ROESY spectrum

Detailed NMR conformational analysis of y -peptides 139-141 (Fig. 2.35) in pyri-dine-d5 revealed that y-peptides as short as four residues adopt a 2.6-hehcal fold stabilized by H-bonds between C=0 and NH +3 which close 14-membered pseudocycles [200, 201]. The 2.614-helical structure of a low energy conformer of y-hex-apeptide 141 as determined from NMR measurements in pyridine-d5 [200], is shown in Fig. 2.36A and B). Determination of the structure of y" -peptides in CD3OH was hampered by the much lower dispersion of the diasterotopic H-C(a) protons compared to their dispersion in pyridine-d5. However, the characteristic and properly resolved i/ir-2 NOE crosspeacks between H-C(y) and NH +2 in the NH/H-C(y) region of the ROESY spectrum were an indication that the 2.6-helical structure is at least partially populated in CD3OH. 

The ROESY spectrum affords homonuclear transverse nOe interactions as cross-peaks between the various dipolarly coupled hydrogens. This... 

The ROESY spectrum of podophyllotoxin exhibits a number of crosspeaks (A-D) representing interactions between dipolarly coupled (space coupling) hydrogens, which can be helpful to determine the stereochemistry at different asymmetric centers. For example, based on the assumption that the C-1 proton (8 4.53) is /3-oriented, we can trace out the stereochemistry of other asymmetric centers. Cross-peak B represents dipolar coupling between the C-1 proton (8 4.53) and the C-2 proton (8 2.8), thereby confirming that the C-2 proton is also... 

Fig. 9.4 The fixation of the Ala proton in cyclosporin A via ROE-derived distance restraints as an illustrative example for NMR structure determination. (A) The amide region of the 2D ROESY spectrum of cyclosporin A at room temperature with the cross-peaks to...

In order to combat this, the rotating frame Overhauser effect spectroscopy (ROESY) techniques can be employed. An in-depth discussion of how this technique works is outside the remit of this book but suffice to say, in the ROESY methods (1- and 2-D), NOE data is acquired as if in a weak r.f. field rather than in a large, static magnetic field and this assures that all NOEs are present and positive, irrespective of tumbling rate and magnet size. It is possible that some TOCSY correlations can break through in ROESY spectra but these will have opposite phase to the genuine ROESY correlations and so should therefore not be a problem - unless they should overlap accidentally with them. A 2-D ROESY spectrum of the naphthalene compound is shown below (Spectrum 8.6). 

At 100 °C, the ROESY spectrum has shown chemical exchange of NH-protons and also pairwise chemical exchange of the methyl groups on the dialdehyde units, methine protons as well as the geminal CH2 protons. This behaviour was explained as inversion of the whole molecule in an... 

Fig. 10.19. IDR-HSQC-TOCSY spectrum of the complex marine polyether toxin brevetoxin-2 (7). The data were recorded overnight using a 500 pg sample of the toxin (MW = 895) dissolved in 30 pi of d6-benzene. The data were recorded at 600 MHz using an instrument equipped with a Nalorac 1.7 mm SMIDG probe. Direct responses are inverted and identified by red contours relayed responses are plotted in black. The IDR-HSQC-TOCSY data shown allows large contiguous protonated segments of the brevetoxin-2 structure to be assembled, with ether linkages established from either long-range connectivities in the HMBC spectrum and/or a homonuclear ROESY spectrum.

Fig. 8. ID ROESY-TOCSY. (a) H spectrum of the oligosaccharide 3 (5 mg/0.5 ml D2O). (b) ID ROESY spectrum of 3 acquired using the pulse sequence of fig. 7(a) with selective excitation of the H-lb proton. Duration of the 270° Gaussian pulse and the spin-lock pulse ( yBi/ K = 2.8 kHz) was 49.2 ms and 0.5 s, respectively. The spin-lock pulse was applied 333.3 Hz downfield from the H-lb resonance. The time used for the frequency change was 3 ms. (c) ID ROESY-TOCSY spectrum acquired using the pulse sequence of fig. 7(c) and the selective ROESY transfer from H-lb followed by a selective TOCSY transfer from H-4c. Parameters for the ROESY part were the same as in (b). A 49.2 ms Gaussian pulse was used at the beginning of the 29.07 ms TOCSY spin lock. 256 scans were accumulated. A partial structure of 3 is given in the inset. Solid and dotted lines represent TOCSY and ROESY...

The easiest way to reduce the amplitude of TOCSY cross peaks in the ROESY spectra is to record a spectrum with minimal spin-lock power [23]. The other possibility is to modulate the frequency of the spin-lock field [25]. However, the most convenient way is to apply a series of 180° pulses instead of a single continuous-wave pulse during the mixing time, as is done in the T-ROESY experiment. Figure 4(D) shows the T-ROESY spectrum of cyclo(Pro-Gly) recorded with = 300 ms. Although the... 

Load the ROESY spectrum of peracetylated glucose D NMRDATA ... 

Load the 2D NOESY spectrum of peracetylated glucose D NMRDATA GLUCOSE 2D HH GHHNO 001999.RR. Process it and set up a layout according to the procedure outlined above for the 2D ROESY spectrum. Plot the spectrum according to your preferred layout ideas. Compare the results with the results obtained with the 2D ROESY and the 1D NOE experiments. 

The structures of these compounds were assigned primarily by spectroscopic methods. The recrystalization of the natural Et A12-oxide (67) and the 21 -G-methyl-A12-formyl derivative of compound 63 gave single crystals and allowed X-ray analysis of these systems [74]. The absolute stereochemistry of 70 was determined by chiral GC of the L-Cys unit and by ROESY spectrum of its acetyl derivative [75]. The structures of Et s are related to the microbially derived safracins and saframycins -antitumor agents first isolated from cultured Streptomyces species [76] -as well as to the sponge metabolites renieramycin and xestomycin [77]. 

The ROESY spectrum for lactose is given in Figure 5.29. Note the overall appearance in the upper part of the figure. In the lower part, the anomeric region is shown as expanded insets. The two glucose... 

FIGURE 5.29 Top the 2-D ROESY spectrum for lactose. Bottom expansions of the three anomeric correlations. 

The other anomer of glucose, in contrast, has its anomeric proton (/31) in the axial position. In the preferred chair conformation of glucose, protons occupy all of the other axial positions leading to presumed diaxial interactions between H-l (/31) and H-3 (/33) and between H-l (/31) and H-5 (/35).The ROESY spectrum reveals three interactions with the anomeric proton, /SI, at 4.67 ppm. The H-2 COSY interaction is, of course, present, and the two-diaxial NOE interactions to H-3 (/33) and H-5 (/S5) are quite evident. 

Fig. 64. 2D-ROESY spectrum of 103a in CDCI3 solution at 315 K, C-Me region. Positive cross signals represent chemical exchange between corresponding methyl groups.88 Reproduced with permission from the American Chemical Society.

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