Formation of a left-handed RNA double helix: energetics of the A-Z transition of poly[r(G-C)] in concentrated NaClO4 solutions.

Ultraviolet spectroscopic and nuclear magnetic resonance (NMR) studies have shown that poly[r(G-C)] in a solution of 4 M NaClO4 undergoes a transition to a left-handed Z-RNA helix upon raising the temperature to 60 degrees C [Hall, K., Cruz, P., Tinoco, I., Jr., Jovin, T. M., & van de Sande, J. H. (1984) Nature (London) 311, 584-586]. In the present report, the transition temperature of this particular order/order transition is shown to increase with decreasing NaClO4 concentration to about 110 degrees C, above which only the helix-to-random coil transition is detectable. The reversibility and cooperativity of the helix/helix conversion has facilitated the quantitative evaluation of the transition enthalpy by means of differential scanning microcalorimetry. In 5 M NaClO4, the transition temperature is 43 degrees C, the conversion enthalpy 4.2 kJ (1.0 kcal) per mole of base pair, and the corresponding entropy change 13 J (3.1 cal) deg-1. The van't Hoff enthalpy for the same process, determined from the temperature dependence of the optical transition, is 0.26 MJ (62 kcal) per mole of cooperative unit. The ratio of the two enthalpy values yields an apparent cooperative length for the A-Z transition of poly[r(G-C)] of approximately 60 base pairs, indicative of a concerted all-or-none process.