A HIGH RESOLUTION 13C NMR STUDY OF SOLID POLY (β-BENZYL-l-ASPARTATE) BY THE CROSS POLARIZATION-MAGIC ANGLE SPINNING METHOD. DISTINCTION OF THE RIGHT-HANDED α-HELIX, LEFT-HANDED α-HELIX, ω-HELIX, AND β-SHEET FORMS BY CONFORMATION-DEPENDENT 13C CHEMICAL SHIFTS