C8 methylation of the title purine nucleotides was achieved in near neutral aqueous solution by reaction with 13C enriched [Co(NH 3) 5(CH 3)] 2+. This cation decomposes upon dissolution in water by release of a methyl radical. The latter was identified by electron spin resonance (ESR) spectroscopy as the PBN/13??H3 adduct (PBN = phenyl-N-tert-butylnitrone). When the purine nucleotide was used in large excess, the efficiency of the trapping by the C8 atom was determined by integration of the 13C NMR signals to be 20-25% for GMP and 15-20% for AMP, respectively, at 37??C. ?? 2004 Elsevier Inc. All rights reserved.