An IR-investigation of the helix-coil conversion of poly U

In the presence of two or polyvalent cations, e. g., Mg²⁺, protonated ethylenediamine, protonated spermidine, and protonated spermine, poly U forms an ordered structure in aqueous solution. At elevated temperatures, the secondary structure is lost by a cooperative process. The melting temperature T_m rises with the size of the cations. Using IR-spectroscopy, we could show that the secondary structure of poly U is stabilized by hydrogen bonds. Both carbonyl groups of the uracil are involved in the hydrogen bonding. This experimental result enables us to choose the right arrangement for UU base pairing.