Conformational studies by dynamic NMR. 89. Stereomutation and cryogenic enantioseparation of conformational antipodes of hindered aryl oximes

Aryl benzyl oximes having the configuration Z give rise to stereolabile atropisomers when a halogen atom is present in the ortho position of the aryl moiety, as a consequence of the restricted aryl-CN bond rotation. By means of dynamic (1)H NMR spectroscopy it has been possible to determine the corresponding rotation barrier, hence the lifetime of the atropisomers that, in the case of the iodine derivative, was found sufficiently long as to allow a physical separation to be achieved on an appropriately cooled enantioselective HPLC column. Comparison of the barriers determined by dynamic NMR and dynamic HPLC proved the equivalence of the two techniques. When the iodine atom was substituted by an alpha-naphthyl group, two dynamic processes were observed. That with the lower barrier could be determined by NMR and that with the higher barrier by HPLC, thus outlining the complementarity of these two techniques.     

NMR investigation of the dynamics of banana shaped molecules in the isotropic phase: a comparison with calamitic mesogens behaviour

The first translational self-diffusion NMR measurements in the isotropic phase of banana-shaped liquid crystals are reported. In this paper, two banana-shaped mesogens, having a similar molecular structure and showing a nematic phase, have been investigated by means of translational self-diffusion NMR, (2)H NMR spin-spin and (1)H NMR spin-lattice relaxation measurements in the isotropic phase. While (1)H diffusion and (2)H relaxation times reveal a peculiar slow dynamic behaviour of banana-shaped mesogens compared with calamitic mesogens, the (1)H relaxation times seem to be affected by fast dynamics only. The origin of these dynamic features is discussed in terms of overall and internal molecular motions, in the frame of recent speculations concerning the formation of molecular clusters or aggregates in the isotropic phase of banana-shaped liquid crystals.     

NMR spectra and dynamics of the sodium salt of per(6-thiobenzoic acid)-gamma-cyclodextrin

The solution-state NMR spectra of a per-6-substituted gamma-cyclodextrin show some interesting dynamic properties. At high temperature (353 K), the (1)H NMR spectrum shows dynamic averaging of the different conformations. This averaging is no longer observed on cooling of the cyclodextrin solution to 278 K, resulting in NMR spectra with a large (1)H and (13)C chemical shift dispersion. The complete assignment of the eight unique glucosyl residues was achieved using COSY, HSQC and exchange spectroscopy. A ROESY spectrum, with a short mixing time to reduce the effects of exchange, gives correlations that lead to the determination of the connectivity of all eight glucosyl residues. On the NMR time-scale, the cyclodextrin is highly dynamic; the lower temperature minimum energy conformation has one of the aromatic rings self-complexed and a distorted cyclodextrin torus.     

Spontaneous self-assembly of water-soluble nucleotide-calixarene conjugates in small micelles coalescing to microspheres

Spontaneous self-assembly of calix[4]arenes bearing four 2'-deoxythymidine or 2'-deoxyadenosine nucleotide pendants is investigated using (1)H NMR, exchange NMR, and diffusion ordered NMR spectroscopies and dynamic light scattering. In aqueous medium, the nucleotide-calixarene conjugates, by noncovalent interactions involving both nucleobases and calixarene skeleton, form dimers which self-organize in micelles by increasing the concentration. Microscopic images (scanning electron microscopy and transmission electron microscopy) show that the nucleobase affects the aggregate morphology in the solid state.     

Dynamic NMR study of cyclic derivatives of pyridoxine

A series of pyridoxine derivatives was investigated by ¹H and 2D nuclear overhauser enhancement spectroscopy (NOESY) NMR. The free energies of activation for the pyridyl‐oxygen rotation of the 2,4‐dinitrophenyl ether of the seven‐membered acetals of pyridoxine were measured by dynamic NMR. A conformational exchange between the chair and twist forms of the seven‐membered acetal ring was confirmed by dynamic NMR and STO3G computations. Copyright © 2014 John Wiley & Sons, Ltd.     

Between Two Chairs: Combination of Theory and Experiment for the Determination of the Conformational Dynamics of Xylosides

The conformational properties of monosaccharides constitute fundamental features of oligosaccharides. While the energy landscape of monosaccharides can be altered by a specific biochemical environment or by chemical modifications, the analysis of resulting dynamic conformational equilibria is not feasible by experimental means alone. In this work, a series of β-d -xylopyranosides is used to outline how a combination of experimental NMR parameters and computed molecular properties can be used to determine conformers and quantify the composition of conformational equilibria. We demonstrate that identifying the most stable conformers using energy calculations is challenging and computing of NMR shieldings is typically not sensitive enough. On the other hand, computed spin-spin coupling constants for the xyloside ring can be used to unambiguously assign experimental NMR data of dynamic conformational equilibria and quantify the ratio of different conformers in the mixture. As a proof of principle, this procedure allowed to analyze a hitherto unknown dynamic equilibrium of a diamino-xyloside as a precursor of a molecular switch.     

Dynamic effects in MAS and MQMAS NMR spectra of half-integer quadrupolar nuclei: calculations and an application to the double perovskite cryolite

Dynamic processes such as chemical exchange or rotations between inequivalent orientations can affect the magic-angle spinning (MAS) and the multiple-quantum (MQ) MAS NMR spectra of half-integer quadrupolar nuclei. The present paper discusses such dynamic multisite MAS and MQMAS effects and applies them to study the dynamic processes that occur in the double perovskite cryolite, Na3AlF6. Dynamic line shape simulations invoking a second-order broadening of the central transition and relying on the semiclassical Bloch-McConnell formalism for chemical exchange were performed for a variety of exchange models possessing different symmetries. Fitting experimental variable-temperature cryolite 23Na NMR data with this formalism revealed that the two inequivalent sodium sites in this mineral undergo an exchange characterized by a broad distribution of rates. To further assess this dynamic process a variety of 27Al and 19F MAS NMR studies were also undertaken; quantitative 27Al-19F dipolar coupling measurements then revealed a dynamic motion of the AlF6 octahedra that were qualitatively consistent with predictions stemming from molecular dynamic simulations on this double perovskite.     

Simultaneous detection of protein phosphorylation and acetylation by high-resolution NMR spectroscopy

Post-translational protein modifications (PTMs) such as phosphorylation and acetylation regulate a large number of eukaryotic signaling processes. In most instances, it is the combination of different PTMs that "encode" the biological outcome of these covalent amendments in a highly dynamic and cell-state-specific manner. Most research tools fail to detect different PTMs in a single experiment and are unable to directly observe dynamic PTM states in complex environments such as cell extracts or intact cells. Here we describe in situ observations of phosphorylation and acetylation reactions by high-resolution liquid-state NMR spectroscopy. We delineate the NMR characteristics of progressive lysine acetylation and provide in vitro examples of joint phosphorylation and acetylation events and how they can be deciphered on a residue-specific basis and in a time-resolved and quantitative manner. Finally, we extend our NMR investigations to cellular phosphorylation and acetylation events in human cell extracts and demonstrate the unique ability of NMR spectroscopy to simultaneously report the establishment of these PTMs by endogenous cellular enzymes.     

Lanthanide paramagnetic probes for NMR spectroscopic studies of fast molecular conformational dynamics and temperature control. Effective six-site proton exchange in 18-crown-6 by exchange spectroscopy

(1)H and (13)C NMR measurements are reported for the CDCl(3) and CD(2)Cl(2) solutions of [La(18-crown-6)(NO(3))(3)] (I), [Pr(18-crown-6) (NO(3))(3)] (II), [Ce(18-crown-6)(NO(3))(3)] (III), and [Nd(18-crown-6)(NO(3))(3)] (IV) complexes. Temperature dependencies of the (1)H NMR spectra of paramagnetic II-IV have been analyzed using the dynamic NMR (DNMR) methods for six-site exchange. Two types of conformational dynamic processes were identified (the first one is conditioned by interconversion of complex enantiomeric forms and pseudorotation of a macrocycle molecule upon the C(2) symmetry axis; the second one is conditioned by macrocycle molecule inversion). Application of exchange spectroscopy (2D-EXSY) of DNMR for investigation of this dynamic system (II-IV) simplifies the assignment of the NMR signals and represents the first experimental study of multisite exchange. In the present work, the methodology of paramagnetic 4f (Ce, Pr, and Nd) probe applications for the study of free-energy, enthalpy, and entropy changes in chemical exchange processes, as well as the advantages of this method in a comparison with DNMR studies of diamagnetic substances, is discussed. In particular, as a result of paramagnetic chemical shifts in 4f complexes, the range of measurable rate constants expands considerably compared to the analogous range in diamagnetic compounds. Coordination compounds investigated in the paper represent new types of thermometric NMR sensors and lanthanide paramagnetic probes for in situ temperature control in solution.     

Bis(thiocarbamates): Synthesis and Substituent Effects on the Barrier to N‐CO Rotation

The synthesis of a new series of variously substituted bis(thiocarbamates) from the addition of dithiols to isocyanates is described. These bis(thiocarbamates) were fully characterized by NMR, IR, and HRMS analyses. In addition, the effect of different substituents on the hindered rotation about the N‐CO bond in these bis‐thiocarbamates was investigated through dynamic NMR experiments.     

A Novel Conformation Investigation on Newly Synthesized Compound of Diethyl Puerarin-7-yl Phosphate

1INTRODUCTION Puerarin(4?,7-dihydroxy-8-β-D-glucosylisoflavo ne),whose chemical structure is shown in Fig.1,is a C-glycoside compound.It is present in a large amount of active components of Puerariae radix,a com-monly used Chinese herb,which exerts sedative and antipyretic actions and is often used to treat influ-enza,wrist stiffness and headache[1].A number of investigations were carried out internationally to iden-tify the physiological activities of puerarin such as antiproliferative…     

利用可逆共价键pH响应性制备聚轮烷

合成了具有可逆酰腙键的2,4-二硝基苯甲醛封端的哑铃型聚乙二醇衍生物. 在60 ℃时将水溶液的pH值调节至酸性, 哑铃型聚合物上的酰腙键发生可逆的“断开”和“生成”. 在这个可逆过程中, 溶液中的α-环糊精逐步与聚乙二醇内含复合. 由于环糊精具有较强疏水作用的内部空腔, 可以与聚乙二醇形成稳定的内含结晶复合物, 在这种超分子作用力下, 哑铃型聚乙二醇衍生物的分子链上会动态地穿入更多的α-环糊精, 最终形成聚轮烷. 综合液体核磁共振、粉末X射线衍射、固体碳-13交叉极化/魔角自旋核磁共振及差示扫描量热分析结果证明, 这种利用可逆共价键pH响应性制备聚轮烷的方法是可行的. 与传统的聚轮烷制备方法不同, 这种利用动态的可逆共价键制备聚轮烷的方法并不需要预先合成准(聚)轮烷.     

Molecular hydrogen tweezers: structure and mechanisms by neutron diffraction, NMR, and deuterium labeling studies in solid and solution

The mechanism of reversible hydrogen activation by ansa-aminoboranes, 1-N-TMPH-CH(2)-2-[HB(C(6)F(5))(2)]C(6)H(4) (NHHB), was studied by neutron diffraction and thermogravimetric mass-spectroscopic experiments in the solid state as well as with NMR and FT-IR spectroscopy in solution. The structure of the ansa-ammonium borate NHHB was determined by neutron scattering, revealing a short N-H···H-B dihydrogen bond of 1.67 ?. Moreover, this intramolecular H-H distance was determined in solution to be also 1.6-1.8 ? by (1)H NMR spectroscopic T(1) relaxation and 1D NOE measurements. The X-ray B-H and N-H distances deviated from the neutron and the calculated values. The dynamic nature of the molecular tweezers in solution was additionally studied by multinuclear and variable-temperature NMR spectroscopy. We synthesized stable, individual isotopic isomers NDDB, NHDB, and NDHB. NMR measurements revealed a primary isotope effect in the chemical shift difference (p)Δ(1)H(D) = δ(NH) - δ(ND) (0.56 ppm), and hence supported dihydrogen bonding. The NMR studies gave strong evidence that the structure of NHHB in solution is similar to that in the solid state. This is corroborated by IR studies providing clear evidence for the dynamic nature of the intramolecular dihydrogen bonding at room temperature. Interestingly, no kinetic isotope effect was detected for the activation of deuterium hydride by the ansa-aminoborane NB. Theoretical calculations attribute this to an "early transition state". Moreover, 2D NOESY NMR measurements support fast intermolecular proton exchange in aprotic CD(2)Cl(2) and C(6)D(6).     

Conformational Analysis of Sulfur-Containing Heterocycles,Part I. Synthesis and Structural Determination of Diastereomerically Pure 4,6-Bis-(phenoxymethyl)-1,2,5-trithiepanes

Some diastereomerically pure 4,6-bis-(phenoxymethyl)-l,2,5-trithiepanes were synthesized and unambiguously assigned. Their conformational properties and dynamic behavior were investigated by various NMR spectroscopic methods and quantum-chemical calculations at the HF/6-31G* level. The ground states of these compounds proved to be twist-chairs. A ring interconversion can occur in the meso-isomers as well as in the (±)-isomers. This interconversion can be described as a simultaneous inversion of the disulfide bridge. In the case of the meso-isomers, both ground states are mirror images of each other and the transition state is a highly symmetrical chair. The barrier heights of interconversion were determined to be in the range of 50 kJ/mol by variable-temperature NMR measurements. The ground states as well as the transition state of the (±)-isomers were found to be nonsymmetrical. However, those dynamic processes that are fast with respect to the NMR time scale lead to averaged NMR spectra at room temperature. A further dynamic process found through the quantum-chemical calculations is a flapping of the meth-ylene groups of the rings. The energy barrier of this flapping was calculated to be very small (< 20 kJ/mol) and could not be observed by low-temperature NMR measurements.     

Synthesis and characterization of cross-linked reverse micelles

The design and synthesis of a new cross-linkable amphiphile is reported. Solutions of the amphiphile in a toluene/water mixture form reverse micelles as indicated by dynamic light scattering and NMR spectroscopy. As indicated by dynamic light scattering, TEM, and NMR spectroscopy data, these reverse micelles can be cross-linked without drastically changing the radius of the reverse micelles. Mixed reverse micelles are also characterized and cross-linked. The cross-linked reverse micelles are demonstrated to facilitate phase transfer and can be used to site isolate a catalyst.     

蓖麻油型聚氨酯IPN的结构与性能研究

本文用蓖麻油,甲苯二异氰酸酯和甲基丙烯酸β-羟乙酯组成的加聚型网络(Ⅰ)和蓖麻油,甲苯二异氰酸酯和聚丙二醇组成的缩聚型聚氨酯(Ⅱ),以同步法形成了一组新型的IPN。用动态力学法和固体核磁共振法研究了它们的结构和性能,形成IPN后,力学数据显示出机械强度的提高,动态力学损耗谱上表现出,tanδ的增宽,固体核磁共振谱计算的结果是界面层的比例增高,但界面层比例并不随配比的改变而变化,它揭示出此IPN可能具有壳核结构。     

Exploring the Potential of Multinuclear Solid-State 1H, 13C,and 35Cl Magnetic Resonance To Characterize Static and Dynamic Disorder in Pharmaceutical Hydrochlorides

Crystallographic disorder, whether static or dynamic, can be detrimental to the physical and chemical stability, ease of crystallization and dissolution rate of an active pharmaceutical ingredient. Disorder can result in a loss of manufacturing control leading to batch-to-batch variability and can lengthen the process of structural characterization. The range of NMR active nuclei makes solid-state NMR a unique technique for gaining nucleus-specific information about crystallographic disorder. Here, we explore the use of high-field ³⁵Cl solid-state NMR at 23.5 T to characterize both static and dynamic crystallographic disorder: specifically, dynamic disorder occurring in duloxetine hydrochloride ( 1 ), static disorder in promethazine hydrochloride ( 2 ), and trifluoperazine dihydrochloride ( 3 ). In all structures, the presence of crystallographic disorder was confirmed by ¹³C cross-polarization magic-angle spinning (CPMAS) NMR and supported by GIPAW-DFT calculations, and in the case of 3 , ¹H solid-state NMR provided additional confirmation. Applying ³⁵Cl solid-state NMR to these compounds, we show that higher magnetic fields are beneficial for resolving the crystallographic disorder in 1 and 3 , while broad spectral features were observed in 2 even at higher fields. Combining the data obtained from ¹H, ¹³C, and ³⁵Cl NMR, we show that 3 exhibits a unique case of disorder involving the ⁺N−H hydrogen positions of the piperazinium ring, driving the chloride anions to occupy three distinct sites.     

Conformational Analysis of Biphenyl-2,2′-Diacetate

Conformational analysis of biphenyl-2,2′-diacetate by dynamic NMR and UV spectra and by plots of enzyme activity vs temperature plots is described. From dynamic NMR spectra of the biphenyl-2,2′-diacetate with a chiral shift reagent, the coalescence temperature (T_c), the Gibbs energy of activation (ΔG^≠), and the rate coefficient (k) of bipbenyl-2,2′-diacetate were ?5 °C, 59.5 kJ/mol, and 13.3 s^?1, respectively. From analysis of the conformational break in the UV spectra and the discontinuity in the plots of enzyme activity vs temperature, the racemerization temperature of bipbenyl-2,2′-diacetate is about 5°C.     

The anisotropic effect of functional groups in 1H NMR spectra is the molecular response property of spatial nucleus independent chemical shifts (NICS)--conformational equilibria of exo/endo tetrahydrodicyclopentadiene derivatives

The inversion of the flexible five-membered ring in tetrahydrodicyclopentadiene (TH-DCPD) derivatives remains fast on the NMR timescale even at 103 K. Since the intramolecular exchange process could not be sufficiently slowed for spectroscopic evaluation, the conformational equilibrium is thus inaccessible by dynamic NMR. Fortunately, the spatial magnetic properties of the aryl and carbonyl groups attached to the DCPD skeleton can be employed in order to evaluate the conformational state of the system. In this context, the anisotropic effects of the functional groups in the (1)H NMR spectra prove to be the molecular response property of spatial nucleus independent chemical shifts (NICS).