Second harmonic generation of myofibrils exhibits a polarization-resolved "gradient" effect

Area of Science:

• Biophysics
• Microscopy
• Biomolecular structure

Background:

• Polarization-resolved second harmonic generation (PSHG) microscopy is a technique used to study the ultrastructure of biological tissues.
• Myofibrils and collagen fibrils are key structural components in muscle and connective tissues, respectively.
• The rho parameter is a measure of nonlinear optical activity.

Purpose of the Study:

• To investigate the ultrastructure of individual myofibrils and collagen fibrils using PSHG microscopy.
• To visualize and analyze the rho parameter within these fibrils.
• To explore the physical mechanisms behind observed gradient effects in the rho parameter.

Main Methods:

• Utilized polarization-resolved second harmonic generation (PSHG) microscopy to image myofibrils and collagen fibrils.
• Performed numerical focal volume simulations to interpret second harmonic generation (SHG) signals.
• Re-imaged fibrils after 180° rotation to study gradient effects.

Main Results:

• Observed a gradient in the rho parameter orthogonal to myofibrils.
• Numerical simulations indicated that myosin SHG possesses an imaginary chiral component, akin to collagen fibrils.
• Fibril rotation revealed directional changes in the gradient, more pronounced in myofibrils than collagen.

Conclusions:

• Myosin SHG exhibits chiral properties that should be considered in PSHG analysis.
• The observed gradient effect is influenced by the fibril type and its orientation.
• Chiral SHG contributions, often overlooked, are significant for interpreting PSHG data in muscle and collagen disease discrimination.