Field Dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) in the Photoreaction of<i>N</i>-Acetyl Histidine with 2,2‘-Dipyridyl in Aqueous Solution

Abstract

Chemically induced dynamic nuclear polarization (CIDNP) effects for the amino acid-dye (histidine-dipyridyl) photoreaction system are measured in the range between 0 and 7 T using a novel mechanical field cycling unit with fast digital positioning of a high-resolution NMR probe in a spatially varying magnetic field. 1 H CIDNP effects are observed for the CH 2 protons in β-position and for two protons (H-2 and H-4) at the imidazole ring. For the protons in β-position a multiplet effect is observed having a polarization pattern that changes with the magnetic field. By analysis of the spin nutation, the non-Boltzmann population differences among the nuclear levels are determined. At a field below 20 mT “zero-field character” of the multiplet effect prevails corresponding to preferentially populated states with symmetric spin wave functions. Likewise, for the two histidine ring-protons strong polarization with an emission/absorption multiplet pattern is found between 20 and 300 mT changing below 20 mT to zero-field character. Superimposed is emissive CIDNP (net effect) for both protons. Above 0.1 T, the ring proton net effect turns absorptive and around 7 T the polarization exhibits its maximum. Numerical simulations of the field dependence in high field approximation are in very good agreement with the experimental data obtained at fields ranging from 0.1 to 7 T. The influence of different dynamic processes on the CIDNP formation and its field dependence is analyzed. Optimization of the magnetic field strength for CIDNP application in studies of protein structure and folding process is discussed.