Determination of the solid-state conformations of polyalanine using magic-angle spinning NMR spectroscopy

Conformations of the powder samples of poly-L-alanine with molecular weights of 356 Da (tetraalanine), 15 000 Da (PLA-200), and 23 600 Da (PLA-333) were characterized by ¹³C cross-polarization magic-angle spinning (CPMAS) and ¹H combined rotation and multiple pulse (CRAMPS) solid-state NMR spectroscopy. From the ¹³C and ¹H isotropic chemical shift values, it is predicted that the main chain conformations of tetraalanine and PLA-200 are mainly β-sheet while the conformation of PLA-333 is mainly an α-helix. It is unusual and interesting that a high molecular weight homopolypeptide, PLA-200, has β-sheet conformation rather than an α-helix conformation. The effect of dichloroacetic acid (DCA) solvent on the backbone conformation of these peptides was also studied. It is inferred from solid-state NMR results that conformations of tetraalanine and PLA-333 are similar before and after crystallization from DCA. On the other hand, the backbone conformation of PLA-200 is 60% α-helix and 40% β-sheet after crystallization from the DCA solvent. Variable temperature studies are also reported.