Thursday, October 6, 2011

Dynamic Nuclear Polarization in NMR

Chandrakumar, N. Journal of the Indian Institure of Science 2010, 90, 133.

Dynamic nuclear polarization was first predicted — and, shortly thereafter, established experimentally — in 1953, the first demonstration being on Lithium metal. The basic approach involves the saturation of the ESR of a paramagnetic species in the system, while the NMR is observed. Initial applications of DNP involved low and moderate field studies that focused especially on investigations of molecular hydrodynamics. Applications to MRI provided a subsequent fillip to the technique. In the meanwhile, the closely related nuclear Overhauser effect (NOE) — which involves saturation, as well as observation of different NMR signals — had become an essential technique for the structure elucidation of both small molecules, as well as biomolecules. Most recently, DNP is witnessing rejuvenation, with high field applications to sensitivity enhancement in NMR. We present in the following an overview of Dynamic nuclear polarization (DNP). The elementary general theory of the phenomenon is discussed. Four different DNP mechanisms that are currently recognized are briefly introduced and different modes of the experiment — involving either cw ESR irradiation, or pulsed ESR excitation — are pointed out. A brief run down of various possible implementations is presented, including our own early work at moderate fields in cw mode, as well as hardware configurations and requirements for high field DNP. Different current implementations of DNP experiments are summarized, including solid state, as well as in situ and ex situ dissolution DNP variants. Typical results of DNP enhanced high resolution NMR are then briefly discussed, including the results of our own early work on differential 19F enhancements at moderate fields. Design of free radicals that satisfy the requirements to establish an efficient cross effect DNP is discussed. Recent experiments that have succeeded in detecting an intermediate in the photocycle of bacteriorhodopsin are alluded to. Finally, the implementation of ultrafast multi-dimensional NMR techniques under DNP conditions is briefly discussed, as an approach to further exploitation of the prospects that are on offer.