Cross-peaks, in COSY spectra

Since many of the signals in COSY spectra are in antiphase, they may not show up as cross-peaks due to the intrinsic nature of the polarization transfer experiment. The intensities of cross-peaks in COSY spectra may be represented by an antiphase triangle (Fig. 5.33B), in contrast to multiplet... 

How do the peaks appearing on the diagonal differ from the off-diagonal cross-peaks in COSY spectra How do they arise ... 

Fig. 8.15. Calculated shapes of cross peaks in COSY spectra characterized by Ti < /2Jjj (conditions Ju = 8 Hz Ti = 5 ms f,m = = 10 ms). (A) Phase-sensitive spectra, phased...

Y Kim, JH Prestegard. Measurement of vicinal couplings from cross peaks in COSY spectra, J Magn Reson 84 9-13, 1989. 

Figure 5-53 (A) JH NMR spectrum of a 17 base-pair DNA segment from the operator sequence OR3 from bacteriophage X in D20 at 37°C. (B) Combined COSY above the diagonal and NOESY (below the diagonal) spectra. C5H and C6H J coupling is established from cross-peaks in box d for cytosines and in box a for thymines. Two unresolved cross-peaks give rise to the more intense spots marked by arrows. Box b contains cross-peaks from scalar coupling of the two H2 protons to the HT protons of the deoxyribose rings. Most of the aromatic proton resonances could be assigned using the NOE cross-peaks in box f. For further details see Wemmer et al.676 See also Bax and Lerner.672 Courtesy of B. Reid.

Today, it is possible to make complete assignments of all proton and carbon atoms in the NMR spectra of most isolated anthocyanins. These assignments are normally based on chemical shifts (8) and coupling constants (J) observed in 1-D H and l3C NMR spectra (Fig. FI.4.2), combined with correlations observed as cross-peaks in various homo- and heteronu-clear 2-D NMR experiments (see below for details on COSY, TOCSY, HSQC, HMBC, NOESY, and ROESY). 

Figure 15.1. (A) COSY, (B) TOCSY, (C) 1H-1T HSQC or HMQC, (D) dl- Y HMBC, for 4-oxopentanal. For clarity, only key assignments have been given as an example. Note that the double-ended arrows indicate how to interpret the spectra. In the case of COSY and TOCSY the information is represented as cross-peaks that are symmetrically oriented with respect to the central diagonal. In the single-bond correlation (HSQC/HMQC) a cross-peak represents in one dimension the carbon chemical shift and in the other dimension the proton chemical shift. Note there is no diagonal in heteronuclear NMR experiments. In the HMBC, lines are drawn vertically to connect the cross-peaks. In HMBC 2-4 bonds, H-13C correlations are often observed. Note that the 4-bond correlation is less common in NMR but has been included here as an example, and 1-bond correlation is commonly filtered from the HMBC experiment to improve detection limits for the weaker 2-4 bond correlations.

We have investigated peptides whose structures were known beforehand from NMR or x-ray spectroscopy and related these structures to 2D-IR spectroscopy. Ultimately, one would like to deduce the structure of an unknown sample from a 2D-IR spectrum. In the case of 2D NMR spectroscopy, two different phenomena are actually needed to determine peptide structures. Essentially, correlation spectroscopy (COSY) is utilized in a first step to assign protons that are adjacent in the chemical structure of the peptide so that J coupling gives rise to cross peaks in these 2D spectra. However, this through-bond effect cannot be directly related to the three-dimensional structure of the sample, since that would require quantum chemistry calculations, which presently cannot be performed with sufficient accuracy. The nuclear Overhauser effect (NOE), which is an incoherent population transfer process and has a simple distance dependence, is used as an additional piece of information in order to measure the distance in... 

TOCSY and COSY spectra for T-2 toxin (7-1) are pictured in Figure 7-13. The greater number of cross peaks in the TOCSY spectrum illustrates the further relaying of coupling information. For example, H-2 (8 3.70) shows connectivity to H-3 (8 4.16) in the COSY spectrum and relayed connectivity to OH-3 (8 3.20) and to H-4 (8 5.31) in the TOCSY spectrum. 

Intensity-based methods. 1H— H RDCs have been obtained by analyzing the intensity ratios of the diagonal and cross-peaks in a series of 2D CT COSY spectra.177 This method can only be applied to resolved resonances, for example, those of anomeric protons.149 Similar limitations apply to /-modulated ID directed COSY,153,178 which uses selective 180° pulses to produce a series of ID spectra for each pair of coupled spins. This approach has recently been extended to include additional selection blocks yielding a versatile method for the measurement of coupling constants in compounds with severely overlapping proton resonances such as those found in carbohydrates.154 The problem of overlapping resonances can also be resolved by involving 13C nuclei, as demonstrated on natural abundance (13C COSMO HSQC)32 or uniformly 13C isotopically enriched carbohydrates (2D-HSQC-(sel C, sel H)-CT COSY experiment).73,158... 

Figure 27. While the information content of the two presentations is identical, the stacked plot is related more easily to a conventional spectrum, and therefore used more widely when identification of individual peaks is of primary interest, as in J-spectroscopy. On the other hand, a contour plot is more economical when one is attempting to establish connectivities between atoms by an observation of cross peaks, as in COSY and NOESY spectra. Here a contour projection of the normal spectrum is plotted on the diagonal, and the cross peaks can be identified readily on the projections of their coordinates, as shown in Figure 26. The cross peaks in the COSY spectrum are indicative of spin-spin coupling between the two groups on the diagonal. The cross peaks in the NOESY spectrum indicate the existence of crossrelaxation between the two groups.

E7-4. The smallest coupling that will gives rise to a discernible cross-peak in a COSY spectra depends on both the linewidth and the signal-to-noise ratio of the spectrum. Explain this observation. 

The complete unambiguous assignments of other carbon resonances, particularly the singlets, were made possible from HMBC and HMQC spectra. Independent assignments of proton and carbon resonances, and cross peaks in HMBC, HMQC, COSY and ROESY were also performed... 

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