Energetics of Nucleic Acid Stability: The Effect of ΔC_p

摘要

We report high-resolution differential scanning calorimetric data on the poly(dAdT)poly(dAdT), poly(dA)poly(dT), poly(dIdC)poly(dIdC), poly(dGdC)poly(dGdC), poly(rA)poly(rU), and poly(rI)poly(rC) nucleic acid duplexes. We use these data to evaluate the melting temperatures, T_M, enthalpy changes, ΔH_M, and heat capacity changes, ΔC_P, accompanying helix-to-coil transitions of each polymeric duplex studied in this work at different NaCl concentrations. In agreement with previous reports, we have found that ΔC_p exhibits a positive, nonzero value, which, on average, equals 268 +- 33 J mol~(-1) K~(-1). With ΔC_P, we have calculated the transition free energies, ΔG, enthalpies, ΔH, and entropies, ΔS, for the duplexes as a function of temperature. Since, ΔG, ΔH, and ΔS all strongly depend on temperature, the thermodynamic comparison between DNA and/or RNA duplexes (that may differ from one another with respect to sequence, composition, conformation, etc.) is physically meaningful only if extrapolated to a common temperature. We have performed such comparative analyses to derive differential thermodynamic parameters of formation of GC versus AT, AU, and IC base pairs as well as B′ versus A and B helix conformations. We have proposed some general microscopic interpretations for the observed sequence-specific and conformation-specific thermodynamic differences between the duplexes.