Question

12. Explain the effect of molecular size and motion on NOE(Nuclear overhauser effect).

The question is from structural biology(NMR spectroscopy).

Answer 1

The size of Nuclear overhauser effect (NOE) is directly proportional to the ratio of magnetogyric ratios of sending and receiving nuclei. For large molecules with high viscosity solvents, molecular motion reduces and become lesser than the Larmor precession frequency. In this situation, negative NOE approaches -100%. Sometimes, it is helpful to manipulate solvent viscosity and temperature to achieve negative NOE's, since these are inherently larger than the positive NOEs seen under conditions of fast molecular motion.

When the molecular weight is around 1000, the NOE effects will be very small, the ROESY experiment can be performed which does not have this limitation.