A model for the interaction of the water-soluble sapphyrin derivative 1 with a variety of nucleic acid species is presented. Three modes of interaction are described : The first mode, seen with all the nucleic acid species, is that of "phosphate chelation". This mode is exemplified by a solid state structure of the complex formed between the monobasic form of cAMP and the sapphyrin species [2H . 2](2+). It involves the specific chelation of the oxyanion of a phosphorylated nucleotide or nucleic acid species with the protonated core of sapphyrin via Coulombic interactions that include H-bonding interactions. Spectroscopically, this interaction is characterized by a visible absorption at 422 nm and corresponds to complexes formed between the dimeric form of 1 and phosphorylated nucleotides. In the case of double-stranded DNA, this mode of binding shows a preference for the more flexible copolymer [poly-(dA-dT)](2) over [poly(dG-dC)](2). The second mode involves a hydrophobic interaction with the nucleobases present in both monomeric and single-stranded polymeric nucleotides. Spectroscopically, this nucleotide-dependent interaction is characterized by the absorption of the monomeric species of 1 at ca. 450 nm. The third mode involves the highly ordered aggregation of 1 on the surface of certain double-stranded, helical nucleic acids at low phosphate ester to sapphyrin (P/S) ratios and is templated by the higher order structure of these nucleic acid polymers. Spectroscopically, this mode is characterized by a visible absorption at ca. 400 nm and a large, conservative induced CD signal for 1.