Multinuclear Quadrupole Resonance Studies of Biological Systems

Abstract

Abstract We have employed multinuclear quadrupole resonance, using the field cycling technique, to study the structure of heterocyclic biomolecules. The electron density at nitrogen was investigated by the Townes-Dailey model of 14N NQR. Large substituent effects were evident especially on the n orbitals. Electron transfer through H-bonds also affects the 14 N quadrupole interaction. Deuterium quadrupole resonance at exchangeable sites was also utilized. Since the deuterium quadrupole correlation coefficient is sensitive to the geometry of H-bonds, the intermolecular distance of the pyrrole-pyridine complex could be determined. The field cycling technique is sensitive enough to detect 14 N ··· 14 N double transitions in parabanic acid thanks to the long spin-lattice relaxation time. However in many biological systems, such as hydrates, the technique is hardly applicable because of the short proton relaxation times. In such cases the solid state NMR technique was utilized. The 2H NMR spectrum of thymidine reveals interesting static and dynamical features of deuterium quadrupole interaction.