In COSY

The sine-bell functions are attractive because, having only one adjustable parameter, they are simple to use. Moreover, they go to zero at the end of the time domain, which is important when zero-filling to avoid artifacts. Generally, the sine-bell squared and the pseudoecho window functions are the most suitable for eliminating dispersive tails in COSY spectra. 

The matrix obtained after the F Fourier transformation and rearrangement of the data set contains a number of spectra. If we look down the columns of these spectra parallel to h, we can see the variation of signal intensities with different evolution periods. Subdivision of the data matrix parallel to gives columns of data containing both the real and the imaginary parts of each spectrum. An equal number of zeros is now added and the data sets subjected to Fourier transformation along I,. This Fourier transformation may be either a Redfield transform, if the h data are acquired alternately (as on the Bruker instruments), or a complex Fourier transform, if the <2 data are collected as simultaneous A and B quadrature pairs (as on the Varian instruments). Window multiplication for may be with the same function as that employed for (e.g., in COSY), or it may be with a different function (e.g., in 2D /-resolved or heteronuclear-shift-correlation experiments). 

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 ... 

The HOHAHA spectrum (100 ms) of podophyllotoxin is presented. The HOHAHA, or TOCSY (total correlation spectroscopy), spectrum (100 ms) shows coupling interactions of all protons within a spin network, irrespective of whether they are directly coupled to one another or not. As in COSY spectra, peaks on the diagonal are ignored as they arise due to magnetization that is not modulated by coupling interactions. Podophyllotoxin has only one large spin system, extending from the C-1 proton to the C4 and 015 protons. Identify all homonuclear correlations of protons within this spin system based on the crosspeaks in the spectrum. 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

Modulation The variation in amplitude and/or phase of an oscillatory signal by another function, e.g., modulation of the nuclear precession frequency of one nucleus by the nuclear precession frequency of a correlated nucleus in COSY spectra. 

Now let us look at the NOESY spectrum (b) just as in COSY, we can identify a diagonal and a series of associated off-diagonal cross peaks. Thus the interpretation of the results is analogous to the method we have already learned for COSY. However, the cross peaks are not due to spin-spin coupling but to NOE effects between the protons concerned. However, if we look more closely we can see one big difference between the diagonal peaks, which look like irregular circles, and the cross peaks, which look just like all the peaks in the COSY spectrum. 

The reason for this is that our experiments are phase-sensitive. What do we mean by this You will remember that in the DEPT and APT spectra the CH/CHj and CH2 peaks are in one case positive (up) and in the other negative (down), which we also refer to as in opposite phase. Here in COSY and NOESY our experiments include such phase information, which is read off from the way the signals look in the plot. 

We can see three sets of cross peaks methyl/methylene and aromatic CH as in COSY, and in addition a clear interaction between the methine proton and the aromatic protons closest to it. This interaction is naturally not visible in the COSY spectrum, as the protons are separated by five bonds. A look back to Section 1.1.6 shows that this NOE was (as must be the case) also visible in the ID experiment. 

Bodenhausen ° developed a pattern-recognition programme (MARCO POLO) in order to extract coupling pathways in COSY spectra. He subsequently described a recursive deconvolution technique for the measurement of couplings, but this seems to offer more benefits to the measurement of scalar couplings in two-dimensional spectra than in one-dimensional spectra. 

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...

We end our discussion of VGSE by comparing the ROESY correlations of the amide protons with the corresponding interactions in COSY and TOCSY. Figure 5.35 shows comparable sections of each spectrum. We have the seen the COSY and TOCSY portions earlier and showed how these spectra, along with the HMQC, can be used for intra-residue assignments. The ROESY correlations, on the other hand,... 

There are typically two kinds of artifacts in COSY spectra t noise (vertical streaks) and DQ artifacts. The t streaks extend up and down from the most intense and sharpest peaks along the diagonal. These result from any instability that can cause random variations... 

As we see in Section 10.3, 2D NMR lines, including those in COSY, have a complex shape. A number of variants of COSY have been developed to improve the spectral presentation and to avoid unwanted signals. We shall return to COSY in Chapter 12. 

As in COSY, modifications in delay times can make HETCOR sensitive to smaller two-bond and three-bond 13C-H couplings, but indirect detection methods described later are usually more effective. 

Note that the information provided here is similar to that found in the INADEQUATE spectrum of sucrose (Fig. 12.4ft), but the sensitivity ofTOCSY is much greater. The upper 2D spectrum shows a TOCSY spectrum with a short contact time, 10 ms, where the cross peaks are restricted largely to protons that are directly coupled (as in COSY). Courtesy of Daron Freedberg (Food and Drug Administration). 

Multidimensional NMR spectra are not restricted to cases where the separate frequency axes encode signals from different nuclear types. Indeed, much of the early work on the development of 2D NMR was performed on cases where both axes involved chemipal shifts. The main value in such spectra comes from the information content in cross peaks between pairs of protons. In COSY-type spectra (COSY = Correlation SpectroscopY) cross peaks occur only between protons that are scalar coupled (i.e., within 2 or 3 bonds) to each other, whereas in NOESY (NOE Spectroscopy) spectra cross peaks occur for protons that are physically close in space (<5 A apart). A combination of these two types of 2D spectra may be used to assign the NMR signals of small proteins and provides sufficient information on internuclear distances to calculate three-dimensional structures. Figure 12.3 includes a panel showing the COSY spectrum of cyclosporin and highlights the relationships between ID H-NMR spectra and corresponding 2D homonuclear (COSY) and heteronuclear (HSQC) spectra. 

EXPLOSIVES Vol 1, pp B202ff of this Ericycl, c n lbs us d in cosi Diinin ... 

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

Ihe number of multidimensional sequences using some form of solvent suppression is almost without bound. Consequently, we have limited the examples drawn from multidimensional sequences to some (randomly) chosen examples of the flip-back method and suppression in COSY, TOCSY, NOESY and ROESY sequences. A final subsection gives two examples of suppression in multidimensional sequences using Bi gradients. 

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