Water-soluble, cationic metalloporphyrins that bind to DNA show promise as artificial nucleases and as sensitizers for photodynamic therapy, but fundamental questions remain about the binding motifs and sequence specificities. To address these issues, we have studied the interactions of Cu(T4) with a series of oligonucleotides that form hairpin structures (H(2)T4 = meso-tetrrkis(4-(N-methylpyridiniumyl))porphyrin). Each oligonucleotide is a 16-mer with a central run of four thymine (T) bases and complementary ends that can combine to form a specific sequence of six adenine-thymine (A=T) and guanine-cytosine (G=C) base pairs. The techniques employed include thermal melting as well as circular dichroism (CD), absorbance, and emission spectroscopies. The number of G=C base pairs in the stem is the most important factor that determines the melting temperature of the hairpin, and in every case investigated, the uptake of Cu(T4) stabilizes the hairpin. Depending on the nature of the adduct that forms, Delta epsilon varies from -22 to +17 M-1 cm(-1) in the Soret region of the CD spectrum, and the emission intensity from Cu(T4) changes by an order of magnitude. The results yield several useful insights regarding the binding interactions. One is that robust hydrogen bonding within a B-form duplex promotes intercalative binding of Cu(T4). Thus, if the composition is at least 50% G=C base pairs, intercalation will occur even in the absense of a G=C step. On the other hand, a run of four A=T base pairs defines a groove-binding site with an affinity comparable to that for intercalation at a G=C step. Finally, at least in solutions containing excess oligonucleotide, there is no sign that either loop binding or hemiintercalation is a prevalent mode of interaction between Cu(T4) and hairpin hosts.