Second Harmonic Generation Mediated by Aligned Water in Starch Granules

Area of Science:

• Biophysics
• Materials Science
• Spectroscopy

Background:

• Second harmonic generation (SHG) is a nonlinear optical process.
• Starch granules exhibit unique optical properties influenced by their molecular structure.
• Understanding the origin of SHG in biological materials like starch is crucial for advanced imaging and material characterization.

Purpose of the Study:

• To investigate the origin of second harmonic generation (SHG) in starch granules.
• To elucidate the role of hydrogen bonds and water organization in SHG.
• To correlate experimental SHG measurements with theoretical calculations.

Main Methods:

• Ab initio quantum mechanical modeling was employed to understand SHG contributions.
• Polarization-in, polarization-out (PIPO) second harmonic generation microscopy was used for experimental analysis.
• Starch granules (maize and potato) were subjected to varying hydration and heat treatments, including deuteration.

Main Results:

• Ab initio calculations identified anisotropic organization of hydroxide and hydrogen bonds mediated by aligned water as the primary source of SHG in starch allomorphs A and B.
• Experimental results showed highest SHG intensity in fully hydrated starch granules, with heat treatment diminishing the signal.
• PIPO SHG imaging revealed significant changes in nonlinear optical susceptibility ratios with hydration and deuteration, confirming the sensitivity to the hydrogen bond network.

Conclusions:

• The dominant contribution to SHG in starch granules originates from the ordered hydroxide and hydrogen bond network, significantly influenced by aligned water.
• Hydration conditions critically affect SHG intensity and nonlinear susceptibility ratios in starch.
• Nonlinear microscopy provides valuable insights into the structural dynamics and optical properties of starch granules.