Thermodynamic and structural properties of pentamer DNA.DNA, RNA.RNA, and DNA.RNA duplexes of identical sequence.

Four pentamers with the general sequence 5'CU(T)GU(T)G/5'CACAG have been prepared by chemical synthesis in order to generate duplex structures with common sequences. The four duplexes studied include the DNA.DNA duplex (5'dCACAG/5'dCTGTG) and the RNA.RNA duplex (5'rCUGUG/5'rCACAG) as well as the two corresponding DNA.RNA heteroduplexes (5'rCUGUG/5'dCACAG and 5'CACAG/5'dCTGTG). The measured entropy, enthalpy, and free energy changes upon melting are reported for each pentamer and compared to the predicted values where possible. Results show that the two DNA.RNA heteroduplexes are destabilized (delta G degrees 25 = -4.2 +/- 0.4 kcal/mol) relative to either the DNA.DNA duplex (delta G degrees 25 = -4.8 +/- 0.5 kcal/mol) or the RNA.RNA duplex (delta G degrees 25 = -5.8 +/- 0.6 kcal/mol). Circular dichroism spectra indicate that the RNA and the two heteroduplexes adopt an A-form conformation, while the DNA conformation is B-form. Imino proton NMR spectra also show that the heteroduplex structures resemble the RNA.RNA duplex.