Polynucleotides, NOESY

Figure 14.24. Illustration of the expected NOESY correlations along the polynucleotide backbone.

Second, in small molecules, the NOE builds up slowly and attains a theoretical maximum of only 50%, as noted earlier in the ID context. (See Section 5-4 and Appendix 5.) Because a single proton may be relaxed by several neighboring protons, the actual maximum normally is much less than 50%. (Of course, the same problem exists in the ID NOE experiment.) Moreover, as the molecular size increases and behavior departs from the extreme narrowing limit, the maximum NOE decreases to zero and becomes negative. Thus, particularly for medium-sized molecules, the NOESY experiment may fail. For larger molecules, whose relaxation is dominated by the Wq term, not only is the maximum NOE —100% rather than +50%, but also the NOE buildup occurs more rapidly. The NOESY experiment thus has been of particular utility in the analysis of the structure and conformation of large molecules such as proteins and polynucleotides. 

NOES Y spectroscopy has become especially nsefnl in the study of large molecules, such as proteins and polynucleotides. Very large molecules tend to tumble more slowly in solution, which means that nuclear Overhauser effect interactions have more time to develop. Small molecules tumble more quickly in solution the nuclei move past one another too qnickly to allow a significant development of dipolar interactions. The resnlt is that NOESY cross peaks may be too weak to be observed. 

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