Chain polarity in polymorphic unit cells of cellulose, amylose, and chitin

The polymorphic structures of crystalline carbohydrate polymers such as cellulose, amylose, and chitin have been extensively studied using x-ray and neutron diffractions. However, the chain polarity, especially antiparallel polarity of polymer chains, within the unit cell determined from structural refinements of diffraction data was often challenged. Although spectroscopic techniques such as infrared, Raman, and nuclear magnetic resonance can distinguish the polymorphic differences, they could not provide the definitive answer for chain polarity within the crystalline unit cell. In order to resolve such disputes, we have applied vibrational sum frequency generation (SFG) spectroscopy which is intrinsically sensitive to non-centrosymmetric arrange of molecules in space and confirmed that cellulose II, alpha-chitin, and V-type amylose have antiparallel arrangement of constituent polymer chains within the crystal unit cell. This talk will explain how such determination is possible with SFG.