Determination of Collagen Nanostructure from Second-Order Susceptibility Tensor Analysis

A model is proposed to describe the polarization dependence of second harmonic generation (SHG) from type I collagen fibrils. The model is based on sum-frequency vibrational spectrum experiments that attribute the molecular origins of collagen second-order susceptibility to the peptide groups in the backbone of the collagen a-helix and the methylene groups in the pyrrolidine rings. Applying our model to a polarization SHG (P-SHG) experiment leads to a predicted collagen I peptide pitch-angle of 45.82 degrees +/- 0.46 degrees and methylene pitch-angle of 94.80 degrees +/- 0.97 degrees. Compared to a previous model that accounts for only the peptide contribution, our results are more consistent with the x-ray diffraction determination of collagen-like peptide. Application of our model to type II collagen from rat trachea cartilage leads to similar results. The peptide pitch-angle of 45.72 degrees +/- 1.17 degrees is similar to that of type I collagen, but a different methylene pitch-angle of 97.87 degrees +/- 1.79 degrees was found. Our work demonstrates that far-field P-SHG measurements can be used to extract molecular structural information of collagen fibers.