Figure 1.

The geometry of experimental SFG-V spectroscopy. (A) The laboratory frame (X, Y, Z): the X-Z plane designates the plane of incidence within which the visible and IR incident beams are oriented orthogonally to each other and at a 45° angle to the collagen fibril, which also lies in the X-Z plane, parallel to the X axis. The polarization states of the input/output beams are defined relative to the X-Z plane, as indicated by the small black arrows; (B) Orientation of LC collagen film in the laboratory frame: a photomicrograph (20×) under polarized light of cholesteric LC collagen film, 100 × 100 microns, shows parallel bands with alternating light and dark stripes oriented longitudinally along the X axis and orthogonal to the Z axis; the inset above the laboratory frame shows that LC collagen lacks the crimp pattern seen in RTT collagen; (C) The molecular frame (a, b, c): the a and c axes designate the orientation of the two orthogonal dipoles of the amide bond, with the a axis parallel to the C-N bond on the polypeptide backbone. The molecular frames corresponding to the three peptide bonds in the collagen tripeptide Gly-X-Y are shown above a model of the polypeptide backbone. The chain is oriented such that the two-dimensional conformation of the chromophores is clearly seen in the Gly amide chromophore.