A thermodynamic study on rA7U7

Abstract Ultraviolet absorbance spectroscopy and differential scanning calorimetry were employed to study the heat-induced helix-to-coil transition of the oligoribonucleotide rA 7 U 7 . The analysis of concentration-dependent ultraviolet ‘melting’ profiles was used to derive the van't Hoff transition enthalpy Δ H vH UV (−458 kJ/mol cooperative unit). From the DSC data we calculated the calorimetric transition enthalpy Δ Hcal (−412.6 kJ/mol duplex) as well as Δ H vH cal (−447.9 kJ/mol cooperative unit). For the size of the cooperative unit we obtained λ ≈ 1. In contrast to this result, by means of statistical numerical deconvolution we show that intermediate states are significantly populated; at the maximum the fraction of these states reaches 25.4% of the total population. Therefore, this DSC-deconvolution technique offers a more appropriate way to register amounts of populated intermediate states which are not sufficient to obtain a value of λ which is essentially lower than unity.