Achiral and chiral sum frequency vibrational spectroscopy of collagen fibrils

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.