2NH2A.cntdot.T helixes in the ribo- and deoxypolynucleotide series. Structural and energetic consequences of 2NH2A substitution

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Abstract
Polynucleotide helices formed by the interaction of (d2NH2A)n, (r2NH2A)n, (dT)n and (rT)n were prepared and their physical and spectroscopic properties examined. Thermal transitions, dependence of Tm [melting temperature] on salt concentration, stoichiometry, phase diagrams and calculated enthalpies are reported. UV, CD [circular dichroism] and IR spectra are reported. All of the deoxy-deoxy helices containing 2NH2A have positive CD 1st extrema near 290 nm and appear to have B-form structure. All the ribo-ribo or hybrid helices have negative 1st extrema in this region and appear to have A-form structure. Elevation of Tm by the 2-NH2 group of 2NH2A is much smaller in the deoxy than in the ribo series. An equation based on the electrostatic theory of Manning was used to calculate ethalpies of the helix-coil transitions of the complexes reported here. These calculated enthalpies are larger for 2NH2A.cntdot.T than for A.cntdot.T helices, but the difference is much smaller in the deoxy than in the ribo series. These effects on Tm and .DELTA.H in the deoxy series are attributed to loss of stabilization of the spine of hydration in B-form structures caused by interference of the 2-NH2 group in the minor groove of the helix. Complete phase diagrams for all 2NH2A,T systems and some A,T systems are reported. The diagrams differ widely and can be placed in 4 groups according to the number of transitions each system possesses.