ESR of sulfur-substituted purines and nucleosides; sulfur- and oxygen-centered radicals in mercaptopurine crystals at 77 K

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Abstract
Single crystals of the sulfur‐substituted adenosine analog, 6‐methylmercaptopurine riboside, have been irradiated by 4.0 MeV electrons and studied by electron spin resonance techniques at 77 K. Three resonances with large g‐value variations were observed. Two of these, R3 and R5, showed no hyperfine structure, but exhibited g‐tensor principal values of: R3 as 2.0608, 2.0211, and 2.004; and R5 as 2.154, 2.003, and 1.989. These are assigned to a π‐electron disulfide radical (R3) and a monosulfide RS, (R5). The third resonance (R4) exhibited a 41.5 G isotropic doublet splitting and the g‐tensor principal values were 2.086, 2.001, and 1.989. This resonance is ascribed to a radical formed by hydrogen abstraction from the O3′ atom in the ribose sugar. The electronic and molecular structures of the radicals are discussed in some detail.