CC/GG Contacts Facilitate the B to A Transition of DMA in Solution

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Abstract
Self-complementary decadeoxynucleotides, CCGATATCGG, CCAGATCTGG, CCCTG- CAGGG, GGGGGCCCCC, were designed and synthesized to estimate the A-philic free energy of CC/GG contacts. First, regions of temperature-stability of the double-stranded conformation were determined for each 10-mer. Then, circular dichroism spectra were recorded for the B-family forms at different temperatures, counter-ion concentrations and trifluoroethanol contents. A cooperative change typical of the B-A transition is observed in the CD spectra at a trifluoroethanol content specific for each duplex. The positions of half-transition points were functions not only of the nucleotide sequence but of the duplex length as well: the B to A transitions were hindered in these 10-mers in comparison with a lengthy DNA. The B-phility value was estimated to be 3 kcal/mol of 10-mer. The B-A transition point was shown to drop with an increase in the number of CC/GG contacts in a duplex. The designed 10-mers made it possible to estimate quantitatively the A- phility of CC/GG contact as compared with an average DNA: (FA-FB)CC=0.2 Kcal/mol, (FA-FB)DNA=0.7 Kcal/mol.