NMR Line Widths of Biopolymers

As we saw in Section 3.5, the line width (peak width at half height) of a Lorentzian line is determined by the reciprocal of the effective spin-spin relaxation time (7 ). For small organic 

Not all large molecules are spherical they can also be long random coils with a certain degree of flexibility along the chain. If the chain is flexible, there are rapid local fluctuations between monomers and the dipole-dipole interactions are effectively averaged. An example of this is polyuridylic acid (poly U), which has a molecular weight of 100,000 daltons but is nonetheless very flexible. The line widths for poly U as a random coil are only a few hertz. If the chain is incorporated into a rigid double or triple helix, these local motions are lost and the resonances are broadened beyond detection (see Section 14.7.2). 

Many small molecules have a tendency to form large aggregates in solution. It is no longer the molecular weight of an individual monomer that determines the line width but the overall mobility of the molecule in the aggregate. An extreme example of this phenomenon is lipid molecules in membranes (Section 14.8). 

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Figure 2.6 illustrates the approximate resonance frequencies for a number of nuclides at a particular value of B0. On this scale, the range of chemical shifts for each nuclide is smaller than the width of the vertical line denoting the nuclide. It is clear that the frequency range is a continuum, so the H resonance of TMS can in principle serve as the reference for all nuclei. In a modern NMR spectrometer all frequencies are derived from a single source, so all chemical shifts can be determined relative to TMS (or relative to DSS in aqueous solutions). IUPAC has formally recommended that chemical shifts of 2H, 13C, 15N, and 3,P in biopolymers measured in aqueous solution be reported relative to the H resonance of DSS.55 It seems likely that this recommendation will be broadened to use TMS as the reference for all nuclides in nonaqueous solutions. 

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