Telomeres at the ends of human chromosomes contain the repeating sequence 5'-d[(TTAGGG)(n)]-3'. Oxidative damage of guanine in DNAs that contain telomeric and nontelomeric sequence generates 7,8-dihydro-8-oxoguanine (8OG) preferentially in the telomeric segment, because GGG sequences are more reactive in duplex DNA. We have developed a general strategy for probing site-specific oxidation reactivity in diverse biological structures through substitution of minimally modified building blocks that are more reactive than the parent residue, but preserve the parent structure. In this study, 8OG was substituted for guanine at G(8), G(9), G(14), or G(15) in the human telomeric oligonucleotide 5'-d[AGGGTTAG(8)G(9)GTT AG(14)G(15)-GTTAGGGTGT]-3'. Replacement of G by 8OG in telomeric DNA can affect the formation of intramolecular G quadruplexes, depending on the position of substitution. When BOG was incorporated in the 5'-position of a GGG triplet, G quadruplex formation was observed; however, substitution of BOG in the middle of a GGG triplet produced multiple structures. A clear correspondence between structure and reactivity was observed when oligonucleoticles containing 8OG in the 5'-position of a GGG triplet were prepared in the quadruplex or duplex forms and interrogated by mediated electrocatalytic oxidation with Os(bPY)(3)(2+) (bpy = 2,2'-bipyridine). The rate constant for one-electron oxidation of a single 8OG in the 5'-position of a GGG triplet was (6.2+/-1.7) x 10(4) M-1 s(-1) in the G quadruplex form. The rate constant was 2-fold lower for the same telomeric sequence in the duplex form ((3.0+/-1.3) x 10(4) M-1 s(-1)). The position of 8OG in the GGG triplet affects telomerase activity and synthesis of telomeric repeat products. Telomerase activity was decreased significantly when 8OG was substituted in the 5'-position of the GGG triplet, but not when 8OG was substituted in the middle of the triplet. Thus, biological oxidation of G to 8OG in telomeres has the potential to modulate telomerase activity. Further, small molecules that inhibit telomerase by stabilizing telomeric G quadruplexes may not be as effective under oxidative stress.