The conformational transitions (helix-coil transitions) of three hairpin triple helices, models 5’.(A-G), + 5’-(T-C),-T,- (hrC-T), [CY]. S-&G), + 5’-(T-h’C),-T,-(C-T), [YC] and 5’.(A-G), + 5’-(T-h’C),-T,-(h’C-T), [YY], are characterized in this work by UV spectroscopy. Melting of these triplexes is biphasic. and the profiles are used to obtain the thermodynamic parameters. The thermodynamic properties of the hairpin triplex are Tm= 19.45°C and Δ Hvh, = 293. I2 kJ molt-’ for CY, Tm= 22.85°C and Δ Hvh, = 256.63 kJ mol- ’ for YC and Tm = 28.47 °Cand ΔHvh, = 234.68 kJ molt ’ for YY at pH 4.4. Those of the duplex are Tm = 30.5O°C and AH,, = 427.09 kJ molt-’ for CY, Tm= 32.96°C and Δ Hvh, = 374.47 kJ molt ’ for YC and Tm = 33.24°C and Δ Hvh, = 329.67 kJ mol- ’ for YY at pH 4.4. The distinct transitions of triplex to duplex and duplex to single strands are analyzed win g the nearest-neighbor Ising model. Electrostatic effects on each conformation are also analyzed. 0 1997 Elsevier Science B.V.