Liquid Crystal NMR Research Articles

Assessing the chirality-dependent conformational distribution of small flexible opposite enantiomers dissolved in weakly ordering enantiopure media by means of liquid crystal NMR techniques

The unique power of Liquid Crystal NMR (LXNMR) in obtaining information about the conformational equilibrium of small solutes dissolved in orienting media can be effectively used to study chirality-related phenomena. In this work we have been able, for the first time in our knowledge, to fully resolve the two different conformational distributions for the R and S enantiomers of a case test molecule (the chiral flexible 4FMBOH: 4-F-α-methyl benzyl alcohol), dissolved in a typical chiral non-racemic aligning solvent, made of PBLG (poly- γ -benzyl-L-glutamate, a helical chiral polypeptide) co-dissolved in CDCl3. The two enantiomers show significant differences in their conformational behaviors, due to the different solute–solvent orientational molecular interactions experienced by the R and S species. In our opinion, fundamental studies of this kind could pave the way for a better understanding of chirality-dependent molecular interactions, often involved in a number of important chemical processes (concerning pharmaceuticals, biochemistry, advanced materials, etc.)

Ring inversion in cyclohexane: a textbook example

ABSTRACT The ring inversion of cyclohexane is a classic textbook example in organic chemistry of large-scale intramolecular interconversion. Early work by Luz using deuterated cyclohexane has shown how the interconversion rate can be studied. Here the complex proton NMR spectrum of cyclohexane is reported. We apply liquid-crystal 1H NMR spectroscopy as a function of temperature to study the interconversion rate process. For low temperatures interconversion is slow, at higher temperatures the fast interconversion limit is approached. For intermediate temperatures significant line broadening is observed as the signature of the interconversion process.

NMR, Raman, and DFT Study of Lyotropic Chromonic Liquid Crystals of Biomedical Interest: Tautomeric Equilibrium and Slow Self-Assembling in Sunset Yellow Aqueous Solutions.

Temperature and composition effects in Sunset Yellow FCF (SSY) aqueous solutions were studied by the 1H, 15N NMR as well as Raman spectroscopy passing through all phase transitions between isotropic phase (I) and chromonic phases-nematic (N) and columnar (M). It was shown that the tautomeric equilibrium in SSY is strongly shifted toward the hydrazone form. The corresponding equilibrium constant p KT = 2.5 was deduced using the density functional theory solvent model density model. The dominance of the hydrazone form was confirmed experimentally using the long-range 1H-15N correlation, widely known as heteronuclear multiple bond correlation. The peak found in the 1H NMR spectra at ca. 14.5 ppm can be attributed to the proton in the intramolecular N-H···O bond. The existence of this signal shows that (i) the growth of the SSY aggregates is accompanied by the segregation of water in the intercolumnar areas with no access for exchange with the N-H protons in the internal layers of the columnar stacks and that (ii) the lifetime of those aggregates is ≥10-8 s or even longer. The temperature dependences of H2O chemical shift and Raman O-H stretching band shape show that water confined in the intercolumnar areas behaves as in the neat substance. When the sample is heated and the transition from M phase to N phase occurs, the molecular motion of water is seen to change in a manner similar to that when water is melting. The equilibration time for N + M→ M is very long because of slow supramolecular restructuring, i.e., the growing of columnar stacks and building of hexagonal arrays. If the sample is cooled down to the temperature below N → M transition relatively fast, the structural changes are behind, and the system falls into supercooled state. In this case, the system evolves via long-lasting self-assembling from the supercooled state to the equilibrium. This process affects the shape of the 1H NMR signal and is easy to monitor.

Xenon NMR of liquid crystals confined to cylindrical nanocavities: a simulation study.

Applications of liquid crystals (LCs), such as smart windows and the ubiquitous display devices, are based on controlling the orientational and translational order in a small volume of LC medium. Hence, understanding the effects of confinement to the liquid crystal phase behaviour is essential. The NMR shielding of (129)Xe atoms dissolved in LCs constitutes a very sensitive probe to the details of LC environment. Linking the experimental results to microscopic phenomena calls for molecular simulations. In this work, the NMR shielding of atomic (129)Xe dissolved in a uniaxial thermotropic LC confined to nanosized cylindrical cavities is computed from coarse-grained (CG) isobaric Monte Carlo (MC) simulations with a quantum-chemically (QC) pre-parameterised pairwise-additive model for the Xe nuclear shielding tensor. We report the results for the (129)Xe nuclear shielding and its connection to the structure and order of the LC appropriate to two different cavity sizes, as well as a comparison to the results of bulk (non-confined) simulations. We find that the confinement changes the LC phase structure dramatically and gives rise to the coexistence of varying degrees of LC order, which is reflected in the Xe shielding. Furthermore, we qualitatively reproduce the behaviour of the mean (129)Xe chemical shift with respect to temperature for atomic Xe dissolved in LC confined to controlled-pore glass materials. In the small-radius cavity the nematic - paranematic phase transition is revealed only by the anisotropic component of the (129)Xe nuclear shielding. In the larger cavity, the nematic - paranematic - isotropic transition is clearly seen in the Xe shielding. The simulated (129)Xe NMR shielding is insensitive to the smectic-A - nematic transition, since in the smectic-A phase, the Xe atoms largely occupy the imperfect layer structure near the cavity walls. The direct contribution of the cavity wall to (129)Xe nuclear shielding is dependent on the cavity size but independent of temperature. Our results show that the combination of CG simulations and a QC pre-parameterised (129)Xe NMR shielding allows efficient studies of the phase behaviour and structure of complex systems containing thousands of molecules, and brings us closer to the simulation of NMR experiments.

The rα structure of C6F5Br determined by 13C NMR in liquid crystal. Distortions of the C6F6 ring induced by single substitution

The 19F with C13-satellites and the 13C NMR spectra of bromopentafluorobenzene C6F5Br dissolved in the nematic phase ZLI 1695 were analyzed and dipolar couplings were obtained. It was shown that extracting carbon-13–fluorine dipolar couplings from the 13C spectrum is more advantageous than from the 19F with C13-satellites one. Excluding the fluorine–fluorine dipolar couplings from the fitting because of their high pseudo-dipolar contributions and neglecting the anisotropy of the carbon-13–fluorine ones seems to be an acceptable approximation which makes it possible to reach the rα structure of bromopentafluorobenzene. Combining obtained results with those we have previously measured for molecules C6F5X (X: H, Cl, I), distortions of hexafluorobenzene ring induced by single substitution were investigated. Experimental NMR structures being modestly accurate, computations using the quantum chemical methods were needed. Investigations have been conducted at the DFT(M062X)/aug-cc-pVTZ-PP level of theory and in the frame of SCRF/PCM(SMD) solvation model. The largest deformations relatively to the reference value 120° were observed in the immediatevicinity of the substituted carbon and consist of a reduction in ring angle at the ipso position and an increase in the adjacent angles. Structural variations are discussed in relation to the electron-donor/electron-acceptor character of the substituent. Substituent effects on the benzene and hexafluorobenzene molecules are compared and are found to be opposite.

Signal intensities in 1H–13C CP and INEPT MAS NMR of liquid crystals

Spectral editing with CP and INEPT in 13C MAS NMR enables identification of rigid and mobile molecular segments in concentrated assemblies of surfactants, lipids, and/or proteins. In order to get stricter definitions of the terms “rigid” and “mobile”, as well as resolving some ambiguities in the interpretation of CP and INEPT data, we have developed a theoretical model for calculating the CP and INEPT intensities as a function of rotational correlation time τc and C–H bond order parameter SCH, taking the effects of MAS into account. According to the model, the range of τc can at typical experimental settings (5kHz MAS, 1ms ramped CP at 80–100kHz B1 fields) be divided into four regimes: fast (τc<1ns), fast-intermediate (τc≈0.1μs), intermediate (τc≈1μs), and slow (τc>0.1ms). In the fast regime, the CP and INEPT intensities are independent of τc, but strongly dependent on |SCH|, with a cross-over from dominating INEPT to dominating CP at |SCH|>0.1. In the intermediate regime, neither CP nor INEPT yield signal on account of fast T1ρ and T2 relaxation. In both the fast-intermediate and slow regimes, there is exclusively CP signal. The theoretical predictions are tested by experiments on the glass-forming surfactant n-octyl-β-d-maltoside, for which τc can be varied continuously in the nano- to millisecond range by changing the temperature and the hydration level. The atomistic details of the surfactant dynamics are investigated with MD simulations. Based on the theoretical model, we propose a procedure for calculating CP and INEPT intensities directly from MD simulation trajectories. While MD shows that there is a continuous gradient of τc from the surfactant polar headgroup towards the methyl group at the end of the hydrocarbon chain, analysis of the experimental CP and INEPT data indicates that this gradient gets steeper with decreasing temperature and hydration level, eventually spanning four orders of magnitude at completely dry conditions.

The stable conformations of 4,4′-dichloro-trans-stilbene in solution by liquid crystal NMR spectroscopy

The mechanisms conferring to the trans-stilbene derivatives their well known important biological properties are not yet completely clear; then, it is reasonable to suppose that their conformational equilibrium could play some kind of role (as found for many other molecules of pharmacological interest). In this scenario, the 4,4′-dichloro-trans-stilbene (DCS) can be considered as the prototype of a series of stilbenes substituted in 4,4′ positions in order to enhance their properties. Following this starting idea, we decided to study the conformational distribution of DCS in solution by Liquid Crystal NMR Spectroscopy (LXNMR), one of the best techniques to obtain this kind of information in the liquid state of matter. Our results show that the molecule is basically not planar: the most populated conformations we found are (a) the couple of symmetry-related, propeller-like structures, belonging to the C2 point group and characterized by the phenyl rings disrotated of about 18.7° with respect to the vinyl plane and (b) the couple of symmetry-related Ci structures (where the rings are parallel, like the pedals of a cycle) having this time the two rings conrotated of an angle of ∼18.7° with respect to the vinyl fragment.

Correction to “Conformational Distribution of trans-Stilbene in Solution Investigated by Liquid Crystal NMR Spectroscopy and Compared with In Vacuo Theoretical Predictions”

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionORIGINAL ARTICLEThis notice is a correctionCorrection to “Conformational Distribution of trans-Stilbene in Solution Investigated by Liquid Crystal NMR Spectroscopy and Compared with In Vacuo Theoretical Predictions”Giorgio Celebre*, Giuseppina De Luca, and Maria Enrica Di PietroCite this: J. Phys. Chem. B 2012, 116, 45, 13583Publication Date (Web):October 31, 2012Publication History Published online31 October 2012Published inissue 15 November 2012https://doi.org/10.1021/jp310376fCopyright © 2012 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views205Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (112 KB) Get e-Alerts Get e-Alerts

Quantum Factorization of 143 on a Dipolar-Coupling Nuclear Magnetic Resonance System

Quantum algorithms could be much faster than classical ones in solving the factoring problem. Adiabatic quantum computation for this is an alternative approach other than Shor's algorithm. Here we report an improved adiabatic factoring algorithm and its experimental realization to factor the number 143 on a liquid-crystal NMR quantum processor with dipole-dipole couplings. We believe this to be the largest number factored in quantum-computation realizations, which shows the practical importance of adiabatic quantum algorithms.

Conformational Distribution oftrans-Stilbene in Solution Investigated by Liquid Crystal NMR Spectroscopy and Compared within VacuoTheoretical Predictions

The basic question about the structure and the conformational distribution of a π-conjugated, flexible organic molecule (interesting in itself, in relation to the balance of forces determining its torsional equilibrium) becomes a really intriguing problem in the case of trans-stilbene (t-St), a "fundamental" molecule from a chemical point of view, as well as the prototype fragment of a series of derivatives endowed with several important biological and technological properties. As a matter of fact, the problem of t-St planarity when the molecule is isolated or in solution is a particularly debated question. In the present paper we studied the conformational distribution of t-St in solution, by resorting to the powerful technique of liquid crystal NMR spectroscopy (LXNMR), and we compared the obtained experimental results with accurate theoretical calculations carried out in vacuo, by using the MP2/6-31G** method (allowing for bond lengths and angles relaxation every 3° torsional steps). Our theoretical and experimental outcomes agree in indicating the nonplanarity of the molecule which, on the contrary, exhibits the coexistence of four stable rotamers, two by two symmetry related. In particular, we have found a couple of global minima corresponding to propeller-like C(2) symmetry conformations, where both the rings are "disrotated", with respect to the vinyl group, of about 17° in solution and of 27° in vacuo (theoretical value). Besides this, the presence of a couple of C(i) local minima, with both the rings "conrotated" of 17° (fluid phase) or of 27° (MP2/6-31G** calculations for the isolated molecule) has been determined.

Quantum decoherence and quasi-equilibrium in open quantum systems with few degrees of freedom: Application to 1H NMR of nematic liquid crystals

Explanation of decoherence and quasi-equilibrium in systems with few degrees of freedom demands a deep theoretical analysis that considers the observed system as an open quantum system. In this work, we study the problem of decoherence of an observed system of quantum interacting particles, coupled to a quantum lattice. Our strategy is based on treating the environment and the system-environment Hamiltonians fully quantum mechanically, which yields a representation of the time evolution operator useful for disentangling the different time scales underlying in the observed system dynamics. To describe the possible different stages of the dynamics of the observed system, we introduce quantum mechanical definitions of essentially isolated, essentially adiabatic, and thermal-contact system-environment interactions. This general approach is then applied to the study of decoherence and quasi-equilibrium in proton nuclear magnetic resonance ((1)H NMR) of nematic liquid crystals. A summary of the original results of this work is as follows. We calculate the decoherence function and apply it to describe the evolution of a coherent spin state, induced by the coupling with the molecular environment, in absence of spin-lattice relaxation. By assuming quantum energy conserving or non-demolition interactions, we identify an intermediate time scale, between those controlled by self-interactions and thermalization, where coherence decays irreversibly. This treatment is also adequate for explaining the buildup of quasi-equilibrium of the proton spin system, via the process we called eigen-selectivity. By analyzing a hypothetical time reversal experiment, we identify two sources of coherence loss which are of a very different nature and give rise to distinct time scales of the spin dynamics: (a) reversible or adiabatic quantum decoherence and (b) irreversible or essentially adiabatic quantum decoherence. Local irreversibility arises as a consequence of the uncertainty introduced by the coupling with an infinite quantum environment. The reversible part can be represented by a semiclassical model, similar to standard line-shape adiabatic models. By exploiting the separation existing between the time scales of the spin coherences and the irreversible decoherence, we present a novel technique to obtain the orientational molecular distribution function for a nematic liquid crystal. The procedure is based on the comparison of the observed coherence time evolution and numerical calculation under the adiabatic quantum decoherence approach. As an example, it is used the experimental free induction decay from a nematic PAA(d6) sample to extract such an orientational distribution. This is the first theoretical description of the experimental liquid crystal NMR signal in the time domain. On the contrary, the irreversible decoherence is intrinsically full-quantum mechanical, as it is governed by the commutation properties of the environment and the spin-lattice Hamiltonians. Consistently, it depends on the molecular correlation in a decisive way, since it vanishes under a mean-field model for the molecular dynamics. The results of this work can contribute to the understanding of the open question of the applicability of the spin-temperature concept in spin systems with few degrees of freedom.

Conformation and Dynamics of 18-Membered Hexathiametacyclophanes: A Two Step Racemization as Studied by Deuterium NMR in Chiral Lyotropic Liquid Crystals

The conformation and interconversion dynamics of two derivatives of the 18-membered hexathia metacyclophane 1 and 2 were studied by (1)H NMR spectroscopy in isotropic solvents and by (2)H NMR in chiral liquid crystalline (CLC) solutions, as well as by molecular structure computations. For the analysis of the dynamic effects, we made use of the concepts of "average symmetry" and "isodynamic groups", introduced by Altmann (Altmann, Proc. R. Soc.1967, 184, A298). Compound 1, which is unsubstituted in the inner aromatic site, has, according to the NMR and molecular force field calculations, a boat shaped ground conformation with C(2) symmetry. It is highly flexible and in the NMR spectrum exhibits two successive dynamic processes. There is a low temperature (170-210 K, E(a) = 10.5 kcal/mol) alternate "wing flipping", which corresponds to interchange between pairs of enantiomers and results, in the fast exchange limit, in an average prochiral molecule with C(2v) symmetry. This process is followed, at higher temperatures (290-320 K, E(a) = 28.5 kcal/mol), by an umbrella flipping type inversion with an average structure of D(2h) symmetry. This second process involves averaging of effective enantiotopic into homotopic sites and can only be studied in chiral solvents. The origin of the chiral discrimination and of their stepwise averaging is discussed. Compound 2, which is substituted with methoxy groups at the inner sites of the benzene rings, is much less flexible and exhibits dynamic effects in the NMR spectrum only at temperatures above 370 K. We were able to study the kinetic parameters of this process in isotropic solvents (E(a) = 21.4 kcal/mol). As for 1, the detailed mechanism of this process can in principle be established using dynamic NMR in CLC; however, experimental limitation precluded us from doing so. Possible alternatives and their effect on the 1D and 2D exchange spectra in CLC are discussed in a concluding section.

Experimental Assessment of the Vibration-Reorientation Contribution to Liquid Crystal NMR Dipolar Couplings: The Case of Tetramethylallene Dissolved in a Nematic Mesophase

In the present paper, the peculiar orientational behavior, studied by liquid crystal NMR (LXNMR) spectroscopy, of the D(2d) symmetry quasi-spherical molecule of tetramethylallene (TMA) dissolved in the nematic solvent I52 is exploited to attempt a quantitative experimental assessment of the correlation between molecular vibrations and overall rotations in weakly oriented molecules. The analysis of the very small D(HH) and (1)D((13)C-H) dipolar couplings, available from the natural abundance LXNMR spectra of TMA at different temperatures, allows for a derivation leading (by making a few approximations) to the quantification of the vibration-reorientation (also called nonrigid) contribution affecting the observed direct (1)D((13)C-H) dipolar coupling. The obtained results show that, under the particular conditions of the studied system (very weak orientational ordering of a highly symmetric molecule), this contribution is particularly important, in order to reproduce the whole value of the "observed" dipolar coupling. This issue is discussed and commented on at length in the work, also, by making reference to the analogy with perfectly symmetric molecules (such as methane and analogues) dissolved in liquid crystalline phases.

Intra- and intermolecular translocation of the bi-domain transcription factor Oct1 characterized by liquid crystal and paramagnetic NMR

The intra- and intermolecular translocation processes whereby the bi-domain transcription factor Oct1 searches for its specific DNA target site have been investigated by residual dipolar coupling (RDC) and paramagnetic relaxation enhancement (PRE) measurements. The RDC data show that the orientation of the POU(S) and POU(HD) domains of Oct1 relative to the long axis of the DNA is the same for specific and nonspecific complexes with DNA. In the context of the specific Oct1-DNA complex, sparsely-populated, spectroscopically "invisible" states reveal their footprints on the PRE profiles observed for the specific complex. Analysis of the PRE data indicates that the POU(HD) domain searches the DNA primarily by rotation-coupled sliding (intramolecular translocation), while the POU(S) domain functions as an antenna to promote intersegment transfer via intermolecular translocation. The latter involves the formation of a bridged intermediate in which the POU(HD) domain is located on the first DNA molecule and the POU(S) domain on the second. The formation of the bridge intermediate promotes the completion of intermolecular translocation of Oct1 via a first order process involving dissociation and association of the POU(HD) domain from one DNA molecule to another. Thus cross-talk between the POU(S) and POU(HD) domains, each fulfilling different and complementary components of the search process ensures efficient sampling of DNA, thereby facilitating the location of specific Oct1 target sites.

Anisotropy of indirect couplings and accurate molecular structures of 1,2- and 1,3-difluorobenzenes by combined analysis of gas electron diffraction, rotational spectroscopy and liquid crystal NMR data

In the combined analysis of gas-phase electron-diffraction scattering data, rotation constants and dipolar coupling constants from NMR experiments in liquid crystal solvents, not only are high-accuracy molecular structures of 1,2- and 1,3-difluorobenzene obtained, but the anisotropic components of some of the CF and FF indirect couplings have been deduced directly from the experimental data. The benefits of combined analyses of data from several different experimental techniques, and factors influencing the accuracy of structures determined in this way, are discussed.

Heteronuclear dipolar decoupling in liquid-crystal NMR using supercycled SWf-TPPM sequences

Heteronuclear dipolar decoupling is an essential requirement for extracting structural information from the (13)C NMR spectra of liquid crystals. Efficient schemes for heteronuclear dipolar decoupling in such systems are formulated here by supercycling SW(f)-TPPM, a sequence introduced recently for this purpose in rotating solids. These sequences are compared with two other commonly used decoupling schemes in liquid-crystal NMR, SPINAL-64 and SW(f)-TPPM, by analyzing the intensities of various resonances in the proton decoupled (13)C spectrum of the liquid-crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). The effectiveness of the decoupling programs with respect to experimental parameters such as RF field strength, decoupler offset frequency and phase angle is also presented.

ChemInform Abstract: The Molecular Structure of 1,2,3-Trichlorobenzene Determined by Combined Analysis of Data from Gas Electron Diffraction and Liquid Crystal NMR and by X-Ray Crystallography

Abstract The molecular structure of 1,2,3-trichlorobenzene (A) has been determined by using data from gas electron diffraction experiments and direct dipolar couplings from 1H NMR experiments in a joint structural analysis. The data are consistent with an assumed molecular model of C2v symmetry and all 11 structural parameters could be refined simultaneously, with standard deviations between 0.3 and 0.7 pm for bond lengths and less than 0.44° for valence angles. The crystal and molecular structures of A have also separately been determined by X-ray crystallography. The mean structural parameters in the solid state are almost equal to those obtained from the joint analysis.

ChemInform Abstract: Conformational Studies by Liquid Crystal NMR and ab initio Calculations: Methyl Nicotinate and Methyl Isonicotinate.

Abstract Conformational properties of methyl nicotinate and methyl isonicotinate have been studied by liquid crystal 1 H-NMR spectroscopy combined with ab initio calculations. The solvent used is a mixture of 80 mol.% of EBBA and 20 mol.% of MBBA.Ab initio calculations have been performed with 4-21G and MINI-4 basis sets to estimate molecular structures and the potential functions for internal rotation. Some structural parameters and the energy difference between rotational isomers have been refined by using observed dipolar coupling constants. The correlation between internal rotation and reorientational molecular motion has been taken into account according to the theory of Emsley, Luckhurst and Stockley. The parameters of the mean external potential are found to take similar values for methyl nicotinate and methyl isonicotinate. The energy difference of the two stable conformers of methyl nicotinate is in agreement with the analysis neglecting the correlation between the two motions.

Cationic Planar Chiral (η6-Arene)Mn(CO)3+ Complexes: Resolution, NMR Study in Chiral-Oriented Solvents, and Applications to the Enantioselective Synthesis of 4-Substituted Cyclohexenones and (η6-Phosphinoarene)Mn(CO)3+ Complexes

An easy round-trip of (D)-(+)-camphor enolate to a racemic mixture of cationic (η6-arene)Mn(CO)3+ complexes is the base of the strategy adopted for the first resolution of such complexes. X-ray structures of one of the (η5-cyclohexadienyl)Mn(CO)3 diastereoisomers obtained after addition of the chiral auxiliary as well as of the corresponding enantiopure η6 cationic complex after rearomatization have been established. Proton-decoupled deuterium 2D NMR in chiral polypeptide liquid crystals proved to be an efficient tool for the determination of the enantiomeric purity of such planar chiral cationic η6 complexes. The potential of this unprecedented resolution is exemplified by enantioselective syntheses in organic and organometallic fields. Thus, starting from the enantiopure (η6-meta-halogenoanisole)Mn(CO)3+ complexes, enantiopure 2,4-disubstituted cyclohexenones were easily generated through a Mn-assisted dearomatization process, and the first examples of enantiopure η5- and η6-phosphino-substituted Mn com...

The Impact of Hydrogen Bonding on Amide 1H Chemical Shift Anisotropy Studied by Cross-Correlated Relaxation and Liquid Crystal NMR Spectroscopy

Site-specific 1H chemical shift anisotropy (CSA) tensors have been derived for the well-ordered backbone amide moieties in the B3 domain of protein G (GB3). Experimental input data include residual chemical shift anisotropy (RCSA), measured in six mutants that align differently relative to the static magnetic field when dissolved in a liquid crystalline Pf1 suspension, and cross-correlated relaxation rates between the 1HN CSA tensor and either the 1H−15N, the 1H−13C′, or the 1H−13Cα dipolar interactions. Analyses with the assumption that the 1HN CSA tensor is symmetric with respect to the peptide plane (three-parameter fit) or without this premise (five-parameter fit) yield very similar results, confirming the robustness of the experimental input data, and that, to a good approximation, one of the principal components orients orthogonal to the peptide plane. 1HN CSA tensors are found to deviate strongly from axial symmetry, with the most shielded tensor component roughly parallel to the N−H vector, and the least shielded component orthogonal to the peptide plane. DFT calculations on pairs of N-methyl acetamide and acetamide in H-bonded geometries taken from the GB3 X-ray structure correlate with experimental data and indicate that H-bonding effects dominate variations in the 1HN CSA. Using experimentally derived 1HN CSA tensors, the optimal relaxation interference effect needed for narrowest 1HN TROSY line widths is found at ∼1200 MHz.