Three-dimensional structural imaging of starch granules by second-harmonic generation circular dichroism

Area of Science:

• Nonlinear Optics
• Biophotonics
• Chiroptical Spectroscopy

Background:

• Chirality is a fundamental property of biological molecules like amino acids.
• Circular dichroism (CD) detects chirality but has low signal contrast and axial resolution.
• Second-harmonic generation circular dichroism (SHG-CD) offers higher contrast but is limited to surfaces.

Purpose of the Study:

• To develop a 3D chiral detection method for biological tissues.
• To investigate the origins of SHG-CD signals in complex biological systems.
• To visualize the 3D organization of starch granules using SHG-CD.

Main Methods:

• Combined laser-scanning microscopy with second-harmonic generation (SHG) and SHG-CD.
• Utilized circularly polarized light for SHG excitation.
• Applied the technique to starch granules composed of chiral amylopectin.

Main Results:

• Achieved 3D visualization of SHG-CD with sub-micrometer resolution.
• Observed ~100% contrast in SHG-CD for starch granules.
• Detected sign changes in SHG-CD across starch granules, indicating macroscopic origin.

Conclusions:

• SHG-CD in thick biological tissues primarily arises from macroscopic molecular packing.
• Developed a novel method for visualizing 3D chiral structures in biological tissues.
• Expanded nonlinear chiroptical studies beyond surfaces to complex biological environments.