Field dependence of chemically induced dynamic nuclear polarization (CIDNP) in the photoreaction of N-acetyl histidine with 2,2 '-dipyridyl in aqueous solution

Chemically induced dynamic nuclear polarization (CIDNP) effects for the ami no acid-dye (histidine-dipyridyl) photoreaction system are measured in the range between 0 and 7 T using a novel mechanical field cycling unit with fa st digital positioning of a high-resolution NMR probe in a spatially varyin g magnetic field. H-1 CIDNP effects are observed for the CH2 protons in bet a -position and for two protons (H-2 and H-4) at the imidazole ring. For th e protons in beta -position a multiplet effect is observed having a polariz ation pattern that changes with the magnetic field. By analysis of the spin nutation, the non-Boltzmann population differences among the nuclear level s are determined. At a field below 20 mT "zero-field character" of the mult iplet effect prevails corresponding to preferentially populated states with Symmetric spin wave functions. Likewise, for the two histidine ring-proton s strong polarization with an emission/absorption multiplet pattern is foun d. between 20 and 300 mT changing below 20 mT to zero-field character. Supe rimposed 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 exh ibits its maximum. Numerical simulations of the field dependence in high fi eld approximation are in very good agreement with the experimental data obt ained 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. Opt imization of the magnetic field strength for CIDNP application in studies o f protein structure and folding process is discussed.