, gentiobiose, raffinose, panose, and isomaltotriose, were the highest at 50°C-60°C, whereas sophorose production was maximal at 70°C. Based on these findings, we initially performed saccharification at 50°C and then switched the temperature to 70°C. The maximum amount of each saccharide including sophorose that was produced was close to the values obtained at these two temperatures Thus, oligosaccharide composition of koji amazake is dependent on saccharification temperature These findings provide useful information for improving the consumer appeal of koji amazake by enhancing oligosaccharide content. This study involved multiple reactions and reactants, such as 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride (cas: 65-22-5Category: alcohols-buliding-blocks).

3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride (cas: 65-22-5) belongs to alcohols. Alkyl halides are often synthesized from alcohols, in effect substituting a halogen atom for the hydroxyl group. The most common reactions of alcohols can be classified as oxidation, dehydration, substitution, esterification, and reactions of alkoxides.Category: alcohols-buliding-blocks

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

Effect of double substitution in cationic chitosan derivatives on dna transfection efficiency was written by Badazhkova, Veronika D.;Raik, Sergei V.;Polyakov, Dmitry S.;Poshina, Daria N.;Skorik, Yury A.. And the article was included in Polymers (Basel, Switzerland) in 2020.Safety of 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride The following contents are mentioned in the article:

In this work, chitosan was chem. modified by a reaction with 4-formyl-N,N,N-trimethylanilinium iodide and pyridoxal hydrochloride and subsequent reduction of the imine bond with NaBH4. This reaction yielded three novel derivatives, N-[4-(N ‘,N’,N’-trimethylammonium)benzyl]chitosan chloride (TMAB-CS), N-[(3-hydroxy-5-(hydroxymethyl)-2-methyl-4-pyridine)methyl]chitosan chloride (Pyr-CS), and N-[4-(N’,N’,N”-trimethylammonium)benzyl]-N-[(3-hydroxy-5-(hydroxymethyl)-2-methyl-4- pyridine)methyl]chitosan chloride (PyrTMAB-CS). Their structures and degrees of substitution were established by 1H NMR spectroscopy as DS1 = 0.22 for TMAB-CS, DS2 = 0.28 for Pyr-CS, and DS1 = 0.21, DS2 = 0.22 for PyrTMAB-CS. Dynamic light scattering measurements revealed that the new polymers formed stable polyplexes with plasmid DNA encoding the green fluorescent protein (pEGFP-N3) and that the particles had the smallest size (110-165 nm) when the polymer:DNA mass ratio was higher than 5:1. Transfection experiments carried out in the HEK293 cell line using the polymer:DNA polyplexes demonstrated that Pyr-CS was a rather poor transfection agent at polymer:DNA mass ratios less than 10:1, but it was still more effective than the TMAB-CS and PyrTMAB-CS derivatives that contained a quaternary ammonium group. Overall, the results show the possibility of combining substituent effects in a single carrier, thereby increasing its efficacy. This study involved multiple reactions and reactants, such as 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride (cas: 65-22-5Safety of 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride).

3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride (cas: 65-22-5) belongs to alcohols. A strong base can deprotonate an alcohol to yield an alkoxide ion (R―O−). For example, sodamide (NaNH2), a very strong base, abstracts the hydrogen atom of an alcohol. Under carefully controlled conditions, simple alcohols can undergo intermolecular dehydration to give ethers. This reaction is effective only with methanol, ethanol, and other simple primary alcohols.Safety of 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts

Bisphenol F exposure affects mouse oocyte in vitro maturation through inducing oxidative stress and DNA damage was written by Ding, Zhi-Ming;Chen, Yang-Wu;Ahmad, Muhammad Jamil;Wang, Yong-Sheng;Yang, Sheng-Ji;Duan, Ze-Qun;Liu, Ming;Yang, Cai-Xia;Liang, Ai-Xin;Hua, Guo-Hua;Huo, Li-Jun. And the article was included in Environmental Toxicology in 2022.Category: alcohols-buliding-blocks The following contents are mentioned in the article:

Bisphenol F (BPF), a substitute for bisphenol A (BPA), is progressively used to manufacture various consumer products. Despite the established reproductive toxicity of BPF, the underlying mechanisms remain to elucidate. This in-vitro study deep in sighted the BPF toxicity on mouse oocyte meiotic maturation and quality. After treating oocytes with BPF (300μM), the oocyte meiotic progression was blocked, accentuated by a reduced rate in the first polar body extrusion (PBE). Next, we illustrated that BPF induced α-tubulin hyper-acetylation disrupted the spindle assembly and chromosome alignment. Concurrently, BPF resulted in severe oxidative stress and DNA damage, which triggered the early apoptosis in mouse oocytes. Further, altered epigenetic modifications following BPF exposure were proved by increased H3K27me3 levels. Concerning the toxic effects on spindle structure, oxidative stress, and DNA damage in mouse oocytes, BPF toxicity was less severe to oocyte maturation and spindle structure than BPA and induced low oxidative stress. However, compared with BPA, oocytes treated with BPF were more prone to DNA damage, indicating not less intense or even more severe toxic effects of BPF than BPA on some aspects of oocytes maturation. In brief, the present study established that like wise to BPA, BPF could inhibit meiotic maturation and reduce oocyte quality, suggesting it is not a safe substitute for BPA. This study involved multiple reactions and reactants, such as 4,4′-Methylenediphenol (cas: 620-92-8Category: alcohols-buliding-blocks).

4,4′-Methylenediphenol (cas: 620-92-8) belongs to alcohols. Because alcohols are easily synthesized and easily transformed into other compounds, they serve as important intermediates in organic synthesis. Tertiary alcohols cannot be oxidized at all without breaking carbon-carbon bonds, whereas primary alcohols can be oxidized to aldehydes or further oxidized to carboxylic acids.Category: alcohols-buliding-blocks

Referemce:
Alcohol – Wikipedia,
Alcohols – Chemistry LibreTexts