CRYSTAL-STRUCTURE OF INTERCALATED 4-STRANDED D(C3T) AT 1.4 ANGSTROM RESOLUTION

The crystal structure of d(C3T), solved at 1.4 Angstrom resolution, re veals that the molecule forms a four-stranded intercalated complex. It consists of two parallel-stranded duplexes, each of which is held tog ether by cytosine-protonated cytosine base pairs. The two duplexes are intercalated with each other and have opposite strand orientation. Th e molecule has a flat, lath like appearance, and the covalently bonded cytosines have a slow right-handed twist of 17.1 degrees. However, th ere is considerable asymmetry. On one of the flat sides, the phosphate groups are rotated away from the center of the molecule. They are hel d in this orientation by bridging water molecules that bind the NH of cytosine and a phosphate group of an opposite chain. There is also con siderable microheterogeneity in the structure. The cytosine hemiproton ation occurs even at pH 7 where stable crystals form.