Scanning tunneling microscopy study of poly-L-proline.

Differences in its peptide bonds allow the imino acid poly-L-proline to exist in two significantly different geometric structures. Form I with cis peptide bonds is supposed to be a right-handed helix and form II with trans peptide bonds a left-handed helix. Cis/trans isomerization about the proline imide is believed to cause the denaturation of a number of proteins and may be a key step in protein folding. Using scanning tunneling microscopy (STM), we present high-resolution images of air-dried poly-L-proline. It is found that the electric conductivity of one monolayer of poly-L-proline is sufficient to allow for STM imaging without significant tip-sample interaction. Only at locations where stacking of poly-L-proline chains occurs, a direct contact of the probing tip to the molecules becomes apparent and prevents us, at present, from resolving the atomic structure of the topmost layer. Our STM images of poly-L-proline show that form II is relatively rigid and forms aggregates in most cases. Form I, which is occasionally observed, is very flexible and exhibits sharp bends as well as 180 degrees backfolding. These observations confirm theoretical predictions on the existence of two peptide bond conformations of poly-L-proline.