Highlighting radical sites through polarized neutron scattering from AFP-modulated polarized protons

Selective nuclear spin reversal by the method of adiabatic fast passage is a way to order a system of dynamically polarized nuclei. Using polarized neutron scattering, it increases the visibility of sources and sinks of proton polarization in radical proteins. Notably, Tyr369 has been confirmed as a potential radical site in bovine liver catalase.

Time-resolved neutron scattering has been used to study dynamically polarized protons in tyrosyl-doped bovine liver catalase. While the evolution of proton polarization and its inversion by the method of adiabatic fast passage (AFP) in a standard dynamic nuclear polarization (DNP) system with organic Cr(V) complexes can be well modelled and understood, the experiments with tyrosyl-doped catalase lead us into the world of extremely dilute paramagnets with only about 10¹⁷ unpaired electrons per cm³. In this regime, the strength of DNP is comparable to the drift of proton polarization towards its thermal equilibrium of P_e = 0.35% at T = 1 K and B = 3.5 T. Negative DNP, which counteracts this drift, is confined to protons very close to the radical site, typically within 5 Å. In contrast, AFP reverses the polarization of protons at a larger distance from the radical. The contrast of a domain of polarized close protons giving rise to neutron scattering is considerably enhanced by AFP. Moreover, the spread of proton polarization is sensitive to magnetic inhomogeneities, like the iron of the heme of the catalase molecule. In short, polarized neutron scattering from AFP-modulated polarized samples provides an excellent tool for mapping sources and sinks of proton polarization in radical proteins.