Two-Dimensional 2D NMR Spectroscopy

Two-dimensional (2D) NMR spectroscopy has recently been used to make absolute tacticity assignments without any other supports.195-208 An early successful example is H COSY analysis of poly(vinyl alcohol).198,199 Figure 17 shows a broad-band decoupled H COSY spectrum of poly (vinyl alcohol)199 and illustrative assignments for the correlations between triad peaks of methine proton and tetrad peaks of methylene protons. Expected connectivities between triad and tetrad are as follows ... 

The idea of two-dimensional (2D) NMR spectroscopy was introduced in 1971 by Jeener, and several experimental demonstrations were soon reported by Ernst and co-workers. Since then many new 2D NMR experiments have been designed to give higher resolution of the resonances and information about the spectral parameters, and therefore the structural details, that would be inaccessible or at least more laborious to determine with ID NMR experiments. The improved resolution in 2D NMR experiments has paved the way for general acceptance that NMR spectroscopy is a valid analytical technique in analysis of complex samples. ... 

On the basis of this, two-dimensional (2D) NMR spectroscopy experiments can be defined as experiments that involve one time domain for signal evolution (fi) and an additional time domain for detection and signal acquisition (t2) together with appropriate radio-frequency... 

Compared with X-ray crystallography, NMR spectroscopy has only recently become established as a technique for protein stmctnre determination. The first complete three-dimensional protein stractnre solved nsing this technique was reported in 1986. The work followed the development of two-dimensional (2D) NMR spectroscopy that allowed the stndy of stractnres of mnch larger molecnles than before, snch as proteins. 

The importance of solution-state NMR today owes much to the extension of the experiment to a second (and higher) dimension [1]. Two-dimensional (2D) NMR spectroscopy is also of much significance in solid-state NMR. In attempting to classify the many important different 2D solid-state NMR experiments which have been proposed to date, we make, in this article, a distinction between homonuclear (i. e., those involving only one kind of nucleus) and heteronuclear experiments. 

Two significant developments in NMR spectroscopy are the use of Fourier transform techniques, and the development of two-dimensional (2D) NMR spectroscopy. Two-dimensional spectra are obtained using a sequence of rf pulses that includes a variable delay or delays. A set of FIDs is acquired and stored. The variable delay is incremented by a small amount of time and a new set of FIDs are obtained and stored, and so on. At the end, the resulting matrix of FID data is Fourier transformed twice once with respect to the acquisition time (as in normal FT-NMR) and second with respect to the time of the variable delay in the pulse sequence. The resulting data represent a surface and are presented as a contour plot of that surface. 

Two-dimensional (2D) NMR spectroscopy is now used routinely for many analytical problems. A convenient way to conceptualize 2D NMR spectroscopic studies is to divide the time of a 2D pulse experiment into four periods ... 

Two-Dimensional NMR. There are excellent reviews of two-dimensional (2D) NMR spectroscopy. " The basis for NMR spectroscopy in two frequency dimensions is spin memory. Once the nuclear spins in a sample are made to precess, they continue to do so for a time of the order of the spin-spin relaxation time Tj. The behavior of the nuclei during the detection period 2 can be related to the behavior of... 

Two-dimensional (2D) NMR is irrefutably the cornerstone of modem structure elucidation methods.1 Despite the inherently low sensitivity of NMR compared to other forms of analytical spectroscopy such as mass spectrometry and vibrational spectroscopy, NMR methods provide the means of establishing atom-to-atom connectivities that cannot be established by other methods. Supplemented by accurate mass measurements and fragmentation pathway information, NMR data can facilitate the elucidation of most small molecule structures. 

In the case of the 7-hydroxy-substituted compounds 44 (Scheme 2), 54 different derivatives were investigated by 13C NMR spectroscopy and, in some cases, also by 1SN NMR spectroscopy <1995JST(335)273>. With the help of proton-coupled 13C NMR spectra, semi-selective INEPT (insensitive nuclei enhanced by polarization transfer) experiments, and heteronuclear multiple bond correlation (HMBC) two-dimensional 2D-NMR spectra, all shifts could be unequivocally assigned. While the C-7 shifts did not allow the existing tautomeric situation to be determined, a clear decision could be made by H NMR spectroscopy in this respect. The 1SN NMR spectra revealed an equilibrium between the N(4)H and N(3)H tautomeric forms, which is fast on the NMR timescale. 

Laurie was one of the first to apply two-dimensional (2D) NMR to carbohydrates. With students Subramaniam Sukumar and Michael Bernstein, and visiting scientist Gareth Morris, he demonstrated and extended the application of many of the directly observed 2D NMR techniques of the time. These included the homo- and hetero-nuclear 2D /-resolved techniques, delayed proton /-resolved NMR that allowed broad resonances to be suppressed, for example, those of dextran in the presence of methyl /Lxvlopyranoside. proton-proton chemical shift correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), proton-carbon chemical shift correlation (known later as HETCOR), and spin-echo correlated spectroscopy (SECSY). Trideuteriomethyl 2,3,4,6-tetrakis-<9-trideuterioacetyl-a-D-glucopyranoside served as a commonly used model compound for these studies. 

The next milestone, in the history of NMR [Frel], was the extension of the NMR spectrum to more than one frequency coordinate. It is called multi-dimensional spectroscopy and is a form of nonlinear spectroscopy. The technique was introduced by Jean Jeener in 1971 [Jeel] with two-dimensional (2D) NMR. It was subsequently explored systematically by the research group of Richard Ernst [Em 1 ] who also introduced Fourier imaging [Kuml]. Today such techniques are valuable tools, for instance, in the structure elucidation of biological macromolecules in solution in competition with X-ray analysis of crystallized molecules as well as in solid state NMR of polymers (cf. Fig. 3.2.7) [Sch2]. 

We are presenting a series of one dimensional (ID) and two dimensional (2D) NMR spectra to exemplify the power of the technique. It is beyond the purpose of the present chapter to cover all types of NMR spectroscopy. Sometimes the same technique is known under several alternative acronyms, and in other cases different techniques provide the same information. We mention some of the most popular techniques listing alternative acronyms, but without detailing the full names or technical aspects. The interested reader should refer to already quoted books for more information [10,11,13]. A good compendium on basic ID and 2D types of NMR spectra is Nakanishi s book [26] whereas some 2D-, 3D- and 4D-spectra are well explained in Evans book [27]. However, as the techniques advance very fast, following more recent reviews and original publications is essential. 

Correlated spectroscopy (COSY) was among the first two-dimensional (2D) NMR experiment realized [447, 448] and it is still among the most useful NMR experiments. COSY generates cross peaks in the 2D spectrum at the intersection of resonances of coupled spins (Fig. 14.48). In proteins cross peaks are observed for gem-inal, i.e. over two bonds, and vicinal, i.e. over three bonds, protons and in small peptides also couplings over four bonds may be detected. Thus the COSY spectrum allows the identification of spin systems for the assignment. However, apart from peptides, the overlap and degeneracy in chemical shifts is likely to prevent one from obtaining entire spin systems exclusively from the COSY spectrum additional experiments are required. 

TROSY (transverse relaxation-optimized spectroscopy) and CRIPT (cross-correlated relaxation-induced polarization transfer) or CRINEPT (cross-correlated relaxation-enhanced polarization transfer) for the two-dimensional (2D) NMR analysis of N-. H-labeled homo-oligomeric macromolecules with masses ranging from 110-800 kDa. Practical applications of these methods are, for instance analyses of intermolecuiar interactions in supramolecular complexes or conformational changes of a single macromolecule upon interactions with other molecules. 

Solution NMR spectroscopy has played a vital role in structural studies of polysaccharides [90,126]. This is an excellent technique for detailed stmcture determination of polysaccharides as long as they are soluble in a suitable solvent. Most polysaccharides have charged residues that increase their interactions with water and other polar molecules and hence they are soluble in water and dimethylsulfoxide (DMSO). It is well known that deuterated solvents (e.g., D2O, CDCI3, and DMSO-Je) are used to run the one-dimensional (ID)- and two-dimensional (2D)-NMR spectra in liquid state [90]. 

Two-dimensional covariance NMR spectroscopy, which was originally established to extract homonuclear correlations (HOMCOR), has been extended to include heteronuclear correlations (HETCOR) by Takeda et al. In a 2D chemical shift correlation experiment, and N... 

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