Abstract
Conformational transitions for a series of imperfect palindromes related to the dodecamer d(CGCGAATTCGCG) have been investigated. These sequences are: two isomeric 13-mers - d(CGCAGAATTCGCG) (13-merI) and d(CGCGAATTACGCG) (13-merII), 17-mer d(CGCGCGAATTACGCGCG) and 15-mer d(CGCGAAATTTACGCG). Insertion of a single adenine nucleotide prevents these sequences from being self-complementary. Analysis of thermodynamic parameters derived from the melting profiles together with other data at higher concentrations (NMR and calorimetry) indicates that the insertion of the additional nucleotide which lacks a complement in the opposite strand does not change the enthalpy of the duplex formation, but does alter the number of stable nucleation configurations. The relative position of the insertion within the self-complementary sequence determines the equilibrium between the duplex form and the single-stranded hairpin loop. C-G segments separated by the insertion from the rest of the molecule can undergo an independent conformational transition at high salt concentration, probably to the Z form.