Poly(dA)·poly(dA) and DNA duplex with four or more adenine bases in a row exhibits a broad, solid-state structural premelting transition at about 35°C. The low-temperature structure is correlated with the phenomena of 'bent DNA.' We have conducted temperature-dependent ultraviolet resonance Raman measurements of the structural transition using poly(dA)·poly(dT) at physiological salt conditions, and are able to identify, between the high and low temperature limits, changes in the vibrational frequencies associated with the C4 carbonyl stretching mode in the thymine ring and the N6 scissors mode of the amine in the adenine ring of poly(dA)·poly(dT). This work supports the model that the oligo-dA tracts' solid-state structural premelting transition is due to a set of cross-strand bifurcated hydrogen bonds between consecutive dA·dT pairs.
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