Double-Stranded Polydeoxyribonucleotides Containing 6-Thiodeoxyguanosine and 6-Thiodeoxyinosine. Poly[d(A-C) . d(T-s6G)], Poly[d(A-s6G) . d(T-C)] and Poly[d(A-C) . d(T-s6I)]

Abstract

Three new double‐stranded polydeoxyribonucleotides containing the nucleoside analogs 6‐thiodeoxyguanosine and 6‐thiodeoxyinosine were synthesized enzymatically. Poly[d(A‐C) · d(T‐s6G)] and poly[d(A‐s6G) · d(T‐C)] were prepared by extensive synthesis with DNA polymerase from Micrococcus luteus, while the same enzyme from Escherichia coli failed in extensive polymerisation but brought about the synthesis of poly[d(T‐s6G)] and poly[d(T‐s6I)] by the repair mechanism using single‐stranded poly[d(A‐C)] template and a minute amount of d(T‐G)n primer. The analog‐containing DNAs were characterized by ultraviolet absorption and circular dichroism spectra as well as by their stability against thermal and alkaline denaturation. The substitution by 6‐thiodeoxyguanosine or 6‐thiodeoxyinosine for deoxyguanosine reduces the stability of the biopolymers considerably. Exchange of the analog‐containing strand by singlestranded poly[d(T‐G)] takes place rapidly even at ambient temperature.

Synthesis of oligonucleotides containing 2′-deoxy-6-thioguanosine at a predetermined site

“…Evidence from a large number of research groups over many years has rather firmly identified incorporation of thioguanine into DNA as the primary locus of action of the mercaptopurine antimetabolites (2, 3, 4, 5, 6, 7). These correlations between incorporation into DNA and cytotoxicity have found some physicochemical basis from studies describing the overall effect of incorporating thioguanine into nucleic acids in place of guanine (8,9,10). It has also been established that NH---S hydrogen bond lengths are greater and hydrogen bond strengths are less than is the case with the NH---O hydrogen bonds (11).…”

“…An appropriate way to study this problem is to create oligonucleotides containing s6G-C base pairs at uniquely defined positions, and then to carry out appropriate physicochemical studies to define the geometry of these base pairs vis-'a-vis the normal G-C base pair. Although several studies have appeared describing incorporation of thioguanine into nucleic acids (8)(9)(10)13), we are aware of only three reports describing approaches to the specific incorporations of 6-thioguanine into chemically synthesized oligonucleotides.…”