Extended knowledge of 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 375-72-4, you can contact me at any time and look forward to more communication. COA of Formula: C4F10O2S.

The reaction rate of a catalyzed reaction is faster than the reaction rate of the uncatalyzed reaction at the same temperature. COA of Formula: C4F10O2S, 375-72-4, Name is 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride, SMILES is O=S(C(F)(F)C(F)(F)C(F)(F)C(F)(F)F)(F)=O, in an article , author is Gegelashvili, Georgi, once mentioned of 375-72-4.

Glutamate transport system as a key constituent of glutamosome: Molecular pathology and pharmacological modulation in chronic pain

Neural uptake of glutamate is executed by the structurally related members of the SLC1A family of solute transporters: GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3, EAAT4, ASCT2. These plasma membrane proteins ensure supply of glutamate, aspartate and some neutral amino acids, including glutamine and cysteine, for synthetic, energetic and signaling purposes, whereas effective removal of glutamate from the synaptic cleft shapes excitatory neurotransmission and prevents glutamate toxicity. Glutamate transporters (GluTs) possess also receptor-like properties and can directly initiate signal transduction. GluTs are physically linked to other glutamate signaling-, transporting- and metabolizing molecules (e.g., glutamine transporters SNAT3 and ASCT2, glutamine synthetase, NMDA receptor, synaptic vesicles), as well as cellular machineries fueling the transmembrane transport of glutamate (e.g., ion gradient-generating Na/K-ATPase, glycolytic enzymes, mitochondrial membrane- and matrix proteins, glucose transporters). We designate this supramolecular functional assembly as ‘glutamosome’. GluTs play important roles in the molecular pathology of chronic pain, due to the predominantly glutamatergic nature of nociceptive signaling in the spinal cord. Down-regulation of GluTs often precedes or occurs simultaneously with development of pain hypersensitivity. Pharmacological inhibition or gene knock-down of spinal GluTs can induce/aggravate pain, whereas enhancing expression of GluTs by viral gene transfer can mitigate chronic pain. Thus, functional up-regulation of GluTs is turning into a prospective pharmacotherapeutic approach for the management of chronic pain. A number of novel positive pharmacological regulators of GluTs, incl. pyridazine derivatives and beta-lactams, have recently been introduced. However, design and development of new analgesics based on this principle will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of the glutamate transport system in nociceptive circuits. This article is part of the issue entitled ‘Special Issue on Neurotransmitter Transporters’.

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 375-72-4, you can contact me at any time and look forward to more communication. COA of Formula: C4F10O2S.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

What I Wish Everyone Knew About 2605-67-6

We¡¯ll also look at important developments in the pharmaceutical industry because understanding organic chemistry is important in understanding health, medicine, 2605-67-6. The above is the message from the blog manager. Safety of Methyl 2-(triphenylphosphoranylidene)acetate.

Chemistry is traditionally divided into organic and inorganic chemistry. The former is the study of compounds containing at least one carbon-hydrogen bonds. 2605-67-6, Name is Methyl 2-(triphenylphosphoranylidene)acetate, molecular formula is C21H19O2P, belongs to pyridazines compound, is a common compound. In a patnet, author is Hameed, R. S. Abdel, once mentioned the new application about 2605-67-6, Safety of Methyl 2-(triphenylphosphoranylidene)acetate.

Study of sulfanyl pyridazine derivatives as efficient corrosion inhibitors for carbon steel in 1.0 M HCl using analytical techniques

Five new sulfur containing heterocyclic derivatives of sulfanyl pyridazine type compounds (1-5) were synthesized, characterized by FT-IR and H-1 NMR, and evaluated as corrosion inhibitors for carbon steel in 1.0 M HCl at various concentration and temperatures. Five different analytical techniques were used in this study, namely, gravimetrical, gasometrical, atomic absorption spectroscopy, AAS, thermometric, and acidimetric techniques. The corrosion inhibition efficiency increases with increasing inhibitor concentration but decreases with temperature. The inhibition of corrosion is due to adsorption on the metal surface and formation of a barrier film that separates the metal from the corrosive medium. The inhibition is due to physicochemical adsorption of a heterocyclic compound on steel surface, and the adsorption obeys the Langmuir isotherm. Thermodynamic parameters for both activation and adsorption were computed and discussed. The reduction in reaction number (RR%) and Atm increased with increasing inhibitor concentrations. The pH increased and the hydrogen ion concentration in the medium decreased upon addition of inhibitors as an inhibitor molecule acts as a ligand for protons in acidic media. The ferrous ion Fe+2 concentrations determined by AAS decreased in the solution with increasing inhibitor concentration. The data obtained from different analytical techniques are in good agreement within (+/- 2%).

We¡¯ll also look at important developments in the pharmaceutical industry because understanding organic chemistry is important in understanding health, medicine, 2605-67-6. The above is the message from the blog manager. Safety of Methyl 2-(triphenylphosphoranylidene)acetate.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

Awesome and Easy Science Experiments about 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride

If you are interested in 375-72-4, you can contact me at any time and look forward to more communication. Formula: C4F10O2S.

In an article, author is Sabt, Ahmed, once mentioned the application of 375-72-4, Formula: C4F10O2S, Name is 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride, molecular formula is C4F10O2S, molecular weight is 302.0906, MDL number is MFCD00007422, category is pyridazines. Now introduce a scientific discovery about this category.

Discovery of 3,6-disubstituted pyridazines as a novel class of anticancer agents targeting cyclin-dependent kinase 2: synthesis, biological evaluation and in silico insights

Human health in the current medical era is facing numerous challenges, especially cancer. So, the therapeutic arsenal for cancer should be unremittingly enriched with novel small molecules that selectively target tumour cells with minimal toxicity towards normal cells. In this context, herein a new series of 3,6-disubstituted pyridazines11a-rhas been synthesised and evaluated forin vitroanticancer activity. They possessed good anti-proliferative action towards human breast cancer T-47D (IC(50)range: 0.43 +/- 0.01 – 35.9 +/- 1.18 mu M) and MDA-MB-231 (IC(50)range: 0.99 +/- 0.03 – 34.59 +/- 1.13 mu M) cell lines, whereas they displayed weak activity against the tested ovarian cancer cell line SKOV-3. Among the studied compounds, the methyltetrahydropyran-bearing pyridazine11memerged as the unique submicromolar growth inhibitor herein reported towards both T-47D (IC50= 0.43 +/- 0.01 mu M) and MDA-MB-231 (IC50= 0.99 +/- 0.03 mu M) cell lines. In addition, the biological results indicated that pyridazines11land11mexerted an efficient alteration within the cell cycle progression as well as induction of apoptosis in both T-47D and MDA-MB-231 cells. Moreover, pyridazines11land11mdisplayed good mean tumour S. I. values of 13.7 and 16.1 upon assessment of their cytotoxicity towards non-tumorigenic breast MCF-10A cells. Furthermore, anin silicostudy proposed CDK2 as a probable enzymatic target for pyridazines11, and explored their binding interactions within the vicinity of CDK2 binding site. Subsequently, pyridazines11e,11h,11l, and11mwere selected to be evaluated for their ability to inhibit CDK2, where they exerted good inhibitory activity (IC50= 151, 43.8, 55.6 and 20.1 nM, respectively). Finally, thein silicostudy implied that target pyridazines11exhibited not only an efficient anticancer activity but also an acceptable ADME, physicochemical and druglikeness properties, specifically pyridazines11land11m. Overall the obtained results from this study quite sustained our strategy and gave us a robust opportunity for further development and optimisation of 3,6-disubstituted pyridazine scaffold to enrich therapeutic arsenal with efficient and safe anticancer CDK inhibitors.

If you are interested in 375-72-4, you can contact me at any time and look forward to more communication. Formula: C4F10O2S.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

Extracurricular laboratory: Discover of 375-72-4

Do you like my blog? If you like, you can also browse other articles about this kind. Thanks for taking the time to read the blog about 375-72-4, Name: 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride.

In an article, author is Sadimenko, Alexander P., once mentioned the application of 375-72-4, Name is 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride, molecular formula is C4F10O2S, molecular weight is 302.0906, MDL number is MFCD00007422, category is pyridazines. Now introduce a scientific discovery about this category, Name: 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride.

Organometallic Complexes of Azines

Organometallic complexes of practically unsubstituted (non-chelating) azines and their benzannulated derivatives, possible N-heterocyclic carbenes containing at least one double bond, and boron-, boronenitrogen, silicon-, tri- and hexaphosphorus analogs of azines are considered in the view of their synthetic availability, coordination modes, and possible application in catalysis and materials chemistry.

Do you like my blog? If you like, you can also browse other articles about this kind. Thanks for taking the time to read the blog about 375-72-4, Name: 1,1,2,2,3,3,4,4,4-Nonafluorobutane-1-sulfonyl fluoride.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

Brief introduction of 3,3,4,4,5,5,6,6,6-Nonafluorohexyl methacrylate

Balanced chemical reaction does not necessarily reveal either the individual elementary reactions by which a reaction occurs or its rate law. In my other articles, you can also check out more blogs about 1799-84-4. Computed Properties of C10H9F9O2.

Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. 1799-84-4, Name is 3,3,4,4,5,5,6,6,6-Nonafluorohexyl methacrylate, molecular formula is C10H9F9O2, belongs to pyridazines compound. In a document, author is La-Placa, Maria-Grazia, introduce the new discover, Computed Properties of C10H9F9O2.

Red Light-Emitting Electrochemical Cells Employing Pyridazine-Bridged Cationic Diiridium Complexes

A rigid dinuclear Ir(III) complex showing high photoluminescence quantum yield in pure films was successfully used to fabricate light-emitting electrochemical cells with and without ionic liquid additives. The devices showed nearly instantaneous electroluminescence after biasing and maximum quantum yield approaching 1%. The lifetime of the devices was found to be limited to approximately 20 hours, which we correlated with the irreversible oxidation of the complex as seen from electrochemical measurements. This work validates the use of highly luminescent dinuclear iridium complexes in light-emitting electrochemical cells. Future studies will pursue materials with more efficient photoluminescence as well as improved electrochemical stability. (c) 2019 The Electrochemical Society.

Balanced chemical reaction does not necessarily reveal either the individual elementary reactions by which a reaction occurs or its rate law. In my other articles, you can also check out more blogs about 1799-84-4. Computed Properties of C10H9F9O2.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

The Absolute Best Science Experiment for Methyl 2-(triphenylphosphoranylidene)acetate

Synthetic Route of 2605-67-6, Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. I hope my blog about 2605-67-6 is helpful to your research.

Synthetic Route of 2605-67-6, Catalysts allow a reaction to proceed via a pathway that has a lower activation energy than the uncatalyzed reaction. 2605-67-6, Name is Methyl 2-(triphenylphosphoranylidene)acetate, SMILES is C3=C([P](C1=CC=CC=C1)(C2=CC=CC=C2)=CC(OC)=O)C=CC=C3, belongs to pyridazines compound. In a article, author is Ahmed, Eman M., introduce new discover of the category.

New pyridazine derivatives as selective COX-2 inhibitors and potential anti-inflammatory agents; design, synthesis and biological evaluation

New pyridazinone and pyridazinthione derivatives were designed, synthesized and identified through performing H-1 NMR, C-13 NMR, IR and MS spectroscopic techniques. All the newly synthesized derivatives were evaluated for cyclooxygenase inhibitory activity and COX-2 selectivity using celecoxib and indomethacin, as reference drugs. All compounds showed highly potent COX-2 inhibitory activity with IC50 values in nano-molar range. Moreover, they demonstrated higher selectivity towards COX-2 inhibition compared to indomethacin. Compounds 3d, 3g and 6a exhibited significantly increased potency towards COX-2 enzyme compared to celecoxib with IC50 values of 67.23, 43.84 and 53.01 nM, respectively. They were 1.1-1.7 folds more potent than celecoxib (IC50 = 73.53 nM) and extremely much more potent than indomethacin (IC50 = 739.2 nM). Of particular interest, Compound 3g showed SI of 11.51 which was as high as that of celecoxib (SI 11.78). This compound was further challenged by in vivo anti-inflammatory activity assay and gastric ulcerogenic effect. It showed comparable anti-inflammatory activity to indomethacin as positive control. Moreover, the anti-inflammatory activity of compound 3g was found to be equipotent to celecoxib. Furthermore, the selective COX-2 inhibitor 3g exhibited a superior gastrointestinal safety profile compared to the reference drugs celecoxib and indomethacin with less number of ulcers and milder ulcer score. The molecular docking study of this compound with COX-2 protein revealed more favorable binding mode compared to celecoxib, explaining its remarkable COX-2 inhibitory potency.

Synthetic Route of 2605-67-6, Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. I hope my blog about 2605-67-6 is helpful to your research.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

Now Is The Time For You To Know The Truth About Methyl 2-(triphenylphosphoranylidene)acetate

The proportionality constant is the rate constant for the particular unimolecular reaction. the reaction rate is directly proportional to the concentration of the reactant. I hope my blog about 2605-67-6 is helpful to your research. SDS of cas: 2605-67-6.

Catalysts are substances that increase the reaction rate of a chemical reaction without being consumed in the process. 2605-67-6, Name is Methyl 2-(triphenylphosphoranylidene)acetate, SMILES is C3=C([P](C1=CC=CC=C1)(C2=CC=CC=C2)=CC(OC)=O)C=CC=C3, belongs to pyridazines compound. In a document, author is Saraswat, Mayank, introduce the new discover, SDS of cas: 2605-67-6.

Through bond and through space interactions in dehydro-diazine radicals: a case study of 3c-5e interactions

Owing to the 3c-5e (3-centred-5-electrons) interactions between two nitrogen lone pairs and a radical electron, the dehydrodiazine radical isomers are very interesting from the fundamental point of view. Among them, pyrimidine has three (1a-1c), pyridazine has two (2a and 2b) and pyrazine has only one (3a) radical isomer. Based on quantum chemical calculations at the (U) B3LYP, (U) M06-2X, (U) BLYP, CBS-QB3 and (U) CCSD(T) levels with cc-pVTZ as the basis set, we found the 4-dehydropyrimidine (1b) radical to be the most stable isomer among the three pyrimidine radicals, followed by the 2-dehydropyrimidine (1a) and 5-dehydropyrimidine (1c) radical isomers. In the case of pyridazine, 3-dehydro radical isomer (2a) is more stable than 4-dehydropyridazine (2b). Bond dissociation energy (BDE) calculations and estimation of radical stabilization energies (RSE) using isodesmic reactions revealed the stability order among the six isomeric diazine radicals as 1c < 2b < 2a < 1a < 3a < 1b. Spin densities at each radical centre and non-zero values at nitrogen centres provided information about the extent of delocalization of radical electrons, which was consistent with the relative stability order of all the isomers. The multiconfigurational CASSCF and natural bond orbital (NBO) calculations suggested the presence of direct through space interactions (between lone pairs and a radical, TS) that play a dominant role over the through bond (through intervening bonds, TB) interactions in deciding the stability order. To confirm these results, we have also estimated the proton affinities (PAs) for each nitrogen atom and compared them with their respective parent diazines, where lowering of the PA values convincingly envisaged the extent and strength of interactions between the nitrogen and radical centre. Atoms-in-molecules (AIM) analysis and estimation of hyperfine coupling constants have also been performed to verify these results. All these results showed that the through space interaction between the lone pair and the radical electron is very important for the electronic structural and stability aspects in dehydrodiazine radicals. The proportionality constant is the rate constant for the particular unimolecular reaction. the reaction rate is directly proportional to the concentration of the reactant. I hope my blog about 2605-67-6 is helpful to your research. SDS of cas: 2605-67-6.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

Final Thoughts on Chemistry for 2605-67-6

The proportionality constant is the rate constant for the particular unimolecular reaction. the reaction rate is directly proportional to the concentration of the reactant. I hope my blog about 2605-67-6 is helpful to your research. Name: Methyl 2-(triphenylphosphoranylidene)acetate.

Chemistry is the science of change. But why do chemical reactions take place? Why do chemicals react with each other? The answer is in thermodynamics and kinetics, 2605-67-6, Name is Methyl 2-(triphenylphosphoranylidene)acetate, SMILES is C3=C([P](C1=CC=CC=C1)(C2=CC=CC=C2)=CC(OC)=O)C=CC=C3, belongs to pyridazines compound. In a document, author is Moslin, Ryan, introduce the new discover, Name: Methyl 2-(triphenylphosphoranylidene)acetate.

Identification of N-Methyl Nicotinamide and N-Methyl Pyridazine-3-Carboxamide Pseudokinase Domain Ligands as Highly Selective Allosteric Inhibitors of Tyrosine Kinase 2 (TYK2)

As a member of the Janus OAK) family of nonreceptor tyrosine kinases, TYK2 plays an important role in mediating the signaling of pro-inflammatory cytokines including IL-12, IL-23, and type 1 interferons. The nicotinamide 4, identified by a SPA-based high-throughput screen targeting the TYK2 pseudokinase domain, potently inhibits IL-23 and IFN alpha signaling in cellular assays. The described work details the optimization of this poorly selective hit (4) to potent and selective molecules such as 47 and 48. The discoveries described herein were critical to the eventual identification of the clinical TYK2 JH2 inhibitor (see following report in this issue). Compound 48 provided robust inhibition in a mouse IL-12-induced IFN gamma pharmacodynamic model as well as efficacy in an IL-23 and IL-12-dependent mouse colitis model. These results demonstrate the ability of TYK2 JH2 domain binders to provide a highly selective alternative to conventional TYK2 orthosteric inhibitors.

The proportionality constant is the rate constant for the particular unimolecular reaction. the reaction rate is directly proportional to the concentration of the reactant. I hope my blog about 2605-67-6 is helpful to your research. Name: Methyl 2-(triphenylphosphoranylidene)acetate.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

Interesting scientific research on 1799-84-4

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 1799-84-4, you can contact me at any time and look forward to more communication. Category: pyridazines.

The reaction rate of a catalyzed reaction is faster than the reaction rate of the uncatalyzed reaction at the same temperature. Category: pyridazines, 1799-84-4, Name is 3,3,4,4,5,5,6,6,6-Nonafluorohexyl methacrylate, SMILES is CC(C(OCCC(F)(F)C(F)(F)C(F)(F)C(F)(F)F)=O)=C, in an article , author is Filali, Mouad, once mentioned of 1799-84-4.

Supramolecular arrangements of novel clickable 4-substituted 3,6-bis(2 ‘-pyridyl)pyridazine molecules

The clickable reaction between the starting 3,6-bis(2′-pyridyl)-1,2,4,5-tetrazine (bptz) with a series of terminal alkynes-containing functional biomolecules [prop-2-yn-1-ol, 4-(prop-2′-yn-1′-yl)morpholine and D-galactose] by means of an inverse electron demand Diels-Alder pathway has been studied and four new 4-substituted 3,6-bis(2′-pyridyl)pyridazine derivatives (4-Rdppn) were isolated, namely 4-(hydroxymethyl)-3,6-di(pyridin-2-yl)pyridazine (1), 4-((prop-2-yn-1-yloxy)methyl)-3,6-di(pyridin-2-yl) pyridazine (2) obtained by post-etherification reaction of 1, 4-(morpholinemethyl)-(3,6-dipyridin-2-yl) pyridazine monohydrate (3) and 3,6-di(pyridin-2-yl)-4-(2,2,7,7-tetramethyltetrahydro-5H-bis([1,3] dioxo)[4,5-b:4′,5’-d]pyran-5-yl)methoxy)methyl)pyridazine (4). The four new compounds were characterized by elemental analysis, IR spectroscopy and H-1/C-13 NMR and the crystal structures of 1-3 were solved by single crystal X-ray diffraction to elucidate their molecular structure. In the light of their structural knowledge, an analysis of the role of the substituent and steric effects on the crystal packing is carried out. Focusing on their dppn fragment of 1-3, an approximate s-trans/s-trans-conformation of the rings occurs as in the free dppn molecule and the bond lengths and angles agree with those reported for this molecule. pi-pi interactions and hydrogen bonds involving the substituents occur in 1 and 2. In addition, the water molecule of crystallization in 3 plays an important role in determining the crystal packing. (C) 2020 Elsevier B.V. All rights reserved.

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 1799-84-4, you can contact me at any time and look forward to more communication. Category: pyridazines.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem

 

What I Wish Everyone Knew About 3,3,4,4,5,5,6,6,6-Nonafluorohexyl methacrylate

Reference of 1799-84-4, Each elementary reaction can be described in terms of its molecularity, the number of molecules that collide in that step. The slowest step in a reaction mechanism is the rate-determining step.you can also check out more blogs about 1799-84-4.

Reference of 1799-84-4, Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. 1799-84-4, Name is 3,3,4,4,5,5,6,6,6-Nonafluorohexyl methacrylate, SMILES is CC(C(OCCC(F)(F)C(F)(F)C(F)(F)C(F)(F)F)=O)=C, belongs to pyridazines compound. In a article, author is Wang, Xiaolin, introduce new discover of the category.

Comprehension of the Effect of a Hydroxyl Group in Ancillary Ligand on Phosphorescent Property for Heteroleptic Ir(III) Complexes: A Computational Study Using Quantitative Prediction

A new Ir(III) complex (dfpypya)(2)Ir(pic-OH) (2) is theoretically designed by introduction of a simple hydroxyl group into the ancillary ligand on the basis of (dfpypya)(2)Ir(pic) (1) with the aim to get the high efficiency and stable blue-emitting phosphors, where dfpypya is 3-methyl-6-(2′,4′-difluoro-pyridinato)pyridazine, pic is picolinate, and pic OH is 3-hydroxypicolinic acid. The other configuration (dfpypya)(2)Ir(pic OH)’ (3) is also investigated to compare with 2. The difference between 2 and 3 is whether the intramolecular hydrogen bond is formed in the (dfpypya)(2)Ir(pic OH). The quantum yield is determined by three different methods including the semiquantitative and quantitative methods. To quantitatively determine the quantum yield is still not an easy task to be completed. This work would provide some useful advices to select the suitable method to reliably evaluate the quantum yield. Complex 2 has larger quantum yield and more stability as compared with 1 and 3. The formation of intramolecular hydrogen bond would become a new method to design new phosphor with the desired properties.

Reference of 1799-84-4, Each elementary reaction can be described in terms of its molecularity, the number of molecules that collide in that step. The slowest step in a reaction mechanism is the rate-determining step.you can also check out more blogs about 1799-84-4.

Reference:
Pyridazine – Wikipedia,
,Pyridazine | C4H4N2 – PubChem