An endor study of X-ray irradiated singlecrystals of two simple peptides

An X-ray irradiated single crystal of N-acetylglycine was studied using the negative technique. The paramagnetic electron wavefunction spreads significantly over the molecules neighboring the damaged molecule in this crystal. The proton is lost by the methylene group of the radiation damaged N-acetylglycine molecule was shown to be trapped in the crystal near the damaged molecule. Conventional studies of X-ray irradiated single crystals of 2,5-piperazinedione were conducted, and the principal hfs tensors for the delta protons were determined. Analysis of these tensors showed that neither of the previously proposed models for the delta proton hfs is correct. The present result suggests that practically all the spin density must be in the vicinity of the alpha carbon atom. The problem of radiation damage in low-sulphur-content proteins is discussed.