和频振转光谱学：胶原蛋白二阶光学非线性效应的分子学起源

The molecular origins of second order nonlinear effects in type I collagen fibrils have been identified with sum-frequency generation vibrational spectroscopy. The dominant contributing molecular groups are: i) the methylene groups associated with a Fermi resonance between the fundamental symmetric stretch and the bending overtone of methylene, and ii) the carbonyl and peptide groups associated with the amide I band. The noncentrosymmetrically aligned methylene groups are characterized by a distinctive tilt relative to the axis perpendicular to the main axis of the collagen fiber, a conformation producing a strong achiral contribution to the second order nonlinear effect. In contrast, the stretching vibration of the carbonyl groups associated with the amide I band results in a strong chiral contribution to the optical second order nonlinear effect. The length scale of these chiral effects ranges from the molecular to the supramolecular.