Spectroscopic and thermodynamic characterization of the interaction of N7-guanyl thioether derivatives of d(TGCTG*CAAG) with potential complements.

The oligomer d(TGCTGCAAG) corresponds to a region of bacteriophage M13mp18 DNA where mutations have been found to be induced by S-(2-chloroethyl)glutathione (glutathione, GSH) [Cmarik, J. L., Humphreys, W. G., Bruner, K. L., Lloyd, R. S., Tibbetts, C., & Guengerich, F. P. (1991) J. Biol. Chem. 267, 6672-6679]. This oligomer was prepared with the central G replaced by S-(2-N7-guanylethyl)-GSH or N-acetyl-S-(2-N7-guanylethyl)Cys methyl ester; these derivatives were purified by HPLC and by affinity chromatography in the latter case. UV mixing and CD spectroscopy studies showed no evidence for preferred pairing of the S-(2-N7-guanylethyl)GSH moiety to any base other than C. UV melting studies of duplexes were performed with complementary strands containing the normal C, as well as the three mismatches (T, A, and G), across from the adducted base. Thermal stabilities were reduced in all cases when G was replaced by either N7-guanyl adduct; the C-containing complement was still the most stable. The reduced stability of the duplex d(TGCTG*CAAG)/d(CTTGCAGCA), where S-(2-N7-guanylethyl)-GSH corresponds to G*, was characterized by an increase in delta G zero of 1.4-2.0 kcal mol-1 (in the range of 25-37 degrees C) relative to the unadducted duplex. van't Hoff analysis of concentration-dependent melting experiments indicated that the delta H zero of the duplex was actually more favorable when this adduct was introduced (delta delta H zero = 13 kcal mol-1), but the decreased thermal stability was due to the entropic component. Similar results were observed when G* was N-acetyl-S-(2-N7-guanylethyl)Cys methyl ester. Under the conditions used, the overall relative stabilities of the oligomeric duplexes containing various base pairs do not indicate that S-(2-N7-guanylethyl)GSH would contribute to a higher frequency of T misinsertion than G. The possibility that ionization at the guanine N1 position may be involved in mutagenesis by N7-guanyl adducts is considered.