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Hydration and Conformational Transitions in DNA, RNA, and Mixed DNA-RNA Triplexes Studied by Gravimetry and FTIR Spectroscopy

Abstract We have studied by gravimetric measurements and FTIR spectroscopy the hydration of duplexes and triplexes formed by combinations of dAn, dTn, rAn, and rUn strands. Results obtained on hydrated films show important differences in their hydration and in the structural transitions which can be induced by varying the water content of the samples. The number of water molecules per nucleotide (w/n) measured at high relative humidity (98% R.H.) is found to be 21 for dAn.dTn and 15 for rAn.rUn. Addition of a third rUn strand does not change the number of water molecules per nucleotide: w/n=21 for rUn*dAn.dTn and w/n=15 for rUn*rAn.rUn. On the contrary, the addition of a third dTn strand changes the water content but in a different way, depending whether the duplex is DNA or RNA. Thus, a loss of four water molecules per nucleotide is measured for dTn*dAn.dTn while an increase of two water molecules per nucleotide is observed for dTn*rAn.rUn. The final hydration is the same for both triplexes (w/n=17). The desorption profiles obtained by gravimetry and FTIR spectroscopy are similar for the rAn.rUn duplex and the rUn*rAn.rUn triplex. On the contrary, the desorption profiles of the dAn.dTn duplex and the triplexes formed with it (rUn*dAn.dTn and dTn*dAn.dTn) are different from each other. This is correlated with conformational transitions induced by varying the hydration content of the different structures, as shown by FTIR spectroscopy. Modifications of the phosphate group hydration and of the sugar conformation (S to N type repuckering) induced by decrease of the water content are observed in the case of triplexes formed on the dAn.dTn duplex.