Solid-State NMR Studies of Structures and Molecular Motions for the Spidroin-Like Polymers

Abstract

Using 13C solid-state nuclear magnetic resonance(NMR) we studied the structures of two spidroin-like polymers which were synthesized by the polymerization of polyalanine ((Ala)5) with oligomers of polystyrene(PS, MW=2000) and polyisoprene(PI, MW=2210). 13C CP/MAS (cross polarization/magic angle spinning) NMR spectra and spin-lattice relaxation time in the rotating frame (T1ρ(13C)) results of the polymers indicated that the chemical shifts of (Ala)5 in both polymers of polystyrene-co-polyalanine (PS-co-PAL) and polyisoprene-co-polyalanine (PI-co-PAL) were almost the same. This means that (Ala)5 peptide segments in the two polymers have similar chemical environments and secondary structures. The similar T1ρ(13C) values for (Ala)5 in the two polymers indicate that (Ala)5 peptide segments also have similar aggregate structures. The mechanical properties of the two spidroin-like polymers are quite different: PS-co-PAL is granular and tough while PI-co-PAL is rubber-like and tensible at room temperature. This indicates that the mechanical performances of spidroin-like polymers are strongly linked to the properties of the chosen polymers. The T1ρ(13C) values of the skeletons —CH2CH— in PI-co-PAL and PS-co-PAL were (5.3±0.4) and (47.0±5.5) ms, respectively, which indicates that PI segments are softer than PS segments in the polymers. In addition, the density functimal theory (DFT) based chemical shift calculation showed that (Ala)5 peptide segments in the polymers of PS- co-PAL and PI-co-PAL had dihedral angles of (-131°, 142°), which correspond to a β-sheet conformation.