The two-dimensional NMR spectroscopy of macromolecules

Two-dimensional (2D) NMR is a versatile technique which exists in many versions. Two broad classes of 2D techniques are (1) correlated spectroscopy and (2) J-resolved spectroscopy. The first of these may be divided into two further subdivisions: COSY, which permits correlations of resonances via J-coupling, and NOESY, which allows direct measurement of intenuclear (usually interproton) distances by the nuclear Overhauser effect. COSY greatly facilitates the interpretation of complex spectra and spectral interpretation in terms of stereochemical sequences is placed on a firmer foundation. NOESY provides direct information concerning the local conformations of polymers in solution. By use of J-resolved 2D NMR, we can separate J-couplings and chemical shifts on different axes and thus achieve a degree of resolution of both these parameters far beyond what is attainable in the 1D spectrum. Finally, we may combine correlated and J-resolved spectroscopy and achieve the benefits of both in 3D NMR, in which the 2D cross peaks exhibit J-coupling fine structure. These techniques are illustrated for a variety of polymers including poly(methyl methacrylate), poly(vinyl fluoride), poly-y-benzyl L-glutamate, and poly(propylene oxide).     

Study of conformation of cryptahemispherands by two-dimensional NMR spectroscopy

Two-dimensional NMR spectroscopy (NOESY, COSY-DQF) has been used in a study of nitrogen-containing cryptahemispherands and their complexes with alkali metals. On the basis of experimentally established interproton distances, the preferred conformations of these molecules have been established. It has been shown that the important features of structure determining the molecular conformation are (a) the orientation of the N-CH₂(Ar) bond relative to the plane of the crown ether and (b) the length of the methylene bridge (spacer). The spatial and torsional angles have been determined. It has been established that as the length of the bridge is increased, the structure of these molecules approaches a betaine structure, with mutually parallel positions of the phenyl rings. Upon complexation, their orientation changes to mutually perpendicular, and this leads to enlargement of the inner cavity of the molecule and a corresponding rearrangement of its dimensions toward the ionic radius of the alkali metal.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 406–411, March, 1993.     

二维NMR测定1,3-双烯膦酸酯的构型

本文报道用1H NMR二维J谱、二维同核相关谱及NOE技术测定了三个共轭双烯膦酸酯的双键构型,并求得各δ和J值。     

Solution conformation of longifolene and its precursor by NMR and ab initio calculations

We describe the conformation and stereospecific 1H and 13C chemical shift assignments of longifolene 1 and its penultimate precursor 2 through the combined use of ab initio calculations and experimental NMR techniques. The predicted stable conformation for both compounds was similar and adopts a twisted chair conformation at the seven-membered ring where C4 lies on top of the exocyclic double bond. The calculated chemical shifts for the stable conformation agree well with the experimental values.     

Principles and features of single-scan two-dimensional NMR spectroscopy

Two-dimensional nuclear magnetic resonance (2D NMR) provides one of the foremost contemporary tools available for the elucidation of molecular structure, function, and dynamics. Execution of a 2D NMR experiment generally involves scanning a series of time-domain signals S(t(2)), as a function of a t(1) time variable which undergoes parametric incrementation throughout independent experiments. Very recently, we proposed and demonstrated a general approach whereby this serial mode of data acquisition is parallelized, enabling the acquisition of complete bidimensional NMR data sets via the recording of a single transient. The present paper discusses in more detail various conceptual and experimental aspects of this novel 2D NMR methodology. The basic principles of the approach are reviewed, various homo- and heteronuclear NMR applications are illustrated, and the main features and artifacts affecting the method are derived. Extensions to higher-dimensional experiments are also briefly noted.     

Sensitivity enhancement in two-dimensional solid-state NMR spectroscopy by transverse mixing.

The sensitivity of two-dimensional (2D) 13C-13C solid-state NMR spectroscopy under magic-angle spinning (MAS) is shown to be enhanced by the use of transverse polarization transfer in place of the conventional longitudinal polarization transfer. Experimental results are reported for 2D spectroscopy of a 20-residue, filament-forming peptide derived from the E. Coli RecA protein, containing five uniformly 13C-labeled residues, performed at 14.1 T with high-speed MAS and with finite-pulse radio-frequency-driven recoupling of dipolar interactions in the mixing period. Significant sensitivity enhancements observed at short mixing periods results from a more rapid build-up of cross-peaks under transverse mixing than under longitudinal mixing and from the 2 gain inherent in 2D measurements in which both orthogonal transverse polarization components in the t1 period contribute to each free-induction decay signal detected in the t2 period.     

Assignment of proton NMR spectra and conformational analysis by two-dimensional heteronuclear relayed correlation NMR spectroscopy

The method of two-dimensional heteronuclear relayed correlation spectroscopy was used to establish the assignment of the severely overcrowded part of the proton spectrum of menthol by relating it to the previously assigned carbon spectrum. Extrapolation of the signal-to-noise ratio obtained with overnight data accumulation on a 10 mM solution suggests that this experiment should be feasible on as little as 10 mg of a moderate-sized organic compound.     

Ultrafast two-dimensional NMR spectroscopy using constant acquisition gradients

Multidimensional NMR spectroscopy plays an important role in the characterization of molecular structure and dynamics. A new methodology for acquiring this kind of spectra has been recently demonstrated, endowed with the potential to compress arbitrary multidimensional NMR acquisitions into a single scan. This "ultrafast" nD acquisition protocol is based on a spatiotemporal encoding of the indirect-domain spin evolution, followed by a repetitive decoding and re-encoding of the information thus stored employing a train of alternating-sign gradients. Such train of switching gradients extending throughout the course of the data acquisition may pose extreme demands on a magnetic resonance system, particularly when dealing with nonshielded gradients, strong eddy currents, or rapidly relaxing spin systems. Limits to the in vivo applicability of such fast-switching scheme may also arise due to gradient-induced perineural stimulation. The present study describes a new approach to ultrafast nD NMR that reduces the number of gradient switchings during the acquisition period to zero, leading in essence to a constant-gradient acquisition scheme. This approach operates on the basis of a novel spatiotemporal encoding including discrete, temporally overlapping, frequency-shifted pulses. Principles and examples of this new approach are given; sensitivity limitations and signal-enhancing prospects of such constant-gradient acquisitions are also discussed and exemplified.     

Orientational ordering of a bent-core mesogen by two-dimensional 13C NMR spectroscopy

Various two-dimensional (2D) NMR techniques are reported on a bent-core mesogen 4,6-dichloro-1,3-phenylenebis[4'-(9-decenyloxy)-1,1'-biphenyl] carboxylate in its nematic and solid phases in order to unambiguously assign its carbon-13 NMR spectrum. The (13)C chemical shifts from the molecular core were studied as a function of temperature to extract its molecular geometry and orientational order tensor. To this end, the chemical shift anisotropy tensors of some carbon sites were measured in the solid state of this mesogen using a recent method called the separation of undistorted powder patterns by effortless recoupling (SUPER). The average bending angle subtended by the two arms of the bent-core structure is determined to be 148.7 degrees. The C-H dipolar couplings obtained from the separated local field (SLF) experiment for the aromatic rings are used to find the local order parameter tensors.     

Poly(vinylpyrrolidone): Configurational assignments by one- and two-dimensional NMR spectroscopy

The configurational assignment of poly(vinylpyrrolidone) (PVP) prepared by peroxide-initiated solution polymerization was studied by the combination of one- and two-dimensional NMR spectroscopy. The broad and overlapping ¹H-NMR and ¹³C{¹H}-NMR spectra of PVP were assigned to the configurational triad, pentad (CH, ²CH₂, ³CH₂, and ⁴CH₂ regions), and tetrad (β-CH₂ region) sequences. The configurational assignments of the various carbon resonances were confirmed with the help of two-dimensional experiments such as heteronuclear single quantum correlation (HSQC), heteronuclear single quantum correlation–total correlation spectroscopy (2-D HSQC–TOCSY). The various geminal and vicinal couplings within the configurational sequences were assigned with the help of total correlation spectroscopy (TOCSY low mixing time). The propagation pathway was studied using the ¹³C{¹H}-NMR (carbonyl carbon) and ¹⁵N{¹H}-NMR spectra. The ¹⁵N{¹H} resonance signals were assigned to pentad-level configurational sequences. The results obtained by the analysis of the area under the resonance signals confirmed that poly(vinylpyrrolidone) obeys Bernoullian statistics. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3922–3928, 1999     

Backbone dynamics of proteins studied by two-dimensional heteronuclear NMR spectroscopy and molecular dynamics simulations

To investigate the backbone dynamics of proteins ¹⁵N longitudinal and transverse relaxation experiments combined with {¹H, ¹⁵N{ NOE measurements together with molecular dynamics simulations were carried out using ribonuclease T₁ and the complex of ribonuclease T₁ with 2′GMP as a model protein. The intensity decay of individual amide cross peaks in a series of (¹H, ¹⁵N)HSQC spectra with appropriate relaxation periods was fitted to a single exponential by using a simplex algorithm in order to obtain ¹⁵N T₁ and T₂ relaxation times. The relaxation times were analyzed in terms of the “model-free” approach introduced by Lipari and Szabo. In addition, a nanosecond molecular dynamics (MD ) simulation of ribonuclease T₁ and its 2′GMP complex in water was carried out. The angular reorientations of the backbone amide groups were classified with several coordinate frames following a transformation of NH vector trajectories. In this study, NH librations and backbone dihedral angle fluctuations were distinguished. The NH bond librations were found to be similar for all amides as characterized by correlation times of librational motions in a subpicosecond scale. The angular amplitudes of these motions were found to be about 10°–12° for out-of-plane displacements and 3°–5° for the in-plane displacement. The contributions from the much slower backbone dihedral angle fluctuations strongly depend on the secondary structure. The dependence of the amplitude of local motion on the residue location in the backbone is in good agreement with the results of NMR relaxation measurements and the X-ray data. The protein dynamics is characterized by a highly restricted local motion of those parts of the backbone with defined secondary structure as well as by a high flexibility in loop regions. Comparison of the MD and NMR data of the free liganded enzyme ribonuclease T₁ clearly indicates a restriction of the mobility within certain regions of the backbone upon inhibitor binding. © 1996 John Wiley & Sons, Inc.     

Multinuclear and two-dimensional NMR spectroscopy of monoazo derivatives of chromotropic acid

By the use of one-dimensional (¹H,¹³C, and¹⁵N) and two-dimensional NMR spectra (H,H-COSY, H,C-COSY, and NOESY), it has been established that the phenylazo derivative of chromotropic acid in DMSO has a quinonehydrazone structure; under normal conditions, there is no azo-quinonehydrazone tautomerism. Spectral evidence has been found for the distribution of intramolecular hydrogen bonds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2530–2535, November, 1989.     

The study of molecular motions in substituted ferrocenes by means of NMR spectroscopy

The proton magnetic resonance spectra for ferrocene derivatives have been investigated in the solid state at a frequency of 27.5 MHz and at temperatures within the range 4.2–300 K. Evidence for re-orientation in various substituent groups and cyclopentadienyl rings has been deduced on the basis of the temperature dependence of the second moment of the PMR lines. The experimentally observed decrease in the second moment has been compared with the calculated contributions of different molecular groups to the second moment thus enabling an identification of these groups. In computing the second moment, account has been taken of both intra- and intermolecular dipole-dipole interactions. It is shown that the introduction of substituents into some ferrocene rings affects the magnitude of the re-orientation energy for both the substituted and unsubstituted rings. The change of the re-orientation energy of the substituted and unsubstituted rings in substituted ferrocenes with the structure and electronic properties of the substituents is discussed.     

The study of photoisomerisation of 1-aryloxyanthraquinones by NMR spectroscopy and UV spectrophotometry

It has been stated by ¹H NMR and UV spectrophotometry that 1-(4-tert-butil-phenoxy)-2-cetylamino-9,10-anthraquinone photoisomerizes to 9-(4-tert-butylphenoxy)-2-cetylamino-1,10-anthraquinon with quantum yield ? = 0.4 ±0.1 under light excitation in the absorption band region in the UV/visible spectrum range with the maximum λ = 441 nm.     

Structure of britannin. Use of two-dimensional1H NMR spectroscopy

Two-dimensional¹H NMR spectroscopy (COSY, NOESY) in combination with the one-dimensional variant has been used to establish the conformation of the molecule of britannin — a sesquiterpene lactone of the ambrosane type. The comparative possibilities of one- and two-dimensional NMR spectroscopies in stereochemical investigations of sesquiterpene lactone with ambrosane skeletons are considered.     

The assignment of diastereotopic menthyl groups by two-dimensional NMR

The ¹³C and ¹H NMR spectra of (–)-bis[1R, 3 R, 4S]menthylphosphine (1) are assigned by two-dimensional double quantum NMR and ¹³C? ¹H shift correlation diagrams. The variable temperature spectra of 1 indicate hindered rotation about the P? C bonds.     

Erratum: Solution conformation of longifolene and its precursor by NMR and ab initio calculations

The original article to which this Erratum refers was published in Magnetic Resonance in Chemistry, in Volume 44 issue 12.     

Sequencing of trans-4-methacryloyloxyazobenzene/vinylidene chloride copolymers by one- and two-dimensional NMR spectroscopy

Trans-4-methacryloyloxyazobenzene/Vinylidene Chloride (M/V) copolymers of different monomer concentrations were prepared by solution polymerization using benzoyl peroxide as an initiator. The copolymer composition was determined from the ¹³C{¹H}-NMR spectrum. The quaternary carbon of M- and V-centered resonances were used for determining the sequences in terms of the distribution of M- and V-centered triads. The sequence distribution of M- and V-centered triads determined from ¹³C{¹H}-NMR spectra of the copolymer is in good agreement with the triad concentration calculated from the statistical model. The comonomer reactivity ratios, determined by both the Kelen Tudos (KT) and the nonlinear error in variables (EVM) methods are r_M = 3.59 ± 0.19, r_V = 0.89 ± 0.07; r_M = 3.76, and r_V = 0.93, respectively. ¹³C Distortionless Enhancement by Polarization Transfer (DEPT) spectrum was used to differentiate between the resonance signals of M- and V-methylene and methyl carbon units. Assignments to the methylene resonance signals have been assigned up to the tetrad levels using 2D HSQC experiments. The geminal couplings in the methylene proton region is shown in the 2D DQF-COSY spectrum. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3179–3185, 1999