Morphological Control of Chromophore Spin State in Zinc Porphyrin-Peptide Assemblies.

Self-assembled peptide micelles and fibers demonstrate unique control over the photophysical properties of the bound, light-activated chromophore, zinc protoporphyrin IX, (PPIX)Zn. Micelles encapsulate either a mixture of uncoordinated and coordinated (PPIX)Zn or all coordinated depending on the ratio of peptide/porphyrin. As the ratio increases toward a 1:1 micelle/porphyrin ratio, providing the chromophore with a discrete coordination environment reminiscent of unstructured proteins, the micelles favor triplet formation. Fibers, however, promote a linear array of porphyrin molecules that dictates exciton hopping and excimer formation at ratios as high as 60:1, peptide/porphyrin. However, even in fibers, the formation of the triplet species increases with increasing peptide/porphyrin ratio due to increased spatial separation between neighboring chromophores facilitating intersystem crossing. Full characterization of the micelles structures and comparison to the fibers lead to the comparison with natural systems and the ability to control the excited populations that have utility in photocatalytic processes. In addition, the incorporation of a second chromophore, heme, yields an electron transfer pathway in both micelles and fibers that highlights the utility of the peptide assemblies when engineering multichromophore arrays as inspired by natural, photosynthetic proteins.

Diverse Bilayer Morphologies Achieved via α-Helix-to-ß-Sheet Transitions in a Short Amphiphilic Peptide.

Transmembrane proteins are functional macromolecules that direct the flow of small molecules and ions across a lipid bilayer. Here, we propose the development of helical peptide amphiphiles that will serve as both the bilayer and the functional unit of a self-assembled peptide bilayer membrane. The peptide, K3L12, was designed not only to possess dimensions similar to that of a lipid bilayer but also to yield a structurally robust, α-helical bilayer. The formation of α-helices is pH-dependent, and upon annealing the sample, a transition from α-helices to ß-sheets can be controlled, as indicated by optical and vibrational spectroscopies. Imaging the materials confirms morphologies similar to that of a lipid bilayer but rich in α-helices. Annealing the samples yields a shift in the morphology from bilayers to curled disks, fibers, and sheets. The structural robustness of the material can facilitate the incorporation of many functions into the bilayer assembly.