Tuning peptide amphiphile supramolecular structure for biomedical applications

The use of biomaterials in regenerative medicine has been an active area of research for more than a decade. Peptide amphiphiles, which are short peptide sequences coupled to alkyl tails, have been studied in the Stupp group since the beginning of the decade and been used for a variety of biomedical applications. Most of the work has focused on the bioactive epitopes places on the periphery of the PA molecules, but the interior amino acids, known as the beta-sheet region, give the PA nanofiber gel much of its mechanical strength. To study the important parameters in the beta-sheet region, six PA molecules were constructed to determine the influence of beta-sheet length and order of the amino acids in the beta-sheet. It was found that having beta-sheet forming amino acids near the center of the fiber improves PA gel stiffness, and that having extra amino acids that have preferences for other secondary structures, like alpha-helix decreased the gels stiffness. Using FTIR and circular dichroism it was found that the mechanical properties are influenced by the amount of twist in the beta-sheet, and PAs that have more twisted beta-sheets form weaker gels. The effect amino acid properties have on peptide amphiphile self-assembly where studied by synthesizining molecules with varying side group size and hydrophobicity. It was found that smaller amino acids lead to stiffer gels and when two amino acids had the same size the amino acid with the larger beta-sheet propensity lead to a stiffer gel. Furthermore, small changes in peptide structure were found to lead to big changes in nanostructure, as leucine and isoleucine, which have the same size but slightly different structures, form flat ribbons and cylindrical nanofibers, respectively. Phenylalanine and alanine were studied more indepth because they represent the effects of adding an aromatic group to amino acids in the beta-sheet regon. These phenylalanine PAs formed short, twisted ribbons when freshly dissolved in water that rapdily elongate to form long twisted ribbons. After being aged for two weeks half of these twisted ribbons turn into helical ribbons and by one month all of them have formed this new nanostructure. As a target in regenerative medicine, spinal cord injury repair presents a daunting challenge that has so far eluded successful pharmaceutical treatment. Previous work showing that PAs bearing the IKVAV epitope were found to increase functional recovery in mice paved the way for the more complex systems studied here. By making a PA that bound growth factors like neurotrophin-3 (NT-3) and glial cell line derived neurotrophic factor (GDNF) in with the PA matrix, it was found that the release of NT-3 could be significantly slowed from an IKVAV with the presence of a novel binding epitope, and that including GDNF into the gel significantly increased neurite outgrowth compared to the standard IKVAV PA.