Self-Organization of Porphyrin-Peptide Units by Metal-Mediated Peptide Assembly

The polypeptide H21(30-mer) folds into a two-stranded coiled-coil in which two solvent-exposed histidine residues reside on opposite sides of its surface. This peptide was allowed to react with cobalt(III) protoporphyrin IX, Co(ppIX), to produce [Co(ppIX){(H21(30-mer)}(2)], as determined by UV-Vis spectroscopy. This bis-axial ligation thus positions a potential coiled-coil oligomerization domain onto each face of the cobalt porphyrin ring. Circular dichroism spectroscopy and high performance size exclusion chromatography provide evidence for the solution-phase self-assembly of these porphyrin-peptide units. Evaporation of the porphyrin-peptide solution on a solid support results in the formation of long rod-like materials having millimeter-scale lengths and micron-scale diameters. The presence of Co(ppIX) in these materials was confirmed by Raman microscopy. However, they were formed only from phosphate buffer, and not from organic buffers or pure water, indicating that their formation might involve a more complicated process than originally anticipated.