The synthesis of a novel sapphyrin-oligonucleotide conjugate is described along with its DNA photocleavage and nucleic acid binding properties. The sapphyrin-oligonucleotide conjugate produces photodamage on a complementary oligonucleotide target when irradiated at wavelengths above 620 mm. Upon piperidine treatment, guanine residues on the target strand in the vicinity of the sapphyrin macrocycle are found to be cleaved more effectively than guanines remote from the sapphyrin subunit. No sequence-specific photomodification was observed when noncomplementary oligonucleotides were used as st target. The duplexes formed between the sapphyrin conjugate and complementary nucleic acids were found to have higher melting temperatures than analogous control systems consisting of unmodified oligonucleotides of the same sequence. Melting temperature studies using variable salt concentrations and oligonucleotide targets indicate that the binding enhancement is due to hydrophobic interactions. The sapphyrin unit attached to an oligonucleotide can thus serve a dual role; it can act as a sequence-specific photomodification agent at irradiation wavelengths >620 nn, and it can increase the affinity of a sapphyrin-bearing oligonucleotide to a complementary sequence.