Investigation of the magnetic field dependence of CIDNP in multinuclear radical pairs. 1. Photoreaction of histidine and comparison of model calculation with experimental data

Magnetic field effects on CIDNP formed in the diamagnetic products of geminate recombination of photo-generated radical pairs are calculated at arbitrary field strength. The simulations are based on a model that uses a full quantum mechanical description of the polarized nuclei and a semiclassical account of all other nuclear spins. The results are compared with CIDNP experiments on the photoreaction between N-acetylhistidine and 2, 2'-dipyridine in fields between 0 and 7 T employing field cycling between a variable field of polarization and a fixed field of detection. To allow quantitative comparison the simulation takes into account the effects of adiabatic field change and of RF excitation pulse angle on the NMR intensity. Both net and multiplet polarization as observed in two coupled spin pairs (CH2 protons in beta position of histidine, and protons in H-2; H-4 position of its ring) are well reproduced over the whole field range. The wide applicability of the theoretical model and the possibility to extend it to larger spin systems make it particularly useful for the analysis of CIDNP in protein systems.