Second Order Optical Nonlinearity in Single and Triple Helical Protein Supramolecular Assemblies
The molecular origins of second order nonlinear optical properties in proteins structures identified by sum-frequency vibrational spectroscopy are discussed. Noncentrosymmetry, existing at a molecular level, is translated to a macroscopic scale through non-covalent intermolecular interactions. In an alpha-helix hydrogen bonds align C=O and N–H groups parallel to a main axis. In contrast, in a collagen triple helix hydrogen bonds align C=O and N–H groups almost perpendicular to a main axis. The hydrogen bonds create a helical ladder. The effect of these unipolar and helical alignments on the second order nonlinearity is to produce achiral and chiral contributions, respectively. In the collagen triple helix, there is also an achiral contribution caused by methylene molecular groups.
Keywords: Sum-frequency generation, second harmonic generation, collagen, alpha-helix, nonlinear optics, biopolymers.