Reduction of Guanosyl Radicals in Reactions with Proteins Studied by TR-CIDNP Olga B. Morozova • Robert Kaptein • Renad Z. Sagdeev • Alexandra V. Yurkovskaya Received: 15 August 2012 / Revised: 5 September 2012 / Published online: 12 October 2012 Ó Springer-Verlag 2012 Abstract 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 radicals were generated photochemically in the reaction of guanosine-5 0 -monophosphate with photosensitizer, triplet-excited 2,2 0 -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 10 7 M -1 s -1 for HEWL and (1.6 ± 0.4) 9 10 7 M -1 s -1 for BLA. Also, exper- iments were carried out at the conditions for denatured HEWL, i.e., at 50 °C in the presence of 10 M urea-d 4 . The rate constant of the reduction of guanosyl radical in this case was (3.6 ± 0.5) 9 10 8 M -1 s -1 . Dedicated to Hans-Martin Vieth on the occasion of his 70th birthday. O. B. Morozova  R. Z. Sagdeev  A. V. Yurkovskaya (&) International Tomography Center, Siberian Branch, Russian Academy of Sciences, Institutskaya 3a, 630090 Novosibirsk, Russia e-mail: yurk@tomo.nsc.ru O. B. Morozova  R. Kaptein  R. Z. Sagdeev  A. V. Yurkovskaya Novosibirsk State University, Pirogova 2, 630090 Novosibirsk, Russia R. Kaptein Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 Utrecht, The Netherlands 123 Appl Magn Reson (2013) 44:233–245 DOI 10.1007/s00723-012-0403-0 Applied Magnetic Resonance