Development of a Field Cycling NMR System for PQR Detection in Biopolymers*

Our goal is to extend the sensitivity of field cycling pure quadrupole resonance (PQR) methods to be of use in biological systems. The nuclei of interest are 2 5 Mg, 6 7 Zn, 4 3 Ca, n B and 1 7 0 . The experiment is based on a field cycling double resonance technique, in which the quadrupole resonance of a rare nucleus is found through its effect on the magnetic order of the abundant nucleus to which the rare nucleus is coupled through dipole-dipole interaction. A field-cycling N M R spectrometer has been developed, based on our existing 500 M H z high resolution spectrometer. The sample can be shuttled pneumatically f rom the high field of a commercial 500 M H z magnet to the magnet 's top, where the residual field and its gradient is canceled out by a pair of Helmholtz coils. Low field homogeneity is within 0.5 gauss. The H signal is observed at high field as a free induction decay (FID) after a 90° pulse. At low field the sample can be irradiated by a digitally tuned R F coil in the 300 k H z 7 M H z range. The sample has to be maintained at low temperature ( ~ 3 0 K) to avoid relaxation via thermal motion of methyl groups in biomolecules. For this purpose field cycling equipment is placed in a variable temperature dewar ( 4 3 0 0 K). We plan to use solutions of biomolecules in s tandard cryoprotective buffer , containing ~ 30% glycerol. Preliminary results on the quadrupole resonance of natural abundance 1 7 0 in the cryoprotective buffer and of natural abundance n B in a protease inhibitor at 50 m M are presented.