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Creators/Authors contains: "Gil, Roberto R."

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  1. Abstract

    Configurational and conformational analysis of the biologically relevant natural product artemisinin was conducted using carbon–carbon residual dipolar couplings (1DCCRDCs) at natural abundance. These RDCs were measured through the 2D‐INADEQUATE NMR experiment using a sample aligned in a compressed poly (methyl methacrylate) (PMMA) gel swollen in CDCl3. Singular value decomposition (SVD) fitting analysis of all carbon–carbon bonds,1DCCRDCs, in relation to the full configuration/conformational space (32 diastereoisomers) of artemisinin, unambiguously identified the correct configuration of artemisinin.

     
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  2. Herein, we demonstrate that homopolymerization and statistical copolymerization of 2-ethylhexyl thiophene-3-carboxylate and 2-ethylhexyl selenophene-3-carboxylate monomers is possible via Suzuki–Miyaura cross-coupling. A commercially available palladium catalyst ([1,3-bis(2,6-di-3-pentylphenyl)imidazol-2-ylidene](3-chloropyridyl)dichloropalladium( ii ) or PEPPSI-IPent) was employed to prepare regioregular conjugated polymers with high molecular weights (∼20–30 kg mol −1 ), and relatively narrow molecular weight distributions. The optical bandgap in the copolymer series could be reduced by increasing the concentration of selenophene-3-carboxylate in the material. Configurational triads were observed in the 1 H NMR spectra of the statistical copolymers, which were assigned using a combination of 2D NMR techniques. The use of a 1 H– 77 Se HSQC spectrum to further examine sequence distribution in the statistical copolymers revealed how 77 Se NMR can be used as a tool to examine the microstructure of Se-containing conjugated polymers. 
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  3. null (Ed.)
    Nuclear Overhauser Effect (NOE) methods in NMR are an important tool for 3D structural analysis of small molecules. Quantitative NOE methods conventionally rely on reference distances, known distances that have to be spectrally separated and are not always available. Here we present a new method for evaluation and 3D structure selection that does not require a reference distance, instead utilizing structures optimized by molecular mechanics, enabling NOE evaluation even on molecules without suitable reference groups. 
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  4. null (Ed.)
  5. The transition from the discrete, excitonic state to the continuous, metallic state in thiolate-protected gold nanoclusters is of fundamental interest and has attracted significant efforts in recent research. Compared with optical and electronic transition behavior, the transition in magnetism from the atomic gold paramagnetism (Au 6s 1 ) to the band behavior is less studied. In this work, the magnetic properties of 1.7 nm [Au 133 (TBBT) 52 ] 0 nanoclusters (where TBBT = 4- tert -butylbenzenethiolate) with 81 nominal “valence electrons” are investigated by electron paramagnetic resonance (EPR) spectroscopy. Quantitative EPR analysis shows that each cluster possesses one unpaired electron (spin), indicating that the electrons fill into discrete orbitals instead of a continuous band, for that one electron in the band would give a much smaller magnetic moment. Therefore, [Au 133 (TBBT) 52 ] 0 possesses a nonmetallic electronic structure. Furthermore, we demonstrate that the unpaired spin can be removed by oxidizing [Au 133 (TBBT) 52 ] 0 to [Au 133 (TBBT) 52 ] + and the nanocluster transforms from paramagnetism to diamagnetism accordingly. The UV-vis absorption spectra remain the same in the process of single-electron loss or addition. Nuclear magnetic resonance (NMR) is applied to probe the charge and magnetic states of Au 133 (TBBT) 52 , and the chemical shifts of 52 surface TBBT ligands are found to be affected by the spin in the gold core. The NMR spectrum of Au 133 (TBBT) 52 shows a 13-fold splitting with 4-fold degeneracy of 52 TBBT ligands, which are correlated to the quasi- D 2 symmetry of the ligand shell. Overall, this work provides important insights into the electronic structure of Au 133 (TBBT) 52 by combining EPR, optical and NMR studies, which will pave the way for further understanding of the transition behavior in metal nanoclusters. 
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  6. Abstract

    Determination of the solution conformation of both small organic molecules and peptides in water remains a substantial hurdle in using NMR solution conformations to guide drug design due to the lack of easy to use alignment media. Herein we report the design of a flexible compressible chemically cross‐linked poly‐4‐acrylomorpholine gel that can be used for the alignment of both small molecules and cyclic peptides in water. To test the new gel, residual dipolar couplings (RDCs) andJ‐coupling constants were used in the configurational analysis of strychnine hydrochloride, a molecule that has been studied extensively in organic solvents as well as a small cyclic peptide that is known to form an α‐helix in water. The conformational ensembles for each molecule with the best fit to the data are reported. Identification of minor conformers in water that cannot easily be determined by conventional NOE measurements will facilitate the use of RDC experiments in structure‐based drug design.

     
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  7. Abstract

    Determination of the solution conformation of both small organic molecules and peptides in water remains a substantial hurdle in using NMR solution conformations to guide drug design due to the lack of easy to use alignment media. Herein we report the design of a flexible compressible chemically cross‐linked poly‐4‐acrylomorpholine gel that can be used for the alignment of both small molecules and cyclic peptides in water. To test the new gel, residual dipolar couplings (RDCs) andJ‐coupling constants were used in the configurational analysis of strychnine hydrochloride, a molecule that has been studied extensively in organic solvents as well as a small cyclic peptide that is known to form an α‐helix in water. The conformational ensembles for each molecule with the best fit to the data are reported. Identification of minor conformers in water that cannot easily be determined by conventional NOE measurements will facilitate the use of RDC experiments in structure‐based drug design.

     
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  8. Abstract

    We present a method to use long‐range CH coupling constants to derive the correct diastereoisomer from the molecular constitution of small molecules. A set of 792JCHand3JCHvalues collected from a single HSQMBC experiment on a sample of strychnine were used in the CASE‐3D (computer‐assisted 3D structure elucidation) protocol. In addition to the most commonly used3JCHcoupling constants, the subset of 322JCHvalues alone showed an excellent degree of configuration selection. The study is mainly based on comparison of DFT‐calculated2,3JCHvalues with experimental ones, critical for the case of2JCH. But the configuration selection also works well using3JCHvalues predicted from a semi‐empirical Karplus‐based equation limited to H−C−C−C fragments. The robustness, shown using strychnine as a proof of concept, makes theJ‐based CASE‐3D analysis a viable option for the application in fields such as peptide and carbohydrate research, organic synthesis, natural‐product identification and analysis, as well as medicinal chemistry.

     
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  9. Abstract

    We present a method to use long‐range CH coupling constants to derive the correct diastereoisomer from the molecular constitution of small molecules. A set of 792JCHand3JCHvalues collected from a single HSQMBC experiment on a sample of strychnine were used in the CASE‐3D (computer‐assisted 3D structure elucidation) protocol. In addition to the most commonly used3JCHcoupling constants, the subset of 322JCHvalues alone showed an excellent degree of configuration selection. The study is mainly based on comparison of DFT‐calculated2,3JCHvalues with experimental ones, critical for the case of2JCH. But the configuration selection also works well using3JCHvalues predicted from a semi‐empirical Karplus‐based equation limited to H−C−C−C fragments. The robustness, shown using strychnine as a proof of concept, makes theJ‐based CASE‐3D analysis a viable option for the application in fields such as peptide and carbohydrate research, organic synthesis, natural‐product identification and analysis, as well as medicinal chemistry.

     
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