Cross Polarization and Dynamic - Angle Spinning of 17 O in L - Alanine

The study of biologically active and other organic compounds by solid-state NMR has for the most part been limited to spin-1/2 nuclei such as H, C, N, F, and P. The study of O, a quadrupolar nucleus (S = 5/2), in solid organic compounds has been limited due to its low natural abundance, low magnetogyric ratio, and strong secondorder quadrupolar interactions. The first two difficulties can be alleviated to some extent through isotopic substitution, the use of high magnetic fields, and through cross polarization (CP) (1) from H to the central (1/2 <-> —1/2) O transition. For a static sample, it is theoretically possible to achieve a one-shot sensitivity enhancement of 7.3 (assuming a large excess of *H compared to O). An alternative approach involving adiabatic slow passage to transfer magnetization from the satellite transitions to the central transition of O could be used to generate an enhancement factor of 5.0 (2). However, when the sample is spun about an axis inclined with respect to the magnetic field, there can be a significant decrease in CP efficiency (3) because the time dependence of the first-order quadrupolar interaction interferes with Hartmann-Hahn matching. Cross polarization while spinning the sample at an angle of 0° (parallel) with respect to the magnetic field is tantamount to operating under static conditions and maximum CP efficiency can be achieved.