Structure of a bis-amidinium derivative of Hoechst 33258 complexed to dodecanucleotide d(CGCGAATTCGCG)2: The role of hydrogen bonding in minor groove drug-DNA recognition

The crystal structure is reported of a complex between the dodecanucleotide sequence d(CGCGAATTCGCG)(2) and an analogue of the DNA binding drug Hoechst 33258, in which the piperazine ring has been replaced by an amidinium group and the phenol ring by a phenylamidinium group, The structure has been refined to an R factor of 19.5% at 2.2 Angstrom resolution. The drug is held in the minor groove by five strong hydrogen bonds, together with bridging water molecules at both ends. There are few other contacts with the floor of the groove, indicating a lack of isohelicity with the groove and suggesting (i) that the observed high DNA affinity of this drug is primarily due to the array of hydrogen bonds and (ii) that these more than compensate for its poor isohelicity.