MOLECULAR AND CRYSTAL-STRUCTURE OF AN INTERCALATION COMPLEX - PROFLAVINE-CYTIDYLYL-(3',5')-GUANOSINE

The high‐resolution crystal and molecular structure of a 3:2 complex of proflavine and cytidylyl‐(3′,5′)‐guanosine is described. The complex exhibits more than one mode of dye binding to the dinucleoside phosphate. One proflavine cation is symmetrically intercalated between the base pairs. The other proflavine cations and ones related by symmetry stack above and below the base pairs and also hydrogen bond externally to the duplex. The conformation of the CpG is most similar to A‐RNA with all C(3′)‐endo sugar puckering. To allow the base pairs to stretch from the normal 3.4‐Å separation to a 6.8‐Å separation, the torsion angles ϕ and χ of the guanosine are increased by about 60° from the values found in RNA. The crystal structure itself contains disordered sulfate anions and is highly solvated, with all but one water molecule involved in a continuous water–sulfate channel. Copyright © 1979 John Wiley & Sons, Inc.