A new Co(III)-hypocrellin B complex with enhanced photonuclease activity.

Hypocrellin B (HB), a naturally occurring photosensitizer, has been extensively and intensively studied as a promising photodynamic therapy (PDT) agent. In this work, a new Co(III) complex [Co(2)(HB)(tmp)(4)](4+) (tmp=3,4,7,8-tetramethyl-1,10-phenanthroline) was designed and synthesized with HB as bridging ligand and tmp as terminal ligand. [Co(2)HB(tmp)(4)](4+) exhibits improved water solubility, enhanced absorptivity in the phototherapeutic window, increased binding affinity and DNA photocleavage capability toward dsDNA with respect to HB. The photodynamic activity of [Co(2)(HB)(tmp)(4)](4+) stems from its (1)O(2) photosensitization ability, in sharp contrast to [Cu(2)(HB)(tmp)(2)](2+) which relies on superoxide anion radical (O(2)(-)) and hydroxyl radical (·OH) to photocleave DNA, though the both complexes possess similar electrochemical properties. The remarkable difference between the photodynamic mechanisms of [Co(2)(HB)(tmp)(4)](4+) and [Cu(2)(HB)(tmp)(2)](2+) was discussed in detail.