Determination of collagen nanostructure from second-order susceptibility tensor analysis

Area of Science:

• Biophysics
• Materials Science
• Spectroscopy

Background:

• Collagen's unique structure influences its optical properties.
• Understanding collagen's molecular arrangement is crucial for various biological and medical applications.
• Previous models for analyzing collagen structure using optical methods had limitations.

Purpose of the Study:

• To develop a model for second harmonic generation (SHG) polarization dependence in type I collagen fibrils.
• To attribute the molecular origins of collagen's second-order susceptibility.
• To extract molecular structural information from collagen fibers using polarization SHG (P-SHG) measurements.

Main Methods:

• Utilizing sum-frequency vibrational spectrum experiments to identify molecular contributors to collagen's second-order susceptibility.
• Developing a theoretical model based on these molecular origins.
• Applying the model to polarization SHG (P-SHG) experiments on type I and type II collagen.

Main Results:

• The model successfully describes the polarization dependence of SHG from collagen fibrils.
• Predicted collagen I peptide pitch-angle: 45.82° ± 0.46°, methylene pitch-angle: 94.80° ± 0.97°.
• Type II collagen showed similar peptide pitch-angle (45.72° ± 1.17°) but a different methylene pitch-angle (97.87° ± 1.79°).

Conclusions:

• The proposed model accurately predicts collagen pitch angles, aligning well with X-ray diffraction data.
• Far-field P-SHG measurements offer a viable method for determining molecular structural parameters of collagen fibers.
• The study highlights the distinct contributions of peptide and methylene groups to collagen's optical properties.