Enthalpy and entropy contributions to the equilibrium of the hydrogen bonding interaction between 1-octylthymine and 9-octyladenine

Abstract

Hydrogen bonding interaction between 1-octylthymine (1) and 9-octyladenine (2) was analyzed by dynamic proton NMR spectroscopy. A mathematical model that takes into account the independent self-association of 1 and 2 was developed in order to determine the equilibrium constant of the interaction between 1-octylthymine and 9-octyladenine K1-2. The measurement of K 1-2, in CDCl3, in the range of temperatures of -30 to 40 °C permitted estimation of the contributions of enthalpy ΔH° = -6.74 kcal/mol, and entropy ΔS° = -13.95 cal mol-1K -1, to the hetero-association process. From the enthalpic point of view the association of 1-octylthymine with 9-octyladenine is 0.56 kcal/mol stronger than the self-association of 1-octylthymine, and 0.09 kcal/mol stronger than the self-association of 9-octyladenine. X-ray diffraction of a monocrystal of mixed 1-octylthymine and 9-octyladenine indicated that the interaction between these nucleobases in the solid state resemble Haschemeyer-Sobell more than Watson-Crick hydrogen bonding.