Electron spin resonance studies have been carried out on frozen aqueous mixtures, containing a macromolecule (Sephadex or Thiogel) and a sulfur compound (reduced or oxidized penicillamine). Prior to freezing, the pH of the solutions was adjusted to 3.8, 7.4, or 9.3. Similar experiments were carried out on the individual components. The samples were irradiated in vacuum at 77°K with X-rays to a dose of 0.5 megarad, and the measurements were carried out immediately after irradiation and after subsequent heat-treatments to temperatures in the range 77° to 263°K. After heat-treatment the samples were brought back to 77°K and the spectra were recorded. The intermolecular transfer of unpaired spins from Sephadex or Thiogel to reduced penicillamine was found to be much greater in the acid than in neutral and alkaline solutions. In contrast, the transfer to oxidized pencillamine was independent of pH. The results indicate that the transfer of unpaired spins from the macromolecule is greater to the undissociated than to the dissociated thiol group, and that hydrogen transfer repair reactions may occur in frozen aqueous mixtures.

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