Nidulaxanthone A, a xanthone dimer with a heptacyclic 6/6/6/6/6/6/6 ring system fromAspergillussp.-F029
Fuqian WangJie JiangSong HuXincai HaoYou‐Sheng CaiYing YeHaoran MaWeiguang SunCheng LuChuanqi HuangHucheng ZhuHong ZhangGeng ZhangYonghui Zhang
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Abstract:
Nidulaxanthone A (Keywords:
Xanthone
Molybdocenes form monomeric and dimeric complexes upon dissolution in water. The solution structures of the molybdocenes at neutral pH are inferred as the monomeric [(CpR)2Mo(OH2)(OH)]+ and the dimeric [(CpR)2Mo(μ-OH)2Mo(CpR)2]2+. The equilibrium constants for the monomer−dimer equilibria were obtained from 1H NMR dilution studies as K'eq = (7.9 × 10-2) ± (1.0 × 10-3) M for R = CH3 at pD 7 and as Keq = (3.5 × 10-2) ± (1.3 × 10-3) M for R = H at pD 3.5. Qualitatively, the monomer of the CpMe molybdocene is more favored than the Cp analogue, compared to their respective dimers.
Dilution
Water dimer
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The structure, interaction energy and vibrational frequencies of the hydrogen bonded cobaltous acid monomer and dimer were investigated at the B3LYP level with the CEP-31G** basis set. The cobaltous acid forms stable cyclic dimer with the dimerization energy of about -14 kcal/mol. The structural parameters of monomers are strongly perturbed upon dimerization, and the vibrational spectra are predicted to show large vibrational shifts compared with the monomer spectra.
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The parallel dimer of deamino-oxytocin has been synthesized by a novel solid-phase route. Successive orthogonal deprotection and oxidation reactions, carried out while the peptide remained anchored to a polymeric support, resulted in the formation of two disulfide bridges and conversion, with minimal side reactions, of the linear monomeric precursor to the dimer. The purified dimer showed approximately 0.1% to 2% of the biological activities of the monomer, as well as prolonged action. The time course of response to the deamino-oxytocin dimer differed from that of the monomer (and of oxytocin), and is probably due to slow formation of monomer under the conditions of biological testing.
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X-ray diffraction studies carried out on single crystals of the monomeric, viz. 5,6-dimethoxy-2H-furo[2,3-h][1]benzopyran-2-one, C13H10O5, and dimeric, viz. 5,5′,6,6′-tetramethoxy-3,3′,4,4′-tetrahydro-2H,2′H-3,3′:4,4′-bi(furo[2,3-h][1]benzopyran)-2,2′-dione, C26H20O10, forms of pimpinellin have revealed that, following cyclodimerization, the carbonyl groups are head-to-head with respect to one another. In the monomer, the heterocyclic ring is planar, but it exhibits a twisted-boat conformation in the dimer. Both the monomer and the dimer interact through C—H⋯O interactions.
Benzopyrans
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The 7S globulin of soybean seed was purified on concanavalin A-Sepharose 4B. It exists as monomers at a high ionic strength or at a low pH, whereas it associates to form dimers under the opposite conditions. The sedimentation coefficient of the monomer is 5.6S, and that of the dimer 10S, at a protein concentration of 0.16%. Since the interconversion between the monomer and the dimer proceeds fairly rapidly, the ultracentrifugal pattern of the mixture shows a single peak. The apparent sedimentation coefficient of the mixture successively changes between 5.6S and lOS, corresponding to the ratio of the amounts of the monomer and the dimer. The monomer reversibly dissociates into subunits with 5M urea or 0.01 N HCl.
Trimer
Sepharose
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Abstract New perthiophosphonic acid anhydrides (R‐PS 2 ) n with R = 2,4,6‐tri‐iso‐propylphenyl and 2,4‐di‐tert‐butyl‐6‐methylphenyl have been prepared. For the latter case, the isolation of the monomer (n = 1), the 2,4‐di‐tertbutyl‐6‐methylphenyl‐dithioxophosphorane, and both dimers (n = 2), cis‐ and trans‐2,4‐bis(2,4‐di‐tertbutyl‐6‐methylphenyl)‐2, 4‐dithioxo‐1, 3, 2, 4‐dithiadiphosphetane, has been determined. The new cis and trans dithiadiphosphetanes and dithioxophosphoranes as well as the known compounds (2,4,6‐trimethylphenyl‐PS 2 ) 2 and 2,4,6‐tri‐tert‐butylphenyl‐PS 2 are characterized by solution and high resolution solid‐state 31 P and 13 C NMR spectroscopy. The existence of a dimer‐monomer equilibrium is directly proved by 2D exchange 31 P NMR spectroscopy. It is shown that the reaction of the monomer with methanol is faster than the reaction of the dimer with methanol.
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Quantum chemical
Chain (unit)
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Abstract The monomer‐dimer equilibrium of nitrosobenzene in CDCI 3 solution was studied in the temperature range ‐50 to 30 °C using 13 C, 15 N and 17 O NMR spectroscopy. Decreasing temperature favours the dimeric azodioxy species, and below ca 10 °C both cis and trans dimers were detected in addition to the monomer. The relative abundance of the trans dimer is low (⩽6%) at all temperatures and this is the first report of its existence in solution. The monomer and dimer species were characterized by their 13 C and 15 N shifts, and exchange between them was measured quantitatively by 13 C 2D‐EXSY spectra. Exchange occurred exclusively between the cis ‐dimer and monomer and between the trans ‐dimer and monomer, activation energies, Δ G ≠ (298.15 K) being 65.8±0.1 and 70.0±0.3 kJ mol −1 , respectively.
Nitrosobenzene
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Abstract 1 H n.m.r. is used to show that α‐mercaptoacetone is formed as a mixture of monomer and a dimer. No evidence is found to support the previously claimed isolation of two dimeric species. The dimer dissociates to the monomer on standing as a solution in chloroform, but the monomer is dimerized by grinding. 1‐Mercapto‐3‐phenylpropan‐2‐one behaves similarly.
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Abstract Compounds (III) and (IV) are characterized by single crystal XRD, FTIR, CPMAS 31 P NMR, and solution 31 P and 183 W NMR spectroscopy.
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