High shape persistence in single polymer chains rigidified with lateral hydrogen bonded networks

Exploiting tapping mode-scanning force microscopy (TM-SFM), we characterized single polymeric chains of poly(isocyanodipeptides) (PICs) equilibrated in quasi two-dimensions on the basal plane of mica surfaces. While the average contour length <L> of an acid-catalyzed PIC bearing L-alanine-D-alanine methyl ester groups was as high as 5.3 mum, the corresponding Ni-catalyzed product exhibited an <L> of 70 nm. With a newly devised method based on the statistical analysis of the curvature of polymeric chains on a length scale up to about 100 nm from SFM images, we determined their persistence length /(p). The measured value of /(p) = 76 +/- 6 nm for both products, independent of the contour length, indicates that the single polymer molecules are very rigid, i.e., even more rigid than the double-stranded DNA. This rigidity is attributed to the helical structure of the polymer backbone and, in particular, to the hydrogen-bonded networks that are present between the alanine moieties in the side chains.