No abstract

No abstract

The noncentrosymmetric molecular structures responsible for the second order nonlinear effects of collagen fibrils are identified by sum frequency vibrational spectroscopy. CH 2 and C=O functional groups dominate the achiral and chiral spectra, respectively. Introduction:Second-harmonic generation (SHG) from collagen provides a promising noninvasive and nondestructive optical imaging approach for obtaining information in situ about structure and orientation of collagen fibrils in biological tissues [1]. Ever since SHG was observed in collagen protein structures three decades ago [2], uncertainty remains concerning the molecular origins of the optical nonlinearity in collagen [3][4][5]. Sum frequency vibrational (SFG-V) spectroscopy can identify the molecular components that produce second-order nonlinear optical effects in proteins [6]. For example, our group has shown that the second order nonlinear effects of poly-alpha-benzyl-L-glutamate (PBLG), a single synthetic α-helical protein, originate from collective radiation of the noncentrosymmetric NH and CO-polar groups oriented by intramolecular hydrogen bonding along the α-helix chain [6]. The conformational structure of the collagen molecule is significantly more complicated than that of the single α-helix. A collagen molecule is composed of three α-chains that twist along a common axis to form a right-handed super triple-helical structure. The amino acid composition of each α-chain is a repeating Gly-X-Y sequence dominated by glycine (33%) and proline/hydroxyproline (25%), where X and Y are frequently proline and hydroxyproline, respectively. The formation of numerous interchain hydrogen bonds between the backbone NH-group of glycine of one α-chain and the backbone CO of a residue in X-position of a neighboring α-chain is the major source of stability of the triple helix.In this work we report SFG-V spectra of collagen type I for various input/output polarization configurations. The existence of both chiral and achiral spectral contributions is shown and for the first time we identify the dominant non-centrosymmetric molecular groups that contribute to collagen's second order nonlinearity.