NOESY polymer conformation

We described the basic aspects of NOESY in Section 10.1 as an introductory example of a 2D experiment. NOESY is very widely used in measuring macro-molecular conformation, as we see in Chapter 13. However, as shown in Fig. 8.4, the H— H nuclear Overhauser enhancement 17 varies from its value of +0.5 in small molecules to a limiting value of — 1 in large polymers with very long Tc, and at intermediate values of rc the NOE may vanish. An alternative is to use the NOE measured in the rotating frame, as this quantity is always positive. By analogy to NOESY, this technique has the acronym ROESY (rotating frame Overhauser enhancement spectroscopy),... 

The homonuclear Overhauser enhancements explained in section 4.2.1 may also in principle be exploited for spatial and conformational information. However, this is only possible when the proton shifts are unusually sensitive to tacticity, because otherwise they cannot be resolved. They can be investigated by either 1-D or 2-D methods. The 1-D method involves selective irradiations, but is relatively quick and easy to quantify. The 2-D method, NOESY [38], is more elegant. In both cases one must take precautions to avoid spin-diffusion if quantitative information is required. Spin-diffusion is discussed in chapters 6 and 7 in the context of solid-state NMR, where its effects are more serious. What happens, in brief, is that the Wq term in (equation 4.6) can become very large for a proton pair, so that spin energy flows freely between most of the protons in the sample. Thus all selectivity is lost. In practice, spin diffusion is not too serious in a typical semi-mobile polymer solution, so that qualitative distance information may easily be obtained. The method is particularly promising for random copolymers, where more resonances are available for selective irradiation, because of the many variations of local sequence, as well as of tacticity. 

Two-dimensional nuclear Overhauser enhancement spectroscopy (2D-NOESY) provided qualitative information about spatial proximities of protons in a molecule. The unimer micelle of poly(A/l-Np(60)) (poly(A/l-Np(x)) where x = 60 mol%) in D2O gave cross-peaks between the aromatic and aliphatic protons owing to dipolar interactions, whereas the same polymer showed no such cross-peaks in DMF-rfv [29], This is indicative of a highly compact conformation of the unimer micelle in water. 

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