In 1954, Edsall postulated that the imino-acid proline, which is a frequently found constituent of protein molecules, is a key determinant to the three-dimensional architecture of proteins. It not only should play a fundamental role in stabilizing helical structures of polypeptides, it should allow for sharp bends and even for a complete reversal of the direction of a helix looping back on itself. No direct evidence has yet been published to prove this prediction. Using scanning tunneling microscopy, we have presented high-resolution, real-space images of two conformations of poly-L-proline, where one structure clearly exhibits the predicted 180° back-folding behavior. The measured length, 1.89 nm, of the repeating unit cells agrees with available X-ray data for poly-L-proline I with cis-peptide bonds. We further observe aggregated poly-L-proline II, consisting of highly-ordered, periodically and parallel-linked trans-peptide chains which are 2.4 nm apart from each other. Stacking of these aggregates with their orientation rotated by 90° is also observed.
Zheng, N. J.; Rau, G.; Hazlewood, C. F.; and Rau, C.
"High-Resolution, Real-Space Imaging of Conformational Structures of Poly-L-Proline Helixes,"
Scanning Microscopy: Vol. 5
, Article 5.
Available at: https://digitalcommons.usu.edu/microscopy/vol5/iss3/5