A comparison of the hairpin stability of the palindromic d(CGCG(A/T)4CGCG) oligonucleotides.

The palindromic deoxyribonucleotides 5'-CGCGA-TATCGCG-3' and 5'-CGCGTTAACGCG-3' have been characterized by 1H NMR spectroscopy. The NMR data identified both B-DNA duplex conformations and hairpin conformations, the latter with loop regions consisting of the four central nucleotides. The resonances of the various conformations were assigned by use of two-dimensional NMR methods. The relative stability of the various conformations was investigated as a function of temperature, ionic strength and nucleotide concentration. The duplexes were found to be stabilized at high ionic strength and at low temperature, while the hairpins were stabilized at low ionic strength and at medium temperature. The thermodynamics of the duplex-hairpin and the hairpin-random coil transitions were examined, and compared to the other two oligonucleotide in the palindromic d(CGCG(A/T)4CGCG) oligonucleotide family. The relative stabilities of the duplex conformations with respect to the random coil conformations are similar for the d(CGCGAATTCGCG), d(CGCGATATCGCG) and d(CGCGTATACGCG) oligonucleotides. The duplex conformation of d(CGCGTTAACGCG) is less stable. The hairpin of d(CGCGTTAACGCG) seems also to be less stable relative to the random coil conformation than in the case of the other oligonucleotides at an equal oligonucleotide concentration. A cruciform intermediate between the duplex and hairpin conformations is suggested to explain some discrepancies observed in this work in case of the d(CGCGTTAACGCG) oligonucleotide. This is similar to what has been reported for the d(CGCGTATACGCG) oligonucleotide.