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Sum Frequency Vibrational Spectroscopy: The Molecular Origins of the Optical Second-Order Nonlinearity of Collagen

Israel Rocha-Mendoza ^*, Diego R. Yankelevich ^*, Mingshi Wang ^*, Karen M. Reiser , Curt W. Frank  and André Knoesen ^*

^* Department of Electrical and Computer Engineering, Department of Neurological Surgery, University of California, Davis, California; and Department of Chemical Engineering, Stanford University, Palo Alto, California

Correspondence: Address reprint requests to Andre Knoesen, Tel.: 530-758-9165; E-mail: knoesen{at}ece.ucdavis.edu.

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: 1), the methylene groups associated with a Fermi resonance between the fundamental symmetric stretch and the bending overtone of methylene; and 2), 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.