Solid-state NMR studies of structures and molecular motions for the spidroin-like polymers

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 (T_1ρ(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 T_1ρ(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 T_1ρ(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.