The thermal stability of a new polynucleotide complex has been used to establish the hydrogen-bonding structure of three-stranded C-G-CH+ helices. In the Hoogsteen structure, the 8NH2 group of 8NH2GMP can form a third hydrogen bond to the CH+ strand, but in the alternative structure, the 8NHz group can form no interbase hydrogen bonds. For the new complex, 8NH2GMP.2 poly(C), a transition temperature of 8OoC is observed under conditions in which the corresponding complex formed with 5'-GMP has a T , of 20°C. We conclude from this 60" elevation of transition temperature that a third hydrogen bond is formed by the 8NHz group and that the structure must have Hoogsteen bonding. In order to be compatible with this structure in regular helices formed by U,C copolymers, A-2U bonding would also have to have a Hoogsteen structure.
* Abbreviations: Poly(C), polycytidylic acid; SNHzGMP, 8-amino-5'-guanylic acid; G.2C, a 1:2 complex of poly(C) with a guanylic acid derivative (degree of polymerization of G not specified; state of protonation of C not specified); C-G-CH+, as with G.2C except that protonation of one strand of poly(C) is specified; A-2U, a 1:2 complex of an adenine derivative (degree of polymerization unspecified) with poly(U).