Evaluation of Drug-Nucleic Acid Interactions by Thermal Melting Curves
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If the temperature of a solution containing a helical nucleic acid is raised sufficiently, strand separation, or “melting,” occurs (1 –3 ) (Fig. 1 ) (see Note 1 ) The temperature that marks the midpoint of the melting process is called the melting temperature (or T m ) (Fig. 2 ). At the T m , half of the nucleic acid exists in the helical state and the other half exists in the single-stranded state and the two species are in equilibrium (Eq. 1 ).
Fig 1 Scheme descrlbing the melting process of a DN A or RN A double-helix A-T-rich regions melt first, then additional basepairs melt and unwind the helix Finally G-C-regions melt and the twist is taken up by the single-stranded regions
Fig 2 The effect of increasing temperature on the 260 nm absorbance of polyd(A-T)2 The left ordinate shows normalized absorbance (—), where A norlnallzed = (A ss – A t )/ (A T –A h ); A ss is the absorbance for the single-stranded state (occurs at the htghest temperature),A h is the absorbance for the helical state (occurs at the lowest temperature), and A T is the absorbance at temperature T . The right ordinate shows an approxrmation of the first derivative of normalized absorbance (ΔA normalized /ΔT , 
) The temperature that marks the midpoint of the melting process is called the melting temperature (T m )
) The temperature that marks the midpoint of the melting process is called the melting temperature (T m )
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