Chlorosome-Inspired Synthesis of Templated Metallochlorin-Lipid Nanoassemblies for Biomedical Applications.

Chlorosomes are vesicular light-harvesting organelles found in photosynthetic green sulfur bacteria. These organisms thrive in low photon flux environments due to the most efficient light-to-chemical energy conversion, promoted by a protein-less assembly of chlorin pigments. These assemblies possess collective absorption properties and can be adapted for contrast-enhanced bioimaging applications, where maximized light absorption in the near-infrared optical window is desired. Here, we report a strategy for tuning light absorption towards the near-infrared region by engineering a chlorosome-inspired assembly of synthetic metallochlorins in a biocompatible lipid scaffold. In a series of synthesized chlorin analogues, we discovered that lipid-conjugation, central coordination of a zinc metal into the chlorin ring and a 3(1)-methoxy substitution were critical for the formation of dye assemblies in lipid nanovesicles. The substitutions result in a specific optical shift, characterized by a bathochromically-shifted (74 nm) Qy absorption band, along with an increase in absorbance and circular dichroism as the ratio of dye-conjugated lipid was increased. These alterations in optical spectra are indicative of the formation of delocalized excitons states across each molecular assembly. This strategy of tuning absorption by mimicking the structures found in photosynthetic organisms may spur new opportunities in the development of biophotonic contrast-agents for medical applications.