Thermodynamic characterization of the stability and the melting behavior of a DNA triplex: a spectroscopic and calorimetric study.
We report a complete thermodynamic characterization of the stability and the melting behavior of an oligomeric DNA triplex. The triplex chosen for study forms by way of major-groove Hoogsteen association of an all-pyrimidine 15-mer single strand (termed y15) with a Watson-Crick 21-mer duplex composed of one purine-rich strand (termed u21) and one pyrimidine-rich strand (termed y21). We find that the near-UV CD spectrum of the triplex can be duplicated by the addition of the B-like CD spectrum of the isolated 21-mer duplex and the CD spectrum of the 15-mer single strand. Spectroscopic and calorimetric measurements show that the triplex (y15.u21.y21) melts by two well-resolved sequential transitions. The first transition (melting temperature, Tm, approximately 30 degrees C) is pH-dependent and involves the thermal expulsion of the 15-mer strand to form the free duplex u21.y21 and the free single strand y15. The second transition (Tm approximately 65 degrees C) is pH-independent between pH 6 and 7 and reflects the thermal disruption of the u21.y21 Watson-Crick duplex to form the component single strands. The thermal stability of the y15.u21.y21 triplex increases with increasing Na+ concentration but is nearly independent of DNA strand concentration. Differential scanning calorimetric measurements at pH 6.5 show the triplex to be enthalpically stabilized by only 2.0 +/- 0.1 kcal/mol of base triplets (1 cal = 4.184 J), whereas the duplex is stabilized by 6.3 +/- 0.3 kcal/mol of base pairs. From the calorimetric data, we calculate that at 25 degrees C the y15.u21.y21 triplex is stabilized by a free energy of only 1.3 +/- 0.1 kcal/mol relative to its component u21.y21 duplex and y15 single strand, whereas the 21-mer duplex is stabilized by a free energy of 17.2 +/- 1.2 kcal/mol relative to its component single strands. The y15 single strand modified by methylation of cytosine at the C-5 position forms a triplex with the u21.y21 duplex, which exhibits enhanced thermal stability. The spectroscopic and calorimetric data reported here provide a quantitative measure of the influence of salt, temperature, pH, strand concentration, and base modification on the stability and the melting behavior of a DNA triplex. Such information should prove useful in designing third-strand oligonucleotides and in defining solution conditions for the effective use of triplex structure formation as a tool for modulating biochemical events.
Plum
G E
GE
Department of Chemistry, Rutgers, State University of New Jersey, New Brunswick 08903.
Park
Y W
YW
Singleton
S F
SF
Dervan
P B
PB
Breslauer
K J
KJ
D001483:Base Sequence; D002152:Calorimetry, Differential Scanning; D002942:Circular Dichroism; D004247:DNA / Q000737:chemistry*; D006860:Hydrogen Bonding; D007700:Kinetics; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D009838:Oligodeoxyribonucleotides / Q000138:chemical synthesis / Q000737:chemistry*
D009838:Oligodeoxyribonucleotides; D004247:DNA