Sum-frequency spectroscopy and imaging of aligned helical polypeptides
The sum-frequency spectroscopy signatures of NH-(amide A) and C = O (amide I) groups, the amide segments in all proteins, are measured in thin films that consist of an ensemble of right-handed, helical poly-gamma-benzyl-L-glutamate (PBLG) macromolecules that are endgrafted and self-organized into a monomolecular film with a large degree of unidirectional order. Distinct sum-frequency spectral signatures associated with the amide A and the amide I bands are observed because of a strong noncentro-symmetry produced by intra- and intermolecular forces. Hydrogen bonding self-organizes amino and acidic groups within the molecular helical scaffold. In an endgrafted thin film, repulsive electrostatic forces between PBLG macromolecules stabilize the organization between molecules. The average orientation of the PBLG chain was measured. Imaging scans using sum-frequency generation, complemented by atomic force microscopy, were used to investigate the uniformity of orientation of the PBLG chains.