Secondary structure of peptides, 10. FT‐IR and ¹³C NMR CP/MAS study on tacticity and secondary structure of poly(D,L‐leucine)s

A series of poly ( L ‐leucine)s with average degrees of polymerization in the range between 10 and 100 and various poly ( D,L ‐leucine)s were prepared by polymerization of L ‐leucine‐ N ‐carb‐oxyanhydride (NCA) or D,L ‐Leu‐NCA. The FT‐IR spectra measured in KBr show that the amide I and II bands are exclusively sensitive to the secondary structure of the poly (leucine)s and not to their tacticity. The tacticities of the poly( D,L ‐leucines) were determined in solution of trifluoroacetic acid by means of 13 C NMR spectra, while 13 C NMR CP/MAS spectra of the solid polymers revealed that the poly( D,L ‐leucine)s contain 40 – 60 wt.‐ % α‐helix structure. For this reason the broad amide I and II bands do not enable one to distinguish isotactic and more or less atactic poly(leucine)s. However, the IR‐bands between 500 and 900 cm −1 allow a differentiation between poly( L ‐leucine) and poly( D,L ‐leucine)s and also between poly( D,L ‐leucine)s of different tacticity. The IR‐spectra confirm that primary amine‐initiated polymerizations of D,L ‐Leu‐NCA do not lead to long stereo blocks.