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Hello All I have two examples of what appears to be 6J coupling between 1H and 15N using gradient HMBC experiments with 15N at natural abundance. For such an experiment, 2J and 3J correlations are typical, sometimes 4J (W-type) but I've never seen 6J before. Has anyone else ever seen correlations over this distance? Any other comments? Craig. |
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Gary Martin and I published a paper using gHMBC with H-P in polysaccharides (Zartler, E.R. and G.E Martin. The use of 1H-31P GHMBC and Covariance NMR to unambiguously determine phosphate ester Linkages in Complex Polysaccharide Mixtures. (2011) J. Biomol. NMR, 51:357.. We were able to detect up to 5J couplings. |
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I have measured numerous 1H-15N HMBC spectra for heterocyclic compounds and alkaloids, but observed just 2J and 3J (delay in HMBC sequence set to 5 Hz coupling constant). Were these cross peaks of comparable intensity with others? Were HMBC cross peaks in consistence with other spectral data (1H-13C HMBC, NOESY)? Was the compound's molecule structure known? Unfortunately, t1-artefacts are usual for this experiment, especially when linear prediction was apllied on F1, they can lead to some contradictive assignments (in my experience). |
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Thank-you both for your responses; and TZ for the reference. To answer VVK's questions: The cross-peaks are of comparable intensity (with JXH at 8 Hz); the cross-peaks are consistent with both 1H-13C HMBC and NOESY data (apart from the peculiar distance); the structure is known (NMR & XRD); the other cross-peaks for the 15N of interest are consistent with the structure at 3J, two x 4J (there are no protons at 2J distance); there are T1 ridges in the spectrum but the intensity of the ridge at the the F2 frequency of interest is very, very low; the 6J cross-peak is visible with and without LP; the same 3J, 4J, and 6J cross-peaks are visible in the 1H-15N gHMBC spectrum of a diastereomer of the compound, recorded under identical conditions. Craig, it's better to use either comment tool under the posts or edit your original questions for the follow-up: this helps to keep the content more organized. Thanks. - Evgeny Fadeev (Nov 13 '12 at 09:05) Thanks Evgeny, duly noted. Here are some examples of "abnormal" long-range contacts: 5J (1H/31P) J Bio NMR, 2011, 51, 357–367; 6J (1H/183W) MRC, 1994, 32, 235–241; 6J (1H/117Sn) MRC, 2002, 40, 65–69 and MRC, 2004, 42, 776–780; 7J (1H/31P) MRC, 2001, 39, 684–688. So far no hits for 1H/15N. - Craig (Feb 04 '13 at 04:54) |
