MELTING AND PREMELTING TRANSITIONS OF AN OLIGOMER MEASURED BY DNA-BASE FLUORESCENCE AND ABSORPTION

Incorporation of 2-aminopurine (2AP) in place of adenine gives an opti cal probe of local and global DNA conformation. The temperature depend ence of the absorption of the duplex d[CTGA(2AP)-TTCAG](2) DNA decamer shows that the helix has approximately an all-or-none melting transit ion. Absorbance at wavelengths of 260 and 330 nm monitors the average normal base conformation and the 2AP base local conformation, respecti vely. From this measure, the 2AP base melts less than 1 degrees C belo w the other bases. Temperature-dependent lifetime measurements of 2AP also mirror the melting transition. Absorption spectra show that below T-m most 2AP's are H-bonded. Fluorescence intensity and excitation sp ectra measurements show, on the other hand, that the most highly-fluor escent states correspond to nonH-bonded 2AP's which sense conformation al changes of the helix. The temperature dependence of the fluorescenc e spectral shift shows the conformation and/or dynamics of the 2AP bas e changes 10 degrees C or more below T-m. The data suggest a premeltin g transition which is purely dynamic in nature-transient exposure of m ost 2AP's to water increases, while the average conformation remains B -helical.