Residual dipolar couplings (RDCs) have recently become increasingly important in organic structure determination due to their unique information content. One main limitation for the use of RDCs in organic compounds, however, is that the compound in question needs to be oriented with respect to the magnetic field in order to measure RDCs. So far, there are very few possibilities for modulating the induced degree of orientation. The situation is even worse when chiral orienting media are considered, which could allow absolute configuration determination in the future. We have conducted a systematic investigation into modulating the orientation induced by one chiral orienting medium, namely organic solutions of PBLG (poly‐γ‐benzyl‐L ‐glutamate), as a function of its molecular weight and the organic co‐solvent used, and have obtained significant insights into factors that influence the order induced. With increasing molecular weight of the polypeptide the orientation of the solutes decreases, leading to well‐resolved spectra with improved line shapes. This can be attributed exclusively to the fact that the critical concentration of the liquid‐crystalline phase decreases with increasing molecular weight (pure dilution effect). Any influence of increasing flexibility on the orientation can be ruled out. 相似文献
Residual dipolar couplings (RDCs) are amongst the most powerful NMR parameters for organic structure elucidation. In order to maximize their effectiveness in increasingly complex cases such as flexible compounds, a maximum of RDCs between nuclei sampling a large distribution of orientations is needed, including sign information. For this, the easily accessible one‐bond 1H–13C RDCs alone often fall short. Long‐range 1H–1H RDCs are both abundant and typically sample highly complementary orientations, but accessing them in a sign‐sensitive way has been severely obstructed due to the overflow of 1H–1H couplings. Here, we present a generally applicable strategy that allows the measurement of a large number of 1H–1H RDCs, including their signs, which is based on a combination of an improved PSYCHEDELIC method and a new selective constant‐time β‐COSY experiment. The potential of 1H–1H RDCs to better determine molecular alignment and to discriminate between enantiomers and diastereomers is demonstrated. 相似文献
Summary: Self‐association behaviors of poly(γ‐benzyl L ‐glutamate)‐graft‐poly(ethylene glycol) (PBLG‐graft‐PEG) and its mixtures with PBLG homopolymer in aqueous media were investigated by fluorescence spectroscopy, transmission electron microscopy (TEM), and nuclear magnetic resonance (NMR) spectroscopy. It was revealed that PBLG‐graft‐PEG could self‐assemble to form polymeric micelles with a core‐shell structure in the shape of spindle. The introduction of PBLG homopolymer not only decreases the critical micelle concentration, but also changes the morphology of the micelles.
The excitation fluorescence spectra of pyrene as a function of concentrations for the mixture of PBLG‐graft‐PEG with PBLG and a TEM image of the formed micelles. 相似文献
3‐Allyl‐substituted quinolones undergo a triplet‐sensitized di‐π‐methane rearrangement reaction to the corresponding 3‐cyclopropylquinolones upon irradiation with visible light (λ=420 nm). A chiral hydrogen‐bonding sensitizer (10 mol %) was shown to promote the reaction enantioselectively (88–96 % yield, 32–55 % ee). Surprisingly, it was found that the enantiodifferentiation does not occur at the state of initial product formation but that it is the result of a deracemization event. The individual parameters that control the distribution of enantiomers in the photostationary state have been identified. 相似文献
Syntheses of poly(γ‐benzyl L ‐glutamate)s (PBLGs) labeled with various fluorophores (tryptophan, dansyl, and anthracene) having different molecular weights are reported. Association of PBLG chains was studied by time‐resolved emission anisotropy in the solvents supporting the aggregation process (1,4‐dioxane and tetrahydrofuran) and in N,N‐dimethylformamide, where the aggregates were not formed. The influence of molecular weight and polymer concentration on PBLG association was studied as well. The limiting emission anisotropy (r∞) and rotational correlation times (ϕ) were determined. The chain relaxation dynamics were compared with the fluorescence lifetimes of the fluorophores and spectroscopically suitable labels were selected. Tryptophan was found to be an inconvenient fluorophore for the association study of PBLGs because of its short excited‐state lifetime. Dansyl and anthracene fluorophores, however, proved to be suitable labels for the chain dynamics study of PBLGs in solution. The mobilities of PBLG chains in 1,4‐dioxane were slower than those in tetrahydrofuran and N,N‐dimethylformamide because of PBLG association in this solvent. 相似文献
Together with NOE and J coupling, one‐bond residual dipolar coupling (RDC), which reports on the three‐dimensional orientation of an internuclear vector in the molecular frame, plays an important role in the conformation and configuration analysis of small molecules in solution by NMR spectroscopy. When the molecule has few C? H bonds, or too many bonds are in parallel, the available RDCs may not be sufficient to obtain the alignment tensor used for structure elucidation. Long‐range RDCs that connect nuclei over multiple bonds are normally not parallel to the single bonds and therefore complement one‐bond RDCs. Herein we present a method for extracting the long‐range RDC of a chosen proton or group of protons to all remotely connected carbon atoms, including non‐protonated carbon atoms. Alignment tensors fitted directly to the total long‐range couplings (T=J+D) enabled straightforward analysis of both the long‐range and one‐bond RDCs for strychnine. 相似文献
The concept of using residual dipolar couplings (RDCs) for the structure determination of organic molecules is applied to the simultaneous assignment of all diastereotopic protons in strychnine. To use this important NMR parameter the molecule has to be aligned in the magnetic field. Here we present a new alignment medium for organic substrates. The optimization of the alignment properties of mixtures of poly-gamma-ethyl-L-glutamate (PELG) and CDCl(3) are described and the alignment properties of PELG at different concentrations are evaluated. A comparison of PELG with poly-gamma-benzyl-L-glutamate (PBLG) shows considerable differences in the magnitude of alignment for strychnine in the two alignment media. PELG induces a lower degree of order and makes the measurement of residual dipolar couplings (RDCs) in strychnine possible. All one-bond C-H RDCs of strychnine in PELG were determined by using 2D heteronuclear single quantum coherence (HSQC) spectroscopy. The strategy for the extraction of RDCs for methylene groups is described in detail. The RDCs and order parameters are used to assign pairs of diastereotopic protons. This methodology can distinguish not only one pair of diastereotopic protons but it can be used to assign all pairs of diastereotopic protons simultaneously. Two different calculation approaches to achieve this task are described in detail. 相似文献
The electron microscope study of the isothermal epitaxial crystallization of PBLG on 001 cleavage surface of NaCl or KCl from 0.01% chloroform solution with various n-octane contents has shown that the epitaxial growth process could be divided in two stages. In the first stage a rapid growth of a lamellar layer 20–40 Å thick could be observed; the PBLG molecules in αhelix conformation are oriented along <110> substrate directions. The dimensions and perfectness of alignment of these lamellae along the <110> direction depends on the molecular weight of the PBLG. In the second stage of PBLG epitaxial crystallization the relatively slow thickening of some initial lamellae takes place and one can observe many types of secondary structures. With increasing thickness of the epitaxial structures and increasing molecular weight of the PBLG the degree of orientation in relation to <110> substrate direction diminishes. The two growth stages in PBLG epitaxial crystallization are related to different kinds of orienting and attracting forces of the alkali halide substrate. 相似文献