Reduction of Guanosyl Radicals in Reactions with Proteins Studied by TR-CIDNP

Pulsed-field-gradient nuclear magnetic resonance (NMR) combined with time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP) was applied to study the reduction of guanosyl radicals in reactions with the proteins hen egg white lysozyme (HEWL) and bovine a-lactalbumin (BLA) in their native state.Guanosyl radicalswere generated photochemically in the reaction of guanosine-50-monophosphate with photosensitizer, triplet-excited 2,20-dipyridyl. In this reaction, at pH 5 guanosyl cation radical is formed, which deprotonates to yield the neutral guanosyl radical. To minimize the contribution of the cation radical, phosphate buffer was added, which accelerates the deprotonation of guanosyl cation radical. From model simulations of CIDNP kinetics the rate constants of the reduction were found to be (3.1 ± 0.5) 9 107 M-1s-1 for HEWL and (1.6 ± 0.4) 9 107 M-1s-1 for BLA. Also, experiments were carried out at the conditions for denatured HEWL, i.e., at 50 C in the presence of 10 M urea-d4. The rate constant of the reduction of guanosyl radical in this case was (3.6 ± 0.5) 9 108 M-1s-1.