HUTTERMANN, J., AND MULLER, A. Free Radicals in Gamma-Irradiated 2-Deoxy-n-ribose: Electron Spin Resonance of Single Crystals and Poly crystalline Material. Radiation Res. 33,248-259 (1969). Single crystals of 2-deoxY-D-ribose have been studied by electron spin resonance (ESR) spectroscopyat 8-mm wavelength following 'Y-irradiation. The main feature of the spectra recorded was a quintet pattern of about 10 gauss hyperfine splitting. The splitting as well as the mean g factor of 2.005 was found to be almost independent of crystal orientation. Minor spectral variations were attributed to the presence of other radicals. This conclusion was proved by observation of ESR spectra from irradiated poly crystalline 2-deoxY-D-ribose at 8-mm and 3-cm wavelengths. Heat-treat ment of these sampIes revealed the presence of at least two other radicals. The quintet pattern is attributed to interaction of a radical electron at a carbon atom with four s-protons bound to neighboring carbon atoms in radicals COOHCH 2CHOHCOHCHa, which are assumed to be produced by rupture of a CO bond following initial dissociation of atomic hydrogen. The proposed reaction scheme is discussed as a model for the initial pro cesses of chain ruptures in DNA.