Solid State NMR Structural Analysis of the RADA16-I Designer Self-Assembling Peptide

RADA16-I is a small designer peptide (amino acid sequence Ac-(RADA)₄-NH₂) (1-2) which forms nanofiber matrices through amyloid-like cross-β self-assembly.  It is the essential component of the commercial product BD Puramatrix^TM, which can support tissue regeneration and 3D cell culture. Unlike amyloid fibrils, nanofiber matrices of RADA16-I can dynamically re-grow following nanofiber fragmentation without addition of new peptide.  The highly charged nature of the amino acid sequence at neutral pH (R⁺ and D^-) has lead researchers to hypothesize that RADA16-I nanofibers are characterized by antiparallel β-sheets.  In an effort to fully characterize RADA16-I nanofiber structures and understand mechanisms of self-assembly and self-healing, we have prepared isotopically labeled RADA16-I nanofiber for solid state nuclear magnetic resonance (ssNMR) measurements.  Using ssNMR and supporting techniques (IR spectroscopy and X-ray scattering), we have discovered that the RADA16-I peptide does not self-assemble into nanofiber structures which have been proposed in the literature.  These findings have impact on our understanding of the role of charged sidechains in peptide self-assembly.  Furthermore, we will show that the peptide’s behavior in solution depends on structural changes and inter-molecular organization that can occur in the solid state before the peptide is added to solution.

1.   Ellis-Behnke, R. G., Y. X. Liang, S. W. You, D. K. C. Tay, S. G. Zhang, K. F. So, and G. E. Schneider. 2006. Nano neuro knitting: Peptide nanofiber scaffold for brain repair and axon regeneration with functional return of vision. Proceedings of the National Academy of Sciences of the United States of America 103:5054-5059. 

2.   Yokoi, H., T. Kinoshita, and S. G. Zhang. 2005. Dynamic reassembly of peptide RADA16 nanofiber scaffold. Proceedings of the National Academy of Sciences of the United States of America 102:8414-8419.