Duplex-Guided Refolding into Novel G-Quadruplex (3+1) Hybrid Conformations.

The oligomer d(GCGTG3 TCAG3 TG3 TG3 ACGC) with short complementary flanking sequences at the 5'- and 3'-ends was shown to fold into three different DNA G-quadruplex species. In contrast, a corresponding oligomer that lacks base complementarity between the two overhang sequences folds into a single parallel G-quadruplex. The three coexisting quadruplex structures were unambiguously identified and structurally characterized through detailed spectral comparisons with well-defined G-quadruplexes formed upon the deliberate incorporation of syn-favoring 8-bromoguanosine analogues into specific positions of the G-core. Two (3+1) hybrid structures coexist with the parallel fold and feature a novel lateral-propeller-propeller loop architecture that has not yet been confirmed experimentally. Both hybrid quadruplexes adopt the same topology and only differ in their pattern of anti→syn transitions and tetrad stackings.