Homonuclear 2D correlation

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

TOCSY (Total Correlation Spectroscopy) is another important homonuclear 2D correlation experiment where correlations arise due to the presence of homonuclear scalar coupling.In the standard COSY experiment, crosspeaks appear for spins in which the scalar coupling occurs over typically two to four bonds. In the TOCSY experiment crosspeaks can appear for spins separated by many more bonds as long as they are part of a contiguous network of coupled spins. The correlations are effected by the application of a series of low-power rf pulses termed the spin-lock. The duration of the spin-lock period determines the extent to which the correlations are propagated through the spin system. The TOCSY experiment is a useful complement to the COSY methods for the elucidation of complex structures. 

In Section 9.3.1, the problem of assigning the many resolved resonances in a ID MAS spectrum was mentioned, and ID spectral editing methods were introduced. In this section, we describe homonuclear 2D correlation experi-... 

Another excellent example of the application of NMR techniques in the structure determination of natural products is the structural resolution of rhizobactin, a siderophore isolated from Rhizobium meliloti (354). The proton homonuclear 2D-correlated spectrum revealed the four separate coupled units, whereas the heteronuclear 2D-correlated spectrum established the assignments (Fig. 2.43). Together, these spectra revealed that rhizobactin is composed of one unit each of ethylene diamine, alanine, lysine, and L-malic acid. The sequencing of... 

Homonuclear 2D correlation (COSY) spectra were acquired according to the method of Nagayama et al, and Homonuclear Hartmann-Hahn experiment (HOHAHA) according to Bax. The 2D data were zero filled and weighted prior to Fourier transformation as appropriate. A sine-bell filtering function was used in both dimensions. 

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

In homonuclear-shift-correlated experiments, the Ft domain corresponds to the nucleus under observation in heteronuclear-shift-correlated experiments. Ft relates to the unobserved or decoupled nucleus. It is therefore necessary to set the spectral width SW, after considering the ID spectrum of the nucleus corresponding to the Ft domain. In 2D /-resolved spectra, the value of SW depends on the magnitude of the coupling constants and the type of experiment. In both homonuclear and heteronuclear experiments, the size of the largest multiplet structure, in hertz, determines... 

SWi, which in turn is related to the homonuclear or heteronuclear coupling constants. In homonuclear 2D spectra, the transmitter offset frequency is kept at the center of (i.e., at = 0) and F domains. In heteronuclear-shift-correlated spectra, the decoupler offset frequency is kept at the center (Fi = 0) of thei i domain, with the domain corresponding to the invisible or decoupled nucleus. 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

Another 2D homonuclear shift-correlation experiment that provides the coupling information in a different format is known as SECSY (spin-echo correlation spectroscopy). It is of particular use when the coupled nuclei lie in a narrow chemical shift range and nuclei with large chemical shift differences are not coupled to one another. The experiment differs... 

Applications Useful 2D NMR experiments for identification of surfactants are homonuclear proton correlation (COSY, TOCSY) and heteronuclear proton-carbon correlation (HETCOR, HMQC) spectroscopy [200,201]. 2D NMR experiments employing proton detection can be performed in 5 to 20 min for surfactant solutions of more than 50 mM. Van Gorkum and Jensen [238] have described several 2D NMR techniques that are often used for identification and quantification of anionic surfactants. The resonance frequencies of spin-coupled nuclei are correlated and hence give detailed information on the structure of organic molecules. 

We have characterized a resin-bound pentasaccharide by HR-MAS techniques. A comparison of the solution spectrum of the resin-cleaved pentasaccharide with the HR-MAS spectrum of the resin-bound pentasaccharide is shown in Figure 8.5. It is immediately obvious that the HR-MAS technique provides data of a quality similar to that of the solution technique, but in both cases, only four of the five anomeric protons are visible. However, a 2D homonuclear total correlation spectroscopy (TOCSY) spectrum (Fig. 8.6) of the resin-bound pentasaccharide allowed us to clearly observe the overlapped anomeric protons (demonstrating a resolution of 4.4 Hz). 

The appearance of a homonuclear 2D spectrum is different from what you have seen for HETCOR, the H-13C correlation. Because both frequency scales, F2 and F, are lH chemical-shift scales, we can observe the transfer of coherence from Ha to Hb as a crosspeak at F = 2a, F2 = b (Fig. 9.18, lower right), as well as the opposite sense of transfer from Hb to Ha, a symmetrically disposed crosspeak at F = 2b, F2 = 2a (upper left crosspeak). In HETCOR we observe 13C coherence (in F2) that was transferred from the attached proton, whose chemical shift appears in F. Transfer in the opposite sense (13C to H) is not possible... 

We can resolve this ambiguity with a TOCS Y spectrum. This is just a homonuclear 2D ( H H) experiment with the TOCSY spin lock as the mixing portion of the pulse sequence. With the fragment CH1-CH2-CH3-Cq4-CH5-CH6-CH7 we expect to see HI correlated... 

Although we might be able to get all the H-H coupling information through a long series of homonuclear decoupling experiments, there is a much simpler way the 2D NMR technique known as homonuclear shift correlation spectroscopy, or COSY. 

So far we have discussed homonuclear 2D H,H-shift correlation spectroscopy (H,H-COSY) as well as heteronuclear 2D C,H-shift correlation spectroscopy (C,H-COSY, or C,H-HSC). Let us now consider homonuclear 2D C,C-shift correlation spectroscopy. 

Figure 4 Homonuclear 2D spectra of the 8mer peptide EWTLYWR in 90 % H2O, 10 % D2O. (a) Representation of coherence transfer pathways for COSY (soiid arrows), TOCSY (dotted arrows) and NOESY (dashed arrows) experiments, (b) Section of COSY spectrum displaying the backbone H -H -correlations, additionaiiy the side-chain H -H -correlations of R8 is visible in the upper right-hand corner. The H -H -correlation of El cannot be detected because of soivent-exchange broadening of the N-terminal amino group, (c) Section of TOCSY spectrum that displays the correlations of the backbone with all protons within the amino acid side chain, (d) Section of ROESY spectrum that displays correlations between backbone H -H intraresidual as well as to the neighboring (i-1) amino acid. The cross peaks to the (i-1) amino acid have higher intensities. Thus, a "sequential walk" is possible (arrows) that allows identification of the position of amino acids within the peptide chain. Additionally the H of El can be assigned (first arrow on the left).

Traditionally, homonuclear 2D double quantum filtered correlation spectroscopy (DQF-COSY) and total correlated spectroscopy (TOCSY) spectra are valuable in the identification of resonances of individual monosaccharide units. In the presence of small couplings, through space connectivities detected by NOESY/ROESY (nuclear Overhauser effect spectroscopy/ rotational nuclear Overhauser effect spectroscopy) experiments are also useful in completing the resonance assignment. When the H NMR spectra of complex oligosaccharides are too crowded to fully elucidate the structure by homonuclear correlation methods, it is efficient to use 2D heteronuclear correlation methods, such as heteronuclear single quantum correlation... 

Figure 5 Cross-peaks in homonuclear 2D TOCSY spectra arising due to ROESY effects. Clean TOCSY spectra were acquired with the MLEV-17 spin-lock sequence, (a) Base proton H6-to-methyl correlations in a 27-nt AT-rich DNA stem-loop structure 93 the spectrum was recorded with the 50-ms mixing sequence, (b) and (c) TOCSY spectra acquired for a 31 -nt stem-loop RNA (unpublished data), (b) H5-H6 cross-peaks in pyrimidines and a H1 -H8 cross-peak (boxed) in the syn guanine from the tetraloop UACG the spectrum was recorded with the 30-ms mixing sequence, (c) Sequential H2 -H6/H8 cross-peaks the spectrum was recorded with the 90-ms mixing sequence.

The methodologies based on H- H COSY and NOESY experiments are the most sensitive, as they rely on detection of high-sensitivity, abundant nuclei. COSY provides correlations among atoms that are /-coupled, while NOESY provides those among protons based on their separation in space. The drawback of using these homonuclear 2D methods is their complexity, as two coupled protons can produce many peaks in the 2D spectrum. 

A theoretical treatment of the DREAM adiabatic homonuclear recoupling experiment has been given using Floquet theory. An effective Hamiltonian has been derived analytically and the time evolution of the density operator in the adiabatic limit has been described. Shape cycles have been proposed and characterized experimentally. Application to spin-pair filtering as a mixing period in a 2D correlation experiment has been explored and the experimental results have been compared to theoretical predictions and exact numerical simulations. 

General symmetry principles for rotor-synchronized pulse sequences in MAS solid-state NMR have been presented. The synunetry theory has been extended to the case of generalized Hartmann-Hahn sequences, in which rotor-synchronized r.f. irradiation is applied simultaneously to two isotopic spin species. The symmetry theory has been used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin-spin interactions, and which also suppress CSAs. Experimental demonstrations of heteronuclear 2D correlation spectroscopy, heteronuclear MQ spectroscopy, and the estimation of intemuclear dipolar couplings have been given. 

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