Conformational characterization of polypeptides in the solid state as viewed from the conformation-dependent carbon-13 chemical shifts determined by the carbon-13 cross polarization/magic angle spinning method: oligo(L-alanine), poly(L-alanine), copolymers of L- and D-alanines, and copolymers of L-alanine with N-methyl- or N-benzyl-L-alanine