We have synthesized a series of polyintercalating compounds, including the first known tetraintercalator, based on the 1,4,5,8-naphthalenetetracarboxylic diimide chromophore. The chromophores are attached in a headto-tail arrangement by peptide linkers and are synthesized by standard solid phase peptide synthesis methods. We report evidence, based on UV-visible spectroscopy and viscometry, that the compounds are fully intercalated upon binding to double-stranded DNA. Using DNAse I footprinting experiments, the bisintercalator 2 was found to bind to DNA in a cooperative manner. The footprinting results as well as association and dissociation kinetics data reveal that the compounds exhibit a tremendous preference for GC over AT sequences. A mode of binding is proposed in which the compounds intercalate completely from the major groove, and not in a threading manner as may be suggested by their structures. A kinetic scheme is proposed that takes into account the observed cooperativity and fits the data for the dissociations of the polyintercalators from poly(dAdT), although a similar scheme could not adequately model their dissociations from poly(dGdC) or from calf thymus DNA.

The synthesis and NMR structural studies are reported for a modular threading tetraintercalator bound to DNA. The tetraintercalator design is based on 1,4,5,8-tetracarboxylic naphthalene diimide units connected through flexible peptide linkers. Aided by an overall C(2) symmetry, NMR analysis verified a threading polyintercalation mode of binding, with linkers alternating in the order minor groove, major groove, minor groove, analogous to how a snake might climb a ladder. This study represents the first NMR analysis of a threading tetraintercalator and, as such, structurally characterizes a new topology for molecules that bind to relatively long DNA sequences with extensive access to both DNA grooves.

In view of the modular nature and facile synthesis of our NDI-based polyintercalators, such structural knowledge can be used to improve or alter the specificity of the compounds and design longer polyintercalators that recognize correspondingly longer DNA sequences with alternating access to both DNA grooves.

We have been investigating a modular, threading DNA polyintercalator design based upon the 1,4,5,8-naphthalene tetracarboxylic diimide (NDI) intercalating unit. Previously, we have reported the NMR analysis of a bis-intercalator-DNA complex in which the peptide linker between NDI units was found to occupy the DNA major groove (Guelev, Lee, Sorey, Hoffman, Iverson, Chem. Biol. 2001, 8, 415-425). Here we describe the NMR analysis of a complex between a related bis-intercalator known to display altered DNA sequence specificity. In this case, the linker resides in the DNA minor groove. We have thus shown that within this set of sequence specific bis-intercalators, both DNA grooves can be accessed, setting the stage for longer threading polyintercalators designed to have linkers occupying both grooves in an alternating fashion.