PH-sensitive assembly of light-harvesting dendrimer zinc porphyrin bearing peripheral groups of primary amine with poly(ethylene glycol)-b-poly(aspartic acid) in aqueous solution

The light-harvesting dendrimer zinc porphyrin [NH2CH2CH2NHCO](32)DPZn, a potential photosensitizer in photodynamic therapy, was synthesized and successfully assembled with poly(ethylene glycol)-b-poly(aspartic acid) to form micelles in PBS buffer (10 mM) following the concept of polyion complex micelle. The coupling reaction between the [CO2H](32)DPZn and N-trifluoroacetylethylene-1,2-diamine using the DCC/HOBt method afforded the protected dendrimer [CF3CONHCH2CH2NHCO](32)DPZn, which was then mildly hydrolyzed to remove all the trifluoroacetyl. protecting groups. Light-scattering measurements showed that the [NH2CH2CH2NHCO](32)DPZn has the ability to assemble with poly(ethylene glycol)-b-poly(aspartic acid) to form a spherical micelle with a diameter of about 55 nm, having a narrow size polydispersity in PBS. The micelles were stable both in size and composition up to a 0.90 M (NaCl) salt concentration. The high salt stability is associated with a hydrogen-bonding network formed in the micellar core among the amide groups of the dendrimers and those of poly(aspartic acid) moieties, which could be destroyed by urea, a typical hydrogen-bonding cleaver. Furthermore, the pH range for the stable micelles in physiological saline (0.15 M NaCl) is from 6.2 to 7.4, suggesting the potential of the pH-triggered release of the entrapped dendrimers in the acidic pH environment (pH similar to 5.0) of the intracellular endosomal compartment.